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sched/psi: Optimize task switch inside shared cgroups again
Way back when PSI_MEM_FULL was accounted from the timer tick, task
switching could simply iterate next and prev to the common ancestor to
update TSK_ONCPU and be done.
Then memstall ticks were replaced with checking curr->in_memstall
directly in psi_group_change(). That meant that now if the task switch
was between a memstall and a !memstall task, we had to iterate through
the common ancestors at least ONCE to fix up their state_masks.
We added the identical_state filter to make sure the common ancestor
elimination was skipped in that case. It seems that was always a
little too eager, because it caused us to walk the common ancestors
*twice* instead of the required once: the iteration for next could
have stopped at the common ancestor; prev could have updated TSK_ONCPU
up to the common ancestor, then finish to the root without changing
any flags, just to get the new curr->in_memstall into the state_masks.
This patch recognizes this and makes it so that we walk to the root
exactly once if state_mask needs updating, which is simply catching up
on a missed optimization that could have been done in commit 7fae6c8171
("psi: Use ONCPU state tracking machinery to detect reclaim") directly.
Apart from this, it's also necessary for the next patch "sched/psi: remove
NR_ONCPU task accounting". Suppose we walk the common ancestors twice:
(1) psi_group_change(.clear = 0, .set = TSK_ONCPU)
(2) psi_group_change(.clear = TSK_ONCPU, .set = 0)
We previously used tasks[NR_ONCPU] to record TSK_ONCPU, tasks[NR_ONCPU]++
in (1) then tasks[NR_ONCPU]-- in (2), so tasks[NR_ONCPU] still be correct.
The next patch change to use one bit in state mask to record TSK_ONCPU,
PSI_ONCPU bit will be set in (1), but then be cleared in (2), which cause
the psi_group_cpu has task running on CPU but without PSI_ONCPU bit set!
With this patch, we will never walk the common ancestors twice, so won't
have above problem.
Suggested-by: Johannes Weiner <hannes@cmpxchg.org>
Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Johannes Weiner <hannes@cmpxchg.org>
Link: https://lore.kernel.org/r/20220825164111.29534-6-zhouchengming@bytedance.com
This commit is contained in:
parent
d79ddb069c
commit
65176f59a1
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@ -820,20 +820,15 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
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u64 now = cpu_clock(cpu);
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if (next->pid) {
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bool identical_state;
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psi_flags_change(next, 0, TSK_ONCPU);
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/*
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* When switching between tasks that have an identical
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* runtime state, the cgroup that contains both tasks
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* we reach the first common ancestor. Iterate @next's
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* ancestors only until we encounter @prev's ONCPU.
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* Set TSK_ONCPU on @next's cgroups. If @next shares any
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* ancestors with @prev, those will already have @prev's
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* TSK_ONCPU bit set, and we can stop the iteration there.
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*/
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identical_state = prev->psi_flags == next->psi_flags;
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iter = NULL;
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while ((group = iterate_groups(next, &iter))) {
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if (identical_state &&
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per_cpu_ptr(group->pcpu, cpu)->tasks[NR_ONCPU]) {
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if (per_cpu_ptr(group->pcpu, cpu)->tasks[NR_ONCPU]) {
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common = group;
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break;
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}
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@ -877,10 +872,12 @@ void psi_task_switch(struct task_struct *prev, struct task_struct *next,
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psi_group_change(group, cpu, clear, set, now, wake_clock);
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/*
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* TSK_ONCPU is handled up to the common ancestor. If we're tasked
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* with dequeuing too, finish that for the rest of the hierarchy.
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* TSK_ONCPU is handled up to the common ancestor. If there are
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* any other differences between the two tasks (e.g. prev goes
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* to sleep, or only one task is memstall), finish propagating
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* those differences all the way up to the root.
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*/
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if (sleep) {
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if ((prev->psi_flags ^ next->psi_flags) & ~TSK_ONCPU) {
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clear &= ~TSK_ONCPU;
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for (; group; group = iterate_groups(prev, &iter))
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psi_group_change(group, cpu, clear, set, now, wake_clock);
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