In order to move to an eligibility based scheduling policy, we need
to have a better approximation of the ideal scheduler.
Specifically, for a virtual time weighted fair queueing based
scheduler the ideal scheduler will be the weighted average of the
individual virtual runtimes (math in the comment).
As such, compute the weighted average to approximate the ideal
scheduler -- note that the approximation is in the individual task
behaviour, which isn't strictly conformant.
Specifically consider adding a task with a vruntime left of center, in
this case the average will move backwards in time -- something the
ideal scheduler would of course never do.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20230531124603.654144274@infradead.org
When balancing sibling domains that have different number of cores,
tasks in respective sibling domain should be proportional to the
number of cores in each domain. In preparation of implementing such a
policy, record the number of cores in a scheduling group.
Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/04641eeb0e95c21224352f5743ecb93dfac44654.1688770494.git.tim.c.chen@linux.intel.com
We currently export the total throttled time for cgroups that are given
a bandwidth limit. This patch extends this accounting to also account
the total time that each children cgroup has been throttled.
This is useful to understand the degree to which children have been
affected by the throttling control. Children which are not runnable
during the entire throttled period, for example, will not show any
self-throttling time during this period.
Expose this in a new interface, 'cpu.stat.local', which is similar to
how non-hierarchical events are accounted in 'memory.events.local'.
Signed-off-by: Josh Don <joshdon@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/20230620183247.737942-2-joshdon@google.com
* Whenever cpuset needs to rebuild sched_domain, it walked all tasks looking
for DEADLINE tasks as they need to be accounted on the new domain. Walking
all tasks can be expensive and there may not be any DEADLINE tasks at all.
Task iteration is now omitted if there are no DEADLINE tasks.
* Fixes DEADLINE bandwidth misaccounting after task migration failures.
* When no controller is enabled, -Wstringop-overflow warning is triggered.
The fix patch added an early exit which is too eager and got reverted for
now. Will fix later.
* Everything else are minor cleanups.
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Merge tag 'cgroup-for-6.5' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup
Pull cgroup updates from Tejun Heo:
- Whenever cpuset needs to rebuild sched_domain, it walked all tasks
looking for DEADLINE tasks as they need to be accounted on the new
domain. Walking all tasks can be expensive and there may not be any
DEADLINE tasks at all. Task iteration is now omitted if there are no
DEADLINE tasks
- Fixes DEADLINE bandwidth misaccounting after task migration failures
- When no controller is enabled, -Wstringop-overflow warning is
triggered. The fix patch added an early exit which is too eager and
got reverted for now. Will fix later
- Everything else is minor cleanups
* tag 'cgroup-for-6.5' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/cgroup:
Revert "cgroup: Avoid -Wstringop-overflow warnings"
cgroup/misc: Expose misc.current on cgroup v2 root
cgroup: Avoid -Wstringop-overflow warnings
cgroup: remove obsolete comment on cgroup_on_dfl()
cgroup: remove unused task_cgroup_path()
cgroup/cpuset: remove unneeded header files
cgroup: make cgroup_is_threaded() and cgroup_is_thread_root() static
rdmacg: fix kernel-doc warnings in rdmacg
cgroup: Replace the css_set call with cgroup_get
cgroup: remove unused macro for_each_e_css()
cgroup: Update out-of-date comment in cgroup_migrate()
cgroup: Replace all non-returning strlcpy with strscpy
cgroup/cpuset: remove unneeded header files
cgroup/cpuset: Free DL BW in case can_attach() fails
sched/deadline: Create DL BW alloc, free & check overflow interface
cgroup/cpuset: Iterate only if DEADLINE tasks are present
sched/cpuset: Keep track of SCHED_DEADLINE task in cpusets
sched/cpuset: Bring back cpuset_mutex
cgroup/cpuset: Rename functions dealing with DEADLINE accounting
After commit 8ad075c2eb ("sched: Async unthrottling for cfs
bandwidth"), we may update the rq clock multiple times in the loop of
__cfsb_csd_unthrottle().
A prior (although less common) instance of this problem exists in
unthrottle_offline_cfs_rqs().
Cure both by ensuring update_rq_clock() is called before the loop and
setting RQCF_ACT_SKIP during the loop, to supress further updates.
The alternative would be pulling update_rq_clock() out of
unthrottle_cfs_rq(), but that gives an even bigger mess.
Fixes: 8ad075c2eb ("sched: Async unthrottling for cfs bandwidth")
Reviewed-By: Ben Segall <bsegall@google.com>
Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Hao Jia <jiahao.os@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20230613082012.49615-4-jiahao.os@bytedance.com
According to the GRUB[1] rule, the runtime is depreciated as:
"dq = -max{u, (1 - Uinact - Uextra)} dt" (1)
To guarantee that deadline tasks doesn't starve lower class tasks,
we do not allocate the full bandwidth of the cpu to deadline tasks.
Maximum bandwidth usable by deadline tasks is denoted by "Umax".
Considering Umax, equation (1) becomes:
"dq = -(max{u, (Umax - Uinact - Uextra)} / Umax) dt" (2)
Current implementation has a minor bug in equation (2), which this
patch fixes.
The reclamation logic is verified by a sample program which creates
multiple deadline threads and observing their utilization. The tests
were run on an isolated cpu(isolcpus=3) on a 4 cpu system.
Tests on 6.3.0
==============
RUN 1: runtime=7ms, deadline=period=10ms, RT capacity = 95%
TID[693]: RECLAIM=1, (r=7ms, d=10ms, p=10ms), Util: 93.33
TID[693]: RECLAIM=1, (r=7ms, d=10ms, p=10ms), Util: 93.35
RUN 2: runtime=1ms, deadline=period=100ms, RT capacity = 95%
TID[708]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 16.69
TID[708]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 16.69
RUN 3: 2 tasks
Task 1: runtime=1ms, deadline=period=10ms
Task 2: runtime=1ms, deadline=period=100ms
TID[631]: RECLAIM=1, (r=1ms, d=10ms, p=10ms), Util: 62.67
TID[632]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 6.37
TID[631]: RECLAIM=1, (r=1ms, d=10ms, p=10ms), Util: 62.38
TID[632]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 6.23
As seen above, the reclamation doesn't reclaim the maximum allowed
bandwidth and as the bandwidth of tasks gets smaller, the reclaimed
bandwidth also comes down.
Tests with this patch applied
=============================
RUN 1: runtime=7ms, deadline=period=10ms, RT capacity = 95%
TID[608]: RECLAIM=1, (r=7ms, d=10ms, p=10ms), Util: 95.19
TID[608]: RECLAIM=1, (r=7ms, d=10ms, p=10ms), Util: 95.16
RUN 2: runtime=1ms, deadline=period=100ms, RT capacity = 95%
TID[616]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 95.27
TID[616]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 95.21
RUN 3: 2 tasks
Task 1: runtime=1ms, deadline=period=10ms
Task 2: runtime=1ms, deadline=period=100ms
TID[620]: RECLAIM=1, (r=1ms, d=10ms, p=10ms), Util: 86.64
TID[621]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 8.66
TID[620]: RECLAIM=1, (r=1ms, d=10ms, p=10ms), Util: 86.45
TID[621]: RECLAIM=1, (r=1ms, d=100ms, p=100ms), Util: 8.73
Running tasks on all cpus allowing for migration also showed that
the utilization is reclaimed to the maximum. Running 10 tasks on
3 cpus SCHED_FLAG_RECLAIM - top shows:
%Cpu0 : 94.6 us, 0.0 sy, 0.0 ni, 5.4 id, 0.0 wa
%Cpu1 : 95.2 us, 0.0 sy, 0.0 ni, 4.8 id, 0.0 wa
%Cpu2 : 95.8 us, 0.0 sy, 0.0 ni, 4.2 id, 0.0 wa
[1]: Abeni, Luca & Lipari, Giuseppe & Parri, Andrea & Sun, Youcheng.
(2015). Parallel and sequential reclaiming in multicore
real-time global scheduling.
Signed-off-by: Vineeth Pillai (Google) <vineeth@bitbyteword.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Daniel Bristot de Oliveira <bristot@kernel.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lore.kernel.org/r/20230530135526.2385378-1-vineeth@bitbyteword.org
The responsiveness of the Per Entity Load Tracking (PELT) util_avg in
mobile devices is still considered too low for utilization changes
during task ramp-up.
