Messages in this thread Patch in this message | | | Date | Tue, 8 Nov 2022 11:08:35 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] sched/core: Minor optimize ttwu_runnable() |
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On Tue, Nov 08, 2022 at 10:11:49AM +0100, Peter Zijlstra wrote: > On Mon, Nov 07, 2022 at 03:54:38PM +0000, Valentin Schneider wrote: > > > So that's the part for the p->sched_class->task_woken() callback, which > > only affects RT and DL (and only does something when !p->on_cpu). I *think* > > it's fine to remove it from ttwu_runnable() as any push/pull should have > > happened when other tasks were enqueued on the same CPU - with that said, > > it wouldn't hurt to double check this :-) > > > > > > As for the check_preempt_curr(), since per the above p can be preempted, > > you could have scenarios right now with CFS tasks where > > ttwu_runnable()->check_preempt_curr() causes NEED_RESCHED to be set. > > > > e.g. p0 does > > > > set_current_state(TASK_UNINTERRUPTIBLE) > > > > but then gets interrupted by the tick, a p1 gets selected to run instead > > because of check_preempt_tick(), and then runs long enough to have > > check_preempt_curr() decide to let p0 preempt p1. > > > > That does require specific timing (lower tick frequency should make this > > more likely) and probably task niceness distribution too, but isn't > > impossible. > > > > Maybe try reading p->on_cpu, and only do the quick task state update if > > it's still the current task, otherwise do the preemption checks? > > I'm confused...
I am and Valentin has a point. It could indeed be preempted and in that case check_preempt_curr() could indeed make it get back on.
In that case his suggestion might make sense; something along the lines of so I suppose...
(And I think we can still do the reorg I proposed elsewhere, but I've not yet tried.)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index cb2aa2b54c7a..6944d9473295 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3698,9 +3698,16 @@ static int ttwu_runnable(struct task_struct *p, int wake_flags) rq = __task_rq_lock(p, &rf); if (task_on_rq_queued(p)) { - /* check_preempt_curr() may use rq clock */ - update_rq_clock(rq); - ttwu_do_wakeup(rq, p, wake_flags, &rf); + if (!p->on_cpu) { + /* + * When on_rq && !on_cpu the task is preempted, see if + * it should preempt whatever is current there now. + */ + update_rq_clock(rq); + check_preempt_curr(rq, p, wake_flags); + } + WRITE_ONCE(p->__state, TASK_RUNNING); + trace_sched_wakeup(p); ret = 1; } __task_rq_unlock(rq, &rf);
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