Messages in this thread | | | Date | Fri, 26 Nov 2021 17:18:17 +0000 | From | Vincent Donnefort <> | Subject | Re: [PATCH] sched/fair: Fix detection of per-CPU kthreads waking a task |
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On Fri, Nov 26, 2021 at 04:49:12PM +0000, Valentin Schneider wrote: > On 26/11/21 15:40, Vincent Guittot wrote: > > On Fri, 26 Nov 2021 at 14:32, Valentin Schneider > > <Valentin.Schneider@arm.com> wrote: > >> /* > >> - * Allow a per-cpu kthread to stack with the wakee if the > >> - * kworker thread and the tasks previous CPUs are the same. > >> - * The assumption is that the wakee queued work for the > >> - * per-cpu kthread that is now complete and the wakeup is > >> - * essentially a sync wakeup. An obvious example of this > >> + * Allow a per-cpu kthread to stack with the wakee if the kworker thread > >> + * and the tasks previous CPUs are the same. The assumption is that the > >> + * wakee queued work for the per-cpu kthread that is now complete and > >> + * the wakeup is essentially a sync wakeup. An obvious example of this > >> * pattern is IO completions. > >> + * > >> + * Ensure the wakeup is issued by the kthread itself, and don't match > >> + * against the idle task because that could override the > >> + * available_idle_cpu(target) check done higher up. > >> */ > >> - if (is_per_cpu_kthread(current) && > >> + if (is_per_cpu_kthread(current) && !is_idle_task(current) && > > > > still i don't see the need of !is_idle_task(current) > > > > Admittedly, belts and braces. The existing condition checks rq->nr_running <= 1 > which can lead to coscheduling when the wakeup is issued by the idle task > (or even if rq->nr_running == 0, you can have rq->ttwu_pending without > having sent an IPI due to polling). Essentially this overrides the first > check in sis() that uses idle_cpu(target) (prev