In Android this manifests in the fact that the first frames of a UI
activity are very prone to be jankframes (a frame which doesn't meet
the required frame rendering time, e.g. 16ms@60Hz) since the CPU
frequency is normally low at this point and has to ramp up quickly.
The beginning of an UI activity is also characterized by the occurrence
of CPU contention, especially on little CPUs. Current little CPUs can
have an original CPU capacity of only ~ 150 which means that the actual
CPU capacity at lower frequency can even be much smaller.
Schedutil maps CPU util_avg into CPU frequency request via:
util = effective_cpu_util(..., cpu_util_cfs(cpu), ...) ->
util = map_util_perf(util) -> freq = map_util_freq(util, ...)
CPU contention for CFS tasks can be detected by 'CPU runnable > CPU
utililization' in cpu_util_cfs_boost() -> cpu_util(..., boost = 1).
Schedutil uses 'runnable boosting' by calling cpu_util_cfs_boost().
To be in sync with schedutil's CPU frequency selection, Energy Aware
Scheduling (EAS) also calls cpu_util(..., boost = 1) during max util
detection.
Moreover, 'runnable boosting' is also used in load-balance for busiest
CPU selection when the migration type is 'migrate_util', i.e. only at
sched domains which don't have the SD_SHARE_PKG_RESOURCES flag set.
Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20230515115735.296329-3-dietmar.eggemann@arm.com
There is a lot of code duplication in cpu_util_next() & cpu_util_cfs().
Remove this by allowing cpu_util_next() to be called with p = NULL.
Rename cpu_util_next() to cpu_util() since the '_next' suffix is no
longer necessary to distinct cpu utilization related functions.
Implement cpu_util_cfs(cpu) as cpu_util(cpu, p = NULL, -1).
This will allow to code future related cpu util changes only in one
place, namely in cpu_util().
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20230515115735.296329-2-dietmar.eggemann@arm.com
The default deadline bandwidth control structure has been removed since
commit eb77cf1c15 ("sched/deadline: Remove unused def_dl_bandwidth")
leading to unused init_dl_bandwidth() and struct dl_bandwidth. Remove
them to clean up the code.
Signed-off-by: Miaohe Lin <linmiaohe@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lore.kernel.org/r/20230524102514.407486-1-linmiaohe@huawei.com
These four functions have a normal definition for CONFIG_FAIR_GROUP_SCHED,
and empty one that is only referenced when FAIR_GROUP_SCHED is disabled
but CGROUP_SCHED is still enabled. If both are turned off, the functions
are still defined but the misisng prototype causes a W=1 warning:
kernel/sched/fair.c:12544:6: error: no previous prototype for 'free_fair_sched_group'
kernel/sched/fair.c:12546:5: error: no previous prototype for 'alloc_fair_sched_group'
kernel/sched/fair.c:12553:6: error: no previous prototype for 'online_fair_sched_group'
kernel/sched/fair.c:12555:6: error: no previous prototype for 'unregister_fair_sched_group'
Move the alternatives into the header as static inline functions with
the correct combination of #ifdef checks to avoid the warning without
adding even more complexity.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20230522195021.3456768-6-arnd@kernel.org
Having the prototype next to the caller but not visible to the callee causes
a W=1 warning:
kernel/sched/fair.c:11985:6: error: no previous prototype for 'task_vruntime_update' [-Werror=missing-prototypes]
Move this to a header, as we do for all other function declarations.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20230522195021.3456768-5-arnd@kernel.org
The schedule_user() function is used on powerpc and sparc architectures, but
only ever called from assembler, so it has no prototype, causing a harmless W=1
warning:
kernel/sched/core.c:6730:35: error: no previous prototype for 'schedule_user' [-Werror=missing-prototypes]
Add a prototype in sched/sched.h to shut up the warning.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20230522195021.3456768-3-arnd@kernel.org
While moving a set of tasks between exclusive cpusets,
cpuset_can_attach() -> task_can_attach() calls dl_cpu_busy(..., p) for
DL BW overflow checking and per-task DL BW allocation on the destination
root_domain for the DL tasks in this set.
This approach has the issue of not freeing already allocated DL BW in
the following error cases:
(1) The set of tasks includes multiple DL tasks and DL BW overflow
checking fails for one of the subsequent DL tasks.
(2) Another controller next to the cpuset controller which is attached
to the same cgroup fails in its can_attach().
To address this problem rework dl_cpu_busy():
(1) Split it into dl_bw_check_overflow() & dl_bw_alloc() and add a
dedicated dl_bw_free().
(2) dl_bw_alloc() & dl_bw_free() take a `u64 dl_bw` parameter instead of
a `struct task_struct *p` used in dl_cpu_busy(). This allows to
allocate DL BW for a set of tasks too rather than only for a single
task.
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Do not assume that all the children of a scheduling domain have a given
flag. Check whether it has the SDF_SHARED_CHILD meta flag.
Suggested-by: Ionela Voinescu <ionela.voinescu@arm.com>
Signed-off-by: Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230406203148.19182-9-ricardo.neri-calderon@linux.intel.com
Introduce per-mm/cpu current concurrency id (mm_cid) to fix a PostgreSQL
sysbench regression reported by Aaron Lu.
Keep track of the currently allocated mm_cid for each mm/cpu rather than
freeing them immediately on context switch. This eliminates most atomic
operations when context switching back and forth between threads
belonging to different memory spaces in multi-threaded scenarios (many
processes, each with many threads). The per-mm/per-cpu mm_cid values are
serialized by their respective runqueue locks.
Thread migration is handled by introducing invocation to
sched_mm_cid_migrate_to() (with destination runqueue lock held) in
activate_task() for migrating tasks. If the destination cpu's mm_cid is
unset, and if the source runqueue is not actively using its mm_cid, then
the source cpu's mm_cid is moved to the destination cpu on migration.
Introduce a task-work executed periodically, similarly to NUMA work,
which delays reclaim of cid values when they are unused for a period of
time.
Keep track of the allocation time for each per-cpu cid, and let the task
work clear them when they are observed to be older than
SCHED_MM_CID_PERIOD_NS and unused. This task work also clears all
mm_cids which are greater or equal to the Hamming weight of the mm
cidmask to keep concurrency ids compact.
Because we want to ensure the mm_cid converges towards the smaller
values as migrations happen, the prior optimization that was done when
context switching between threads belonging to the same mm is removed,
because it could delay the lazy release of the destination runqueue
mm_cid after it has been replaced by a migration. Removing this prior
optimization is not an issue performance-wise because the introduced
per-mm/per-cpu mm_cid tracking also covers this more specific case.
Fixes: af7f588d8f ("sched: Introduce per-memory-map concurrency ID")
Reported-by: Aaron Lu <aaron.lu@intel.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Aaron Lu <aaron.lu@intel.com>
Link: https://lore.kernel.org/lkml/20230327080502.GA570847@ziqianlu-desk2/
When {rt, cfs}_rq or dl task is throttled, since cookied tasks
are not dequeued from the core tree, So sched_core_find() and
sched_core_next() may return throttled task, which may
cause throttled task to run on the CPU.
So we add checks in sched_core_find() and sched_core_next()
to make sure that the return is a runnable task that is
not throttled.
Co-developed-by: Cruz Zhao <CruzZhao@linux.alibaba.com>
Signed-off-by: Cruz Zhao <CruzZhao@linux.alibaba.com>
Signed-off-by: Hao Jia <jiahao.os@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20230316081806.69544-1-jiahao.os@bytedance.com
Remove the capacity inversion detection which is now handled by
util_fits_cpu() returning -1 when we need to continue to look for a
potential CPU with better performance.
This ends up almost reverting patches below except for some comments:
commit da07d2f9c1 ("sched/fair: Fixes for capacity inversion detection")
commit aa69c36f31 ("sched/fair: Consider capacity inversion in util_fits_cpu()")
commit 44c7b80bff ("sched/fair: Detect capacity inversion")
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20230201143628.270912-3-vincent.guittot@linaro.org
With a modified container_of() that preserves constness, the compiler
finds some pointers which should have been marked as const. task_of()
also needs to become const-preserving for the !FAIR_GROUP_SCHED case so
that cfs_rq_of() can take a const argument. No change to generated code.
