linux/kernel/sched
Konstantin Khlebnikov b4fb015eef sched/rt: Optimize checking group RT scheduler constraints
Group RT scheduler contains protection against setting zero runtime for
cgroup with RT tasks. Right now function tg_set_rt_bandwidth() iterates
over all CPU cgroups and calls tg_has_rt_tasks() for any cgroup which
runtime is zero (not only for changed one). Default RT runtime is zero,
thus tg_has_rt_tasks() will is called for almost at CPU cgroups.

This protection already is slightly racy: runtime limit could be changed
between cpu_cgroup_can_attach() and cpu_cgroup_attach() because changing
cgroup attribute does not lock cgroup_mutex while attach does not lock
rt_constraints_mutex. Changing task scheduler class also races with
changing rt runtime: check in __sched_setscheduler() isn't protected.

Function tg_has_rt_tasks() iterates over all threads in the system.
This gives NR_CGROUPS * NR_TASKS operations under single tasklist_lock
locked for read tg_set_rt_bandwidth(). Any concurrent attempt of locking
tasklist_lock for write (for example fork) will stuck with disabled irqs.

This patch makes two optimizations:
1) Remove locking tasklist_lock and iterate only tasks in cgroup
2) Call tg_has_rt_tasks() iff rt runtime changes from non-zero to zero

All changed code is under CONFIG_RT_GROUP_SCHED.

Testcase:

 # mkdir /sys/fs/cgroup/cpu/test{1..10000}
 # echo 0 | tee /sys/fs/cgroup/cpu/test*/cpu.rt_runtime_us

At the same time without patch fork time will be >100ms:

 # perf trace -e clone --duration 100 stress-ng --fork 1

Also remote ping will show timings >100ms caused by irq latency.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lkml.kernel.org/r/157996383820.4651.11292439232549211693.stgit@buzz
2020-01-28 21:37:09 +01:00
..
autogroup.c sched/autogroup: Make autogroup_path() always available 2019-06-24 19:23:40 +02:00
autogroup.h sched/headers: Simplify and clean up header usage in the scheduler 2018-03-04 12:39:29 +01:00
clock.c sched/clock: Use static_branch_likely() with sched_clock_running 2019-11-29 08:10:54 +01:00
completion.c sched/Documentation: Update wake_up() & co. memory-barrier guarantees 2018-07-17 09:30:34 +02:00
core.c x86, sched: Add support for frequency invariance 2020-01-28 21:36:59 +01:00
cpuacct.c sched/headers: Simplify and clean up header usage in the scheduler 2018-03-04 12:39:29 +01:00
cpudeadline.c Linux 5.2-rc5 2019-06-17 12:12:27 +02:00
cpudeadline.h sched/headers: Simplify and clean up header usage in the scheduler 2018-03-04 12:39:29 +01:00
cpufreq.c cpufreq: Avoid leaving stale IRQ work items during CPU offline 2019-12-12 17:59:43 +01:00
cpufreq_schedutil.c sched/uclamp: Rename uclamp_util_with() into uclamp_rq_util_with() 2019-12-25 10:42:08 +01:00
cpupri.c sched/rt: Make RT capacity-aware 2019-12-25 10:42:10 +01:00
cpupri.h sched/rt: Make RT capacity-aware 2019-12-25 10:42:10 +01:00
cputime.c sched/cputime: move rq parameter in irqtime_account_process_tick 2020-01-17 10:19:21 +01:00
deadline.c sched/core: Further clarify sched_class::set_next_task() 2019-11-11 08:35:21 +01:00
debug.c sched/debug: Reset watchdog on all CPUs while processing sysrq-t 2020-01-17 10:19:20 +01:00
fair.c sched/fair: Optimize select_idle_core() 2020-01-28 21:37:08 +01:00
features.h sched/fair/util_est: Implement faster ramp-up EWMA on utilization increases 2019-10-29 10:01:07 +01:00
idle.c idle: fix spelling mistake "iterrupts" -> "interrupts" 2020-01-17 10:19:22 +01:00
isolation.c sched/isolation: Prefer housekeeping CPU in local node 2019-07-25 15:51:55 +02:00
loadavg.c timers/nohz: Update NOHZ load in remote tick 2020-01-28 21:36:44 +01:00
Makefile psi: pressure stall information for CPU, memory, and IO 2018-10-26 16:26:32 -07:00
membarrier.c membarrier: Fix RCU locking bug caused by faulty merge 2019-10-01 21:27:50 +02:00
pelt.c schied/fair: Skip calculating @contrib without load 2019-12-17 13:32:51 +01:00
pelt.h sched/topology: Remove unused 'sd' parameter from arch_scale_cpu_capacity() 2019-06-24 19:23:39 +02:00
psi.c sched/psi: create /proc/pressure and /proc/pressure/{io|memory|cpu} only when psi enabled 2020-01-17 10:19:22 +01:00
rt.c sched/rt: Optimize checking group RT scheduler constraints 2020-01-28 21:37:09 +01:00
sched-pelt.h sched/fair: Fix "runnable_avg_yN_inv" not used warnings 2019-06-17 12:15:58 +02:00
sched.h x86, sched: Add support for frequency invariance 2020-01-28 21:36:59 +01:00
stats.c proc: introduce proc_create_seq{,_data} 2018-05-16 07:23:35 +02:00
stats.h sched/stats: Fix unlikely() use of sched_info_on() 2019-07-25 15:51:55 +02:00
stop_task.c sched/core: Further clarify sched_class::set_next_task() 2019-11-11 08:35:21 +01:00
swait.c kernel/sched/: remove caller signal_pending branch predictions 2019-01-04 13:13:48 -08:00
topology.c sched/topology: Assert non-NUMA topology masks don't (partially) overlap 2020-01-17 10:19:23 +01:00
wait.c Add wake_up_interruptible_sync_poll_locked() 2019-10-31 15:12:23 +00:00
wait_bit.c sched/wait: fix ___wait_var_event(exclusive) 2019-12-17 13:32:50 +01:00