Signed-off-by: Matthew Wilcox (Oracle) <willy@infradead.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221212144946.2657785-1-willy@infradead.org
This feature allows the scheduler to expose a per-memory map concurrency
ID to user-space. This concurrency ID is within the possible cpus range,
and is temporarily (and uniquely) assigned while threads are actively
running within a memory map. If a memory map has fewer threads than
cores, or is limited to run on few cores concurrently through sched
affinity or cgroup cpusets, the concurrency IDs will be values close
to 0, thus allowing efficient use of user-space memory for per-cpu
data structures.
This feature is meant to be exposed by a new rseq thread area field.
The primary purpose of this feature is to do the heavy-lifting needed
by memory allocators to allow them to use per-cpu data structures
efficiently in the following situations:
- Single-threaded applications,
- Multi-threaded applications on large systems (many cores) with limited
cpu affinity mask,
- Multi-threaded applications on large systems (many cores) with
restricted cgroup cpuset per container.
One of the key concern from scheduler maintainers is the overhead
associated with additional spin locks or atomic operations in the
scheduler fast-path. This is why the following optimization is
implemented.
On context switch between threads belonging to the same memory map,
transfer the mm_cid from prev to next without any atomic ops. This
takes care of use-cases involving frequent context switch between
threads belonging to the same memory map.
Additional optimizations can be done if the spin locks added when
context switching between threads belonging to different memory maps end
up being a performance bottleneck. Those are left out of this patch
though. A performance impact would have to be clearly demonstrated to
justify the added complexity.
The credit goes to Paul Turner (Google) for the original virtual cpu id
idea. This feature is implemented based on the discussions with Paul
Turner and Peter Oskolkov (Google), but I took the liberty to implement
scheduler fast-path optimizations and my own NUMA-awareness scheme. The
rumor has it that Google have been running a rseq vcpu_id extension
internally in production for a year. The tcmalloc source code indeed has
comments hinting at a vcpu_id prototype extension to the rseq system
call [1].
The following benchmarks do not show any significant overhead added to
the scheduler context switch by this feature:
* perf bench sched messaging (process)
Baseline: 86.5±0.3 ms
With mm_cid: 86.7±2.6 ms
* perf bench sched messaging (threaded)
Baseline: 84.3±3.0 ms
With mm_cid: 84.7±2.6 ms
* hackbench (process)
Baseline: 82.9±2.7 ms
With mm_cid: 82.9±2.9 ms
* hackbench (threaded)
Baseline: 85.2±2.6 ms
With mm_cid: 84.4±2.9 ms
[1] https://github.com/google/tcmalloc/blob/master/tcmalloc/internal/linux_syscall_support.h#L26
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20221122203932.231377-8-mathieu.desnoyers@efficios.com
CFS bandwidth currently distributes new runtime and unthrottles cfs_rq's
inline in an hrtimer callback. Runtime distribution is a per-cpu
operation, and unthrottling is a per-cgroup operation, since a tg walk
is required. On machines with a large number of cpus and large cgroup
hierarchies, this cpus*cgroups work can be too much to do in a single
hrtimer callback: since IRQ are disabled, hard lockups may easily occur.
Specifically, we've found this scalability issue on configurations with
256 cpus, O(1000) cgroups in the hierarchy being throttled, and high
memory bandwidth usage.
To fix this, we can instead unthrottle cfs_rq's asynchronously via a
CSD. Each cpu is responsible for unthrottling itself, thus sharding the
total work more fairly across the system, and avoiding hard lockups.
Signed-off-by: Josh Don <joshdon@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221117005418.3499691-1-joshdon@google.com
It was found that the user requested affinity via sched_setaffinity()
can be easily overwritten by other kernel subsystems without an easy way
to reset it back to what the user requested. For example, any change
to the current cpuset hierarchy may reset the cpumask of the tasks in
the affected cpusets to the default cpuset value even if those tasks
have pre-existing user requested affinity. That is especially easy to
trigger under a cgroup v2 environment where writing "+cpuset" to the
root cgroup's cgroup.subtree_control file will reset the cpus affinity
of all the processes in the system.
That is problematic in a nohz_full environment where the tasks running
in the nohz_full CPUs usually have their cpus affinity explicitly set
and will behave incorrectly if cpus affinity changes.
Fix this problem by looking at user_cpus_ptr in __set_cpus_allowed_ptr()
and use it to restrcit the given cpumask unless there is no overlap. In
that case, it will fallback to the given one. The SCA_USER flag is
reused to indicate intent to set user_cpus_ptr and so user_cpus_ptr
masking should be skipped. In addition, masking should also be skipped
if any of the SCA_MIGRATE_* flag is set.
All callers of set_cpus_allowed_ptr() will be affected by this change.
A scratch cpumask is added to percpu runqueues structure for doing
additional masking when user_cpus_ptr is set.
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220922180041.1768141-4-longman@redhat.com
Unconditionally preserve the user requested cpumask on
sched_setaffinity() calls. This allows using it outside of the fairly
narrow restrict_cpus_allowed_ptr() use-case and fix some cpuset issues
that currently suffer destruction of cpumasks.
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220922180041.1768141-3-longman@redhat.com
In order to prepare for passing through additional data through the
affinity call-chains, convert the mask and flags argument into a
structure.
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Waiman Long <longman@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220922180041.1768141-5-longman@redhat.com
Check each performance domain to see if thermal pressure is causing its
capacity to be lower than another performance domain.
We assume that each performance domain has CPUs with the same
capacities, which is similar to an assumption made in energy_model.c
We also assume that thermal pressure impacts all CPUs in a performance
domain equally.
If there're multiple performance domains with the same capacity_orig, we
will trigger a capacity inversion if the domain is under thermal
pressure.
The new cpu_in_capacity_inversion() should help users to know when
information about capacity_orig are not reliable and can opt in to use
the inverted capacity as the 'actual' capacity_orig.
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220804143609.515789-9-qais.yousef@arm.com
As reported by Yun Hsiang [1], if a task has its uclamp_min >= 0.8 * 1024,
it'll always pick the previous CPU because fits_capacity() will always
return false in this case.
The new util_fits_cpu() logic should handle this correctly for us beside
more corner cases where similar failures could occur, like when using
UCLAMP_MAX.
We open code uclamp_rq_util_with() except for the clamp() part,
util_fits_cpu() needs the 'raw' values to be passed to it.
Also introduce uclamp_rq_{set, get}() shorthand accessors to get uclamp
value for the rq. Makes the code more readable and ensures the right
rules (use READ_ONCE/WRITE_ONCE) are respected transparently.
[1] https://lists.linaro.org/pipermail/eas-dev/2020-July/001488.html
Fixes: 1d42509e47 ("sched/fair: Make EAS wakeup placement consider uclamp restrictions")
Reported-by: Yun Hsiang <hsiang023167@gmail.com>
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220804143609.515789-4-qais.yousef@arm.com
So that the new uclamp rules in regard to migration margin and capacity
pressure are taken into account correctly.
Fixes: a7008c07a5 ("sched/fair: Make task_fits_capacity() consider uclamp restrictions")
Co-developed-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Qais Yousef <qais.yousef@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220804143609.515789-3-qais.yousef@arm.com
Introduce distinct struct balance_callback instead of performing function
pointer casting which will trip CFI. Avoids warnings as found by Clang's
future -Wcast-function-type-strict option:
In file included from kernel/sched/core.c:84:
kernel/sched/sched.h:1755:15: warning: cast from 'void (*)(struct rq *)' to 'void (*)(struct callback_head *)' converts to incompatible function type [-Wcast-function-type-strict]
head->func = (void (*)(struct callback_head *))func;
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
No binary differences result from this change.
This patch is a cleanup based on Brad Spengler/PaX Team's modifications
to sched code in their last public patch of grsecurity/PaX based on my
understanding of the code. Changes or omissions from the original code
are mine and don't reflect the original grsecurity/PaX code.
Reported-by: Sami Tolvanen <samitolvanen@google.com>
Signed-off-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Nathan Chancellor <nathan@kernel.org>
Link: https://github.com/ClangBuiltLinux/linux/issues/1724
Link: https://lkml.kernel.org/r/20221008000758.2957718-1-keescook@chromium.org
In commit 97886d9dcd ("sched: Migration changes for core scheduling"),
sched_group_cookie_match() was added to help determine if a cookie
matches the core state.
However, while it iterates the SMT group, it fails to actually use the
RQ for each of the CPUs iterated, use cpu_rq(cpu) instead of rq to fix
things.
Fixes: 97886d9dcd ("sched: Migration changes for core scheduling")
Signed-off-by: Lin Shengwang <linshengwang1@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221008022709.642-1-linshengwang1@huawei.com
linux-next for a couple of months without, to my knowledge, any negative
reports (or any positive ones, come to that).
- Also the Maple Tree from Liam R. Howlett. An overlapping range-based
tree for vmas. It it apparently slight more efficient in its own right,
but is mainly targeted at enabling work to reduce mmap_lock contention.
Liam has identified a number of other tree users in the kernel which
could be beneficially onverted to mapletrees.
Yu Zhao has identified a hard-to-hit but "easy to fix" lockdep splat
(https://lkml.kernel.org/r/CAOUHufZabH85CeUN-MEMgL8gJGzJEWUrkiM58JkTbBhh-jew0Q@mail.gmail.com).
This has yet to be addressed due to Liam's unfortunately timed
vacation. He is now back and we'll get this fixed up.
- Dmitry Vyukov introduces KMSAN: the Kernel Memory Sanitizer. It uses
clang-generated instrumentation to detect used-unintialized bugs down to
the single bit level.
KMSAN keeps finding bugs. New ones, as well as the legacy ones.
- Yang Shi adds a userspace mechanism (madvise) to induce a collapse of
memory into THPs.
- Zach O'Keefe has expanded Yang Shi's madvise(MADV_COLLAPSE) to support
file/shmem-backed pages.
- userfaultfd updates from Axel Rasmussen
- zsmalloc cleanups from Alexey Romanov
- cleanups from Miaohe Lin: vmscan, hugetlb_cgroup, hugetlb and memory-failure
- Huang Ying adds enhancements to NUMA balancing memory tiering mode's
page promotion, with a new way of detecting hot pages.
- memcg updates from Shakeel Butt: charging optimizations and reduced
memory consumption.
- memcg cleanups from Kairui Song.
- memcg fixes and cleanups from Johannes Weiner.
- Vishal Moola provides more folio conversions
- Zhang Yi removed ll_rw_block() :(
- migration enhancements from Peter Xu
- migration error-path bugfixes from Huang Ying
- Aneesh Kumar added ability for a device driver to alter the memory
tiering promotion paths. For optimizations by PMEM drivers, DRM
drivers, etc.
- vma merging improvements from Jakub Matěn.
- NUMA hinting cleanups from David Hildenbrand.
- xu xin added aditional userspace visibility into KSM merging activity.
- THP & KSM code consolidation from Qi Zheng.
- more folio work from Matthew Wilcox.
- KASAN updates from Andrey Konovalov.
- DAMON cleanups from Kaixu Xia.
- DAMON work from SeongJae Park: fixes, cleanups.
- hugetlb sysfs cleanups from Muchun Song.
- Mike Kravetz fixes locking issues in hugetlbfs and in hugetlb core.
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Merge tag 'mm-stable-2022-10-08' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
Pull MM updates from Andrew Morton:
- Yu Zhao's Multi-Gen LRU patches are here. They've been under test in
linux-next for a couple of months without, to my knowledge, any
negative reports (or any positive ones, come to that).
- Also the Maple Tree from Liam Howlett. An overlapping range-based
tree for vmas. It it apparently slightly more efficient in its own
right, but is mainly targeted at enabling work to reduce mmap_lock
contention.
Liam has identified a number of other tree users in the kernel which
could be beneficially onverted to mapletrees.
Yu Zhao has identified a hard-to-hit but "easy to fix" lockdep splat
at [1]. This has yet to be addressed due to Liam's unfortunately
timed vacation. He is now back and we'll get this fixed up.
- Dmitry Vyukov introduces KMSAN: the Kernel Memory Sanitizer. It uses
clang-generated instrumentation to detect used-unintialized bugs down
to the single bit level.
KMSAN keeps finding bugs. New ones, as well as the legacy ones.
- Yang Shi adds a userspace mechanism (madvise) to induce a collapse of
memory into THPs.
- Zach O'Keefe has expanded Yang Shi's madvise(MADV_COLLAPSE) to
support file/shmem-backed pages.
- userfaultfd updates from Axel Rasmussen
- zsmalloc cleanups from Alexey Romanov
- cleanups from Miaohe Lin: vmscan, hugetlb_cgroup, hugetlb and
memory-failure
- Huang Ying adds enhancements to NUMA balancing memory tiering mode's
page promotion, with a new way of detecting hot pages.
- memcg updates from Shakeel Butt: charging optimizations and reduced
memory consumption.
- memcg cleanups from Kairui Song.
- memcg fixes and cleanups from Johannes Weiner.
- Vishal Moola provides more folio conversions
- Zhang Yi removed ll_rw_block() :(
- migration enhancements from Peter Xu
- migration error-path bugfixes from Huang Ying
- Aneesh Kumar added ability for a device driver to alter the memory
tiering promotion paths. For optimizations by PMEM drivers, DRM
drivers, etc.
- vma merging improvements from Jakub Matěn.
- NUMA hinting cleanups from David Hildenbrand.
- xu xin added aditional userspace visibility into KSM merging
activity.
- THP & KSM code consolidation from Qi Zheng.
- more folio work from Matthew Wilcox.
- KASAN updates from Andrey Konovalov.
- DAMON cleanups from Kaixu Xia.
- DAMON work from SeongJae Park: fixes, cleanups.
- hugetlb sysfs cleanups from Muchun Song.
- Mike Kravetz fixes locking issues in hugetlbfs and in hugetlb core.
Link: https://lkml.kernel.org/r/CAOUHufZabH85CeUN-MEMgL8gJGzJEWUrkiM58JkTbBhh-jew0Q@mail.gmail.com [1]
* tag 'mm-stable-2022-10-08' of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm: (555 commits)
hugetlb: allocate vma lock for all sharable vmas
hugetlb: take hugetlb vma_lock when clearing vma_lock->vma pointer
hugetlb: fix vma lock handling during split vma and range unmapping
mglru: mm/vmscan.c: fix imprecise comments
mm/mglru: don't sync disk for each aging cycle
mm: memcontrol: drop dead CONFIG_MEMCG_SWAP config symbol
mm: memcontrol: use do_memsw_account() in a few more places
mm: memcontrol: deprecate swapaccounting=0 mode
mm: memcontrol: don't allocate cgroup swap arrays when memcg is disabled
mm/secretmem: remove reduntant return value
mm/hugetlb: add available_huge_pages() func
mm: remove unused inline functions from include/linux/mm_inline.h
selftests/vm: add selftest for MADV_COLLAPSE of uffd-minor memory
selftests/vm: add file/shmem MADV_COLLAPSE selftest for cleared pmd
selftests/vm: add thp collapse shmem testing
selftests/vm: add thp collapse file and tmpfs testing
selftests/vm: modularize thp collapse memory operations
selftests/vm: dedup THP helpers
mm/khugepaged: add tracepoint to hpage_collapse_scan_file()
mm/madvise: add file and shmem support to MADV_COLLAPSE
...
sched_nr_migrate_break is set to a fix value and never changes so we can
replace it by a define SCHED_NR_MIGRATE_BREAK.
Also, we adjust SCHED_NR_MIGRATE_BREAK to be aligned with the init value
of sysctl_sched_nr_migrate which can be init to different values.
Then, use SCHED_NR_MIGRATE_BREAK to init sysctl_sched_nr_migrate.
The behavior stays unchanged unless you modify sysctl_sched_nr_migrate
trough debugfs.
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220825122726.20819-3-vincent.guittot@linaro.org
Patch series "memory tiering: hot page selection", v4.
To optimize page placement in a memory tiering system with NUMA balancing,
the hot pages in the slow memory nodes need to be identified.
Essentially, the original NUMA balancing implementation selects the mostly
recently accessed (MRU) pages to promote. But this isn't a perfect
algorithm to identify the hot pages. Because the pages with quite low
access frequency may be accessed eventually given the NUMA balancing page
table scanning period could be quite long (e.g. 60 seconds). So in this
patchset, we implement a new hot page identification algorithm based on
the latency between NUMA balancing page table scanning and hint page
fault. Which is a kind of mostly frequently accessed (MFU) algorithm.
In NUMA balancing memory tiering mode, if there are hot pages in slow
memory node and cold pages in fast memory node, we need to promote/demote
hot/cold pages between the fast and cold memory nodes.
A choice is to promote/demote as fast as possible. But the CPU cycles and
memory bandwidth consumed by the high promoting/demoting throughput will
hurt the latency of some workload because of accessing inflating and slow
memory bandwidth contention.
A way to resolve this issue is to restrict the max promoting/demoting
throughput. It will take longer to finish the promoting/demoting. But
the workload latency will be better. This is implemented in this patchset
as the page promotion rate limit mechanism.
The promotion hot threshold is workload and system configuration
dependent. So in this patchset, a method to adjust the hot threshold
automatically is implemented. The basic idea is to control the number of
the candidate promotion pages to match the promotion rate limit.
We used the pmbench memory accessing benchmark tested the patchset on a
2-socket server system with DRAM and PMEM installed. The test results are
as follows,
pmbench score promote rate
(accesses/s) MB/s
------------- ------------
base 146887704.1 725.6
hot selection 165695601.2 544.0
rate limit 162814569.8 165.2
auto adjustment 170495294.0 136.9
From the results above,
With hot page selection patch [1/3], the pmbench score increases about
12.8%, and promote rate (overhead) decreases about 25.0%, compared with
base kernel.
With rate limit patch [2/3], pmbench score decreases about 1.7%, and
promote rate decreases about 69.6%, compared with hot page selection
patch.
With threshold auto adjustment patch [3/3], pmbench score increases about
4.7%, and promote rate decrease about 17.1%, compared with rate limit
patch.
Baolin helped to test the patchset with MySQL on a machine which contains
1 DRAM node (30G) and 1 PMEM node (126G).
sysbench /usr/share/sysbench/oltp_read_write.lua \
......
--tables=200 \
--table-size=1000000 \
--report-interval=10 \
--threads=16 \
--time=120
The tps can be improved about 5%.
This patch (of 3):
To optimize page placement in a memory tiering system with NUMA balancing,
the hot pages in the slow memory node need to be identified. Essentially,
the original NUMA balancing implementation selects the mostly recently
accessed (MRU) pages to promote. But this isn't a perfect algorithm to
identify the hot pages. Because the pages with quite low access frequency
may be accessed eventually given the NUMA balancing page table scanning
period could be quite long (e.g. 60 seconds). The most frequently
accessed (MFU) algorithm is better.
So, in this patch we implemented a better hot page selection algorithm.
Which is based on NUMA balancing page table scanning and hint page fault
as follows,
- When the page tables of the processes are scanned to change PTE/PMD
to be PROT_NONE, the current time is recorded in struct page as scan
time.
- When the page is accessed, hint page fault will occur. The scan
time is gotten from the struct page. And The hint page fault
latency is defined as
hint page fault time - scan time
The shorter the hint page fault latency of a page is, the higher the
probability of their access frequency to be higher. So the hint page
fault latency is a better estimation of the page hot/cold.
It's hard to find some extra space in struct page to hold the scan time.
Fortunately, we can reuse some bits used by the original NUMA balancing.
NUMA balancing uses some bits in struct page to store the page accessing
CPU and PID (referring to page_cpupid_xchg_last()). Which is used by the
multi-stage node selection algorithm to avoid to migrate pages shared
accessed by the NUMA nodes back and forth. But for pages in the slow
memory node, even if they are shared accessed by multiple NUMA nodes, as
long as the pages are hot, they need to be promoted to the fast memory
node. So the accessing CPU and PID information are unnecessary for the
slow memory pages. We can reuse these bits in struct page to record the
scan time. For the fast memory pages, these bits are used as before.
For the hot threshold, the default value is 1 second, which works well in
our performance test. All pages with hint page fault latency < hot
threshold will be considered hot.
It's hard for users to determine the hot threshold. So we don't provide a
kernel ABI to set it, just provide a debugfs interface for advanced users
to experiment. We will continue to work on a hot threshold automatic
adjustment mechanism.
The downside of the above method is that the response time to the workload
hot spot changing may be much longer. For example,
- A previous cold memory area becomes hot
- The hint page fault will be triggered. But the hint page fault
latency isn't shorter than the hot threshold. So the pages will
not be promoted.
- When the memory area is scanned again, maybe after a scan period,
the hint page fault latency measured will be shorter than the hot
threshold and the pages will be promoted.
To mitigate this, if there are enough free space in the fast memory node,
the hot threshold will not be used, all pages will be promoted upon the
hint page fault for fast response.
Thanks Zhong Jiang reported and tested the fix for a bug when disabling
memory tiering mode dynamically.
Link: https://lkml.kernel.org/r/20220713083954.34196-1-ying.huang@intel.com
Link: https://lkml.kernel.org/r/20220713083954.34196-2-ying.huang@intel.com
Signed-off-by: "Huang, Ying" <ying.huang@intel.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Tested-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Cc: Johannes Weiner <hannes@cmpxchg.org>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Rik van Riel <riel@surriel.com>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Yang Shi <shy828301@gmail.com>
Cc: Zi Yan <ziy@nvidia.com>
Cc: Wei Xu <weixugc@google.com>
Cc: osalvador <osalvador@suse.de>
Cc: Shakeel Butt <shakeelb@google.com>
Cc: Zhong Jiang <zhongjiang-ali@linux.alibaba.com>
Cc: Oscar Salvador <osalvador@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
There is some ambiguity about task_running() in that it is unrelated
to TASK_RUNNING but instead tests ->on_cpu. As such, rename the thing
task_on_cpu().
Suggested-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/Yxhkhn55uHZx+NGl@hirez.programming.kicks-ass.net
Wrap repeated code in helper function update_current_exec_runtime for
update the exec time of the current.
Signed-off-by: Shang XiaoJing <shangxiaojing@huawei.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220824082856.15674-1-shangxiaojing@huawei.com
We use cpu_cgrp_subsys->fork() to set task group for the new fair task
in cgroup_post_fork().
Since commit b1e8206582 ("sched: Fix yet more sched_fork() races")
has already set_task_rq() for the new fair task in sched_cgroup_fork(),
so cpu_cgrp_subsys->fork() can be removed.
cgroup_can_fork() --> pin parent's sched_task_group
sched_cgroup_fork()
__set_task_cpu()
set_task_rq()
cgroup_post_fork()
ss->fork() := cpu_cgroup_fork()
sched_change_group(..., TASK_SET_GROUP)
task_set_group_fair()
set_task_rq() --> can be removed
After this patch's change, task_change_group_fair() only need to
care about task cgroup migration, make the code much simplier.
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://lore.kernel.org/r/20220818124805.601-3-zhouchengming@bytedance.com
Previously we only maintain task se depth in task_move_group_fair(),
if a !fair task change task group, its se depth will not be updated,
so commit eb7a59b2c8 ("sched/fair: Reset se-depth when task switched to FAIR")
fix the problem by updating se depth in switched_to_fair() too.
Then commit daa59407b5 ("sched/fair: Unify switched_{from,to}_fair()
and task_move_group_fair()") unified these two functions, moved se.depth
setting to attach_task_cfs_rq(), which further into attach_entity_cfs_rq()
with commit df217913e7 ("sched/fair: Factorize attach/detach entity").
This patch move task se depth maintenance from attach_entity_cfs_rq()
to set_task_rq(), which will be called when CPU/cgroup change, so its
depth will always be correct.
This patch is preparation for the next patch.
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20220818124805.601-2-zhouchengming@bytedance.com
There's no good reason to crash a user's system with a BUG_ON(),
chances are high that they'll never even see the crash message on
Xorg, and it won't make it into the syslog either.
By using a WARN_ON_ONCE() we at least give the user a chance to report
any bugs triggered here - instead of getting silent hangs.
None of these WARN_ON_ONCE()s are supposed to trigger, ever - so we ignore
cases where a NULL check is done via a BUG_ON() and we let a NULL
pointer through after a WARN_ON_ONCE().
There's one exception: WARN_ON_ONCE() arguments with side-effects,
such as locking - in this case we use the return value of the
WARN_ON_ONCE(), such as in:
- BUG_ON(!lock_task_sighand(p, &flags));
+ if (WARN_ON_ONCE(!lock_task_sighand(p, &flags)))
+ return;
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/YvSsKcAXISmshtHo@gmail.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Merge tag 'sched-urgent-2022-08-06' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull scheduler fixes from Ingo Molnar:
"Various fixes: a deadline scheduler fix, a migration fix, a Sparse fix
and a comment fix"
* tag 'sched-urgent-2022-08-06' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip:
sched/core: Do not requeue task on CPU excluded from cpus_mask
sched/rt: Fix Sparse warnings due to undefined rt.c declarations
exit: Fix typo in comment: s/sub-theads/sub-threads
sched, cpuset: Fix dl_cpu_busy() panic due to empty cs->cpus_allowed
There are several symbols defined in kernel/sched/sched.h but get wrapped
in CONFIG_CGROUP_SCHED, even though dummy versions get built in rt.c and
therefore trigger Sparse warnings:
kernel/sched/rt.c:309:6: warning: symbol 'unregister_rt_sched_group' was not declared. Should it be static?
kernel/sched/rt.c:311:6: warning: symbol 'free_rt_sched_group' was not declared. Should it be static?
kernel/sched/rt.c:313:5: warning: symbol 'alloc_rt_sched_group' was not declared. Should it be static?
Fix this by moving them outside the CONFIG_CGROUP_SCHED block.
[ mingo: Refreshed to the latest scheduler tree, tweaked changelog. ]
Signed-off-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20220721145155.358366-1-ben-linux@fluff.org
This pull request contains the following branches:
doc.2022.06.21a: Documentation updates.
fixes.2022.07.19a: Miscellaneous fixes.
nocb.2022.07.19a: Callback-offload updates, perhaps most notably a new
RCU_NOCB_CPU_DEFAULT_ALL Kconfig option that causes all CPUs to
be offloaded at boot time, regardless of kernel boot parameters.
This is useful to battery-powered systems such as ChromeOS
and Android. In addition, a new RCU_NOCB_CPU_CB_BOOST kernel
boot parameter prevents offloaded callbacks from interfering
with real-time workloads and with energy-efficiency mechanisms.
poll.2022.07.21a: Polled grace-period updates, perhaps most notably
making these APIs account for both normal and expedited grace
periods.
rcu-tasks.2022.06.21a: Tasks RCU updates, perhaps most notably reducing
the CPU overhead of RCU tasks trace grace periods by more than
a factor of two on a system with 15,000 tasks. The reduction
is expected to increase with the number of tasks, so it seems
reasonable to hypothesize that a system with 150,000 tasks might
see a 20-fold reduction in CPU overhead.
torture.2022.06.21a: Torture-test updates.
ctxt.2022.07.05a: Updates that merge RCU's dyntick-idle tracking into
context tracking, thus reducing the overhead of transitioning to
kernel mode from either idle or nohz_full userspace execution
for kernels that track context independently of RCU. This is
expected to be helpful primarily for kernels built with
CONFIG_NO_HZ_FULL=y.
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Merge tag 'rcu.2022.07.26a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu
Pull RCU updates from Paul McKenney:
- Documentation updates
- Miscellaneous fixes
- Callback-offload updates, perhaps most notably a new
RCU_NOCB_CPU_DEFAULT_ALL Kconfig option that causes all CPUs to be
offloaded at boot time, regardless of kernel boot parameters.
This is useful to battery-powered systems such as ChromeOS and
Android. In addition, a new RCU_NOCB_CPU_CB_BOOST kernel boot
parameter prevents offloaded callbacks from interfering with
real-time workloads and with energy-efficiency mechanisms
- Polled grace-period updates, perhaps most notably making these APIs
account for both normal and expedited grace periods
- Tasks RCU updates, perhaps most notably reducing the CPU overhead of
RCU tasks trace grace periods by more than a factor of two on a
system with 15,000 tasks.
The reduction is expected to increase with the number of tasks, so it
seems reasonable to hypothesize that a system with 150,000 tasks
might see a 20-fold reduction in CPU overhead
- Torture-test updates
- Updates that merge RCU's dyntick-idle tracking into context tracking,
thus reducing the overhead of transitioning to kernel mode from
either idle or nohz_full userspace execution for kernels that track
context independently of RCU.
This is expected to be helpful primarily for kernels built with
CONFIG_NO_HZ_FULL=y
* tag 'rcu.2022.07.26a' of git://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu: (98 commits)
rcu: Add irqs-disabled indicator to expedited RCU CPU stall warnings
rcu: Diagnose extended sync_rcu_do_polled_gp() loops
rcu: Put panic_on_rcu_stall() after expedited RCU CPU stall warnings
rcutorture: Test polled expedited grace-period primitives
rcu: Add polled expedited grace-period primitives
rcutorture: Verify that polled GP API sees synchronous grace periods
rcu: Make Tiny RCU grace periods visible to polled APIs
rcu: Make polled grace-period API account for expedited grace periods
rcu: Switch polled grace-period APIs to ->gp_seq_polled
rcu/nocb: Avoid polling when my_rdp->nocb_head_rdp list is empty
rcu/nocb: Add option to opt rcuo kthreads out of RT priority
rcu: Add nocb_cb_kthread check to rcu_is_callbacks_kthread()
rcu/nocb: Add an option to offload all CPUs on boot
rcu/nocb: Fix NOCB kthreads spawn failure with rcu_nocb_rdp_deoffload() direct call
rcu/nocb: Invert rcu_state.barrier_mutex VS hotplug lock locking order
rcu/nocb: Add/del rdp to iterate from rcuog itself
rcu/tree: Add comment to describe GP-done condition in fqs loop
rcu: Initialize first_gp_fqs at declaration in rcu_gp_fqs()
rcu/kvfree: Remove useless monitor_todo flag
rcu: Cleanup RCU urgency state for offline CPU
...
Save a multiplication in dl_task_fits_capacity() by using already
maintained per-sched_dl_entity (i.e. per-task) `dl_runtime/dl_deadline`
(dl_density).
cap_scale(dl_deadline, cap) >= dl_runtime
dl_deadline * cap >> SCHED_CAPACITY_SHIFT >= dl_runtime
cap >= dl_runtime << SCHED_CAPACITY_SHIFT / dl_deadline
cap >= (dl_runtime << BW_SHIFT / dl_deadline) >>
BW_SHIFT - SCHED_CAPACITY_SHIFT
cap >= dl_density >> BW_SHIFT - SCHED_CAPACITY_SHIFT
__sched_setscheduler()->__checkparam_dl() ensures that the 2 corner
cases (if conditions) `runtime == RUNTIME_INF (-1)` and `period == 0`
of to_ratio(deadline, runtime) are not met when setting dl_density in
__sched_setscheduler()-> __setscheduler_params()->__setparam_dl().
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20220729111305.1275158-4-dietmar.eggemann@arm.com
Create an inline helper for conditional code to be only executed on
asymmetric CPU capacity systems. This makes these (currently ~10 and
future) conditions a lot more readable.
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20220729111305.1275158-2-dietmar.eggemann@arm.com
Load-balancing improvements:
============================
- Improve NUMA balancing on AMD Zen systems for affine workloads.
- Improve the handling of reduced-capacity CPUs in load-balancing.
- Energy Model improvements: fix & refine all the energy fairness metrics (PELT),
and remove the conservative threshold requiring 6% energy savings to
migrate a task. Doing this improves power efficiency for most workloads,
and also increases the reliability of energy-efficiency scheduling.
- Optimize/tweak select_idle_cpu() to spend (much) less time searching
for an idle CPU on overloaded systems. There's reports of several
milliseconds spent there on large systems with large workloads ...
[ Since the search logic changed, there might be behavioral side effects. ]
- Improve NUMA imbalance behavior. On certain systems
with spare capacity, initial placement of tasks is non-deterministic,
and such an artificial placement imbalance can persist for a long time,
hurting (and sometimes helping) performance.
The fix is to make fork-time task placement consistent with runtime
NUMA balancing placement.
Note that some performance regressions were reported against this,
caused by workloads that are not memory bandwith limited, which benefit
from the artificial locality of the placement bug(s). Mel Gorman's
conclusion, with which we concur, was that consistency is better than
random workload benefits from non-deterministic bugs:
"Given there is no crystal ball and it's a tradeoff, I think it's
better to be consistent and use similar logic at both fork time
and runtime even if it doesn't have universal benefit."
- Improve core scheduling by fixing a bug in sched_core_update_cookie() that
caused unnecessary forced idling.
- Improve wakeup-balancing by allowing same-LLC wakeup of idle CPUs for newly
woken tasks.
- Fix a newidle balancing bug that introduced unnecessary wakeup latencies.
ABI improvements/fixes:
=======================
- Do not check capabilities and do not issue capability check denial messages
when a scheduler syscall doesn't require privileges. (Such as increasing niceness.)
- Add forced-idle accounting to cgroups too.
- Fix/improve the RSEQ ABI to not just silently accept unknown flags.
(No existing tooling is known to have learned to rely on the previous behavior.)
- Depreciate the (unused) RSEQ_CS_FLAG_NO_RESTART_ON_* flags.
Optimizations:
==============
- Optimize & simplify leaf_cfs_rq_list()
- Micro-optimize set_nr_{and_not,if}_polling() via try_cmpxchg().
Misc fixes & cleanups:
======================
- Fix the RSEQ self-tests on RISC-V and Glibc 2.35 systems.
- Fix a full-NOHZ bug that can in some cases result in the tick not being
re-enabled when the last SCHED_RT task is gone from a runqueue but there's
still SCHED_OTHER tasks around.
- Various PREEMPT_RT related fixes.
- Misc cleanups & smaller fixes.
Signed-off-by: Ingo Molnar <mingo@kernel.org>
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Merge tag 'sched-core-2022-08-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip
Pull scheduler updates from Ingo Molnar:
"Load-balancing improvements:
- Improve NUMA balancing on AMD Zen systems for affine workloads.
- Improve the handling of reduced-capacity CPUs in load-balancing.
- Energy Model improvements: fix & refine all the energy fairness
metrics (PELT), and remove the conservative threshold requiring 6%
energy savings to migrate a task. Doing this improves power
efficiency for most workloads, and also increases the reliability
of energy-efficiency scheduling.
- Optimize/tweak select_idle_cpu() to spend (much) less time
searching for an idle CPU on overloaded systems. There's reports of
several milliseconds spent there on large systems with large
workloads ...
[ Since the search logic changed, there might be behavioral side
effects. ]
- Improve NUMA imbalance behavior. On certain systems with spare
capacity, initial placement of tasks is non-deterministic, and such
an artificial placement imbalance can persist for a long time,
hurting (and sometimes helping) performance.
The fix is to make fork-time task placement consistent with runtime
NUMA balancing placement.
Note that some performance regressions were reported against this,
caused by workloads that are not memory bandwith limited, which
benefit from the artificial locality of the placement bug(s). Mel
Gorman's conclusion, with which we concur, was that consistency is
better than random workload benefits from non-deterministic bugs:
"Given there is no crystal ball and it's a tradeoff, I think
it's better to be consistent and use similar logic at both fork
time and runtime even if it doesn't have universal benefit."
- Improve core scheduling by fixing a bug in
sched_core_update_cookie() that caused unnecessary forced idling.
- Improve wakeup-balancing by allowing same-LLC wakeup of idle CPUs
for newly woken tasks.
- Fix a newidle balancing bug that introduced unnecessary wakeup
latencies.
ABI improvements/fixes:
- Do not check capabilities and do not issue capability check denial
messages when a scheduler syscall doesn't require privileges. (Such
as increasing niceness.)
- Add forced-idle accounting to cgroups too.
- Fix/improve the RSEQ ABI to not just silently accept unknown flags.
(No existing tooling is known to have learned to rely on the
previous behavior.)
- Depreciate the (unused) RSEQ_CS_FLAG_NO_RESTART_ON_* flags.
Optimizations:
- Optimize & simplify leaf_cfs_rq_list()
- Micro-optimize set_nr_{and_not,if}_polling() via try_cmpxchg().
Misc fixes & cleanups:
- Fix the RSEQ self-tests on RISC-V and Glibc 2.35 systems.
- Fix a full-NOHZ bug that can in some cases result in the tick not
being re-enabled when the last SCHED_RT task is gone from a
runqueue but there's still SCHED_OTHER tasks around.
- Various PREEMPT_RT related fixes.
- Misc cleanups & smaller fixes"
* tag 'sched-core-2022-08-01' of git://git.kernel.org/pub/scm/linux/kernel/git/tip/tip: (32 commits)
rseq: Kill process when unknown flags are encountered in ABI structures
rseq: Deprecate RSEQ_CS_FLAG_NO_RESTART_ON_* flags
sched/core: Fix the bug that task won't enqueue into core tree when update cookie
nohz/full, sched/rt: Fix missed tick-reenabling bug in dequeue_task_rt()
sched/core: Always flush pending blk_plug
sched/fair: fix case with reduced capacity CPU
sched/core: Use try_cmpxchg in set_nr_{and_not,if}_polling
sched/core: add forced idle accounting for cgroups
sched/fair: Remove the energy margin in feec()
sched/fair: Remove task_util from effective utilization in feec()
sched/fair: Use the same cpumask per-PD throughout find_energy_efficient_cpu()
sched/fair: Rename select_idle_mask to select_rq_mask
sched, drivers: Remove max param from effective_cpu_util()/sched_cpu_util()
sched/fair: Decay task PELT values during wakeup migration
sched/fair: Provide u64 read for 32-bits arch helper
sched/fair: Introduce SIS_UTIL to search idle CPU based on sum of util_avg
sched: only perform capability check on privileged operation
sched: Remove unused function group_first_cpu()
sched/fair: Remove redundant word " *"
selftests/rseq: check if libc rseq support is registered
...
The RCU dynticks counter is going to be merged into the context tracking
subsystem. Start with moving the idle extended quiescent states
entrypoints to context tracking. For now those are dumb redirections to
existing RCU calls.
[ paulmck: Apply kernel test robot feedback. ]
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Uladzislau Rezki <uladzislau.rezki@sony.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Nicolas Saenz Julienne <nsaenz@kernel.org>
Cc: Marcelo Tosatti <mtosatti@redhat.com>
Cc: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Cc: Yu Liao <liaoyu15@huawei.com>
Cc: Phil Auld <pauld@redhat.com>
Cc: Paul Gortmaker<paul.gortmaker@windriver.com>
Cc: Alex Belits <abelits@marvell.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Reviewed-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
Tested-by: Nicolas Saenz Julienne <nsaenzju@redhat.com>
effective_cpu_util() already has a `int cpu' parameter which allows to
retrieve the CPU capacity scale factor (or maximum CPU capacity) inside
this function via an arch_scale_cpu_capacity(cpu).
A lot of code calling effective_cpu_util() (or the shim
sched_cpu_util()) needs the maximum CPU capacity, i.e. it will call
arch_scale_cpu_capacity() already.
But not having to pass it into effective_cpu_util() will make the EAS
wake-up code easier, especially when the maximum CPU capacity reduced
by the thermal pressure is passed through the EAS wake-up functions.
Due to the asymmetric CPU capacity support of arm/arm64 architectures,
arch_scale_cpu_capacity(int cpu) is a per-CPU variable read access via
per_cpu(cpu_scale, cpu) on such a system.
On all other architectures it is a a compile-time constant
(SCHED_CAPACITY_SCALE).
Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Vincent Guittot <vincent.guittot@linaro.org>
Tested-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://lkml.kernel.org/r/20220621090414.433602-4-vdonnefort@google.com
Before being migrated to a new CPU, a task sees its PELT values
synchronized with rq last_update_time. Once done, that same task will also
have its sched_avg last_update_time reset. This means the time between
the migration and the last clock update will not be accounted for in
util_avg and a discontinuity will appear. This issue is amplified by the
PELT clock scaling. It takes currently one tick after the CPU being idle
to let clock_pelt catching up clock_task.
This is especially problematic for asymmetric CPU capacity systems which
need stable util_avg signals for task placement and energy estimation.
Ideally, this problem would be solved by updating the runqueue clocks
before the migration. But that would require taking the runqueue lock
which is quite expensive [1]. Instead estimate the missing time and update
the task util_avg with that value.
To that end, we need sched_clock_cpu() but it is a costly function. Limit
the usage to the case where the source CPU is idle as we know this is when
the clock is having the biggest risk of being outdated.
See comment in migrate_se_pelt_lag() for more details about how the PELT
value is estimated. Notice though this estimation doesn't take into account
IRQ and Paravirt time.
[1] https://lkml.kernel.org/r/20190709115759.10451-1-chris.redpath@arm.com
Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://lkml.kernel.org/r/20220621090414.433602-3-vdonnefort@google.com
Introducing macro helpers u64_u32_{store,load}() to factorize lockless
accesses to u64 variables for 32-bits architectures.
Users are for now cfs_rq.min_vruntime and sched_avg.last_update_time. To
accommodate the later where the copy lies outside of the structure
(cfs_rq.last_udpate_time_copy instead of sched_avg.last_update_time_copy),
use the _copy() version of those helpers.
Those new helpers encapsulate smp_rmb() and smp_wmb() synchronization and
therefore, have a small penalty for 32-bits machines in set_task_rq_fair()
and init_cfs_rq().
Signed-off-by: Vincent Donnefort <vincent.donnefort@arm.com>
Signed-off-by: Vincent Donnefort <vdonnefort@google.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Tested-by: Lukasz Luba <lukasz.luba@arm.com>
Link: https://lkml.kernel.org/r/20220621090414.433602-2-vdonnefort@google.com
Wakelist can help avoid cache bouncing and offload the overhead of waker
cpu. So far, using wakelist within the same llc only happens on
WF_ON_CPU, and this limitation could be removed to further improve
wakeup performance.
The commit 518cd62341 ("sched: Only queue remote wakeups when
crossing cache boundaries") disabled queuing tasks on wakelist when
the cpus share llc. This is because, at that time, the scheduler must
send IPIs to do ttwu_queue_wakelist. Nowadays, ttwu_queue_wakelist also
supports TIF_POLLING, so this is not a problem now when the wakee cpu is
in idle polling.
Benefits:
Queuing the task on idle cpu can help improving performance on waker cpu
and utilization on wakee cpu, and further improve locality because
the wakee cpu can handle its own rq. This patch helps improving rt on
our real java workloads where wakeup happens frequently.
Consider the normal condition (CPU0 and CPU1 share same llc)
Before this patch:
CPU0 CPU1
select_task_rq() idle
rq_lock(CPU1->rq)
enqueue_task(CPU1->rq)
notify CPU1 (by sending IPI or CPU1 polling)
resched()
After this patch:
CPU0 CPU1
select_task_rq() idle
add to wakelist of CPU1
notify CPU1 (by sending IPI or CPU1 polling)
rq_lock(CPU1->rq)
enqueue_task(CPU1->rq)
resched()
We see CPU0 can finish its work earlier. It only needs to put task to
wakelist and return.
While CPU1 is idle, so let itself handle its own runqueue data.
This patch brings no difference about IPI.
This patch only takes effect when the wakee cpu is:
1) idle polling
2) idle not polling
For 1), there will be no IPI with or without this patch.
For 2), there will always be an IPI before or after this patch.
Before this patch: waker cpu will enqueue task and check preempt. Since
"idle" will be sure to be preempted, waker cpu must send a resched IPI.
After this patch: waker cpu will put the task to the wakelist of wakee
cpu, and send an IPI.
Benchmark:
We've tested schbench, unixbench, and hachbench on both x86 and arm64.
On x86 (Intel Xeon Platinum 8269CY):
schbench -m 2 -t 8
Latency percentiles (usec) before after
50.0000th: 8 6
75.0000th: 10 7
90.0000th: 11 8
95.0000th: 12 8
*99.0000th: 13 10
99.5000th: 15 11
99.9000th: 18 14
Unixbench with full threads (104)
before after
Dhrystone 2 using register variables 3011862938 3009935994 -0.06%
Double-Precision Whetstone 617119.3 617298.5 0.03%
Execl Throughput 27667.3 27627.3 -0.14%
File Copy 1024 bufsize 2000 maxblocks 785871.4 784906.2 -0.12%
File Copy 256 bufsize 500 maxblocks 210113.6 212635.4 1.20%
File Copy 4096 bufsize 8000 maxblocks 2328862.2 2320529.1 -0.36%
Pipe Throughput 145535622.8 145323033.2 -0.15%
Pipe-based Context Switching 3221686.4 3583975.4 11.25%
Process Creation 101347.1 103345.4 1.97%
Shell Scripts (1 concurrent) 120193.5 123977.8 3.15%
Shell Scripts (8 concurrent) 17233.4 17138.4 -0.55%
System Call Overhead 5300604.8 5312213.6 0.22%
hackbench -g 1 -l 100000
before after
Time 3.246 2.251
On arm64 (Ampere Altra):
schbench -m 2 -t 8
Latency percentiles (usec) before after
50.0000th: 14 10
75.0000th: 19 14
90.0000th: 22 16
95.0000th: 23 16
*99.0000th: 24 17
99.5000th: 24 17
99.9000th: 28 25
Unixbench with full threads (80)
before after
Dhrystone 2 using register variables 3536194249 3537019613 0.02%
Double-Precision Whetstone 629383.6 629431.6 0.01%
Execl Throughput 65920.5 65846.2 -0.11%
File Copy 1024 bufsize 2000 maxblocks 1063722.8 1064026.8 0.03%
File Copy 256 bufsize 500 maxblocks 322684.5 318724.5 -1.23%
File Copy 4096 bufsize 8000 maxblocks 2348285.3 2328804.8 -0.83%
Pipe Throughput 133542875.3 131619389.8 -1.44%
Pipe-based Context Switching 3215356.1 3576945.1 11.25%
Process Creation 108520.5 120184.6 10.75%
Shell Scripts (1 concurrent) 122636.3 121888 -0.61%
Shell Scripts (8 concurrent) 17462.1 17381.4 -0.46%
System Call Overhead 4429998.9 4435006.7 0.11%
hackbench -g 1 -l 100000
before after
Time 4.217 2.916
Our patch has improvement on schbench, hackbench
and Pipe-based Context Switching of unixbench
when there exists idle cpus,
and no obvious regression on other tests of unixbench.
This can help improve rt in scenes where wakeup happens frequently.
Signed-off-by: Tianchen Ding <dtcccc@linux.alibaba.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Valentin Schneider <vschneid@redhat.com>
Link: https://lore.kernel.org/r/20220608233412.327341-3-dtcccc@linux.alibaba.com
The purpose of balance_push() is to act as a filter on task selection
in the case of CPU hotplug, specifically when taking the CPU out.
It does this by (ab)using the balance callback infrastructure, with
the express purpose of keeping all the unlikely/odd cases in a single
place.
In order to serve its purpose, the balance_push_callback needs to be
(exclusively) on the callback list at all times (noting that the
callback always places itself back on the list the moment it runs,
also noting that when the CPU goes down, regular balancing concerns
are moot, so ignoring them is fine).
And here-in lies the problem, __sched_setscheduler()'s use of
splice_balance_callbacks() takes the callbacks off the list across a
lock-break, making it possible for, an interleaving, __schedule() to
see an empty list and not get filtered.
Fixes: ae79270232 ("sched: Optimize finish_lock_switch()")
Reported-by: Jing-Ting Wu <jing-ting.wu@mediatek.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Jing-Ting Wu <jing-ting.wu@mediatek.com>
Link: https://lkml.kernel.org/r/20220519134706.GH2578@worktop.programming.kicks-ass.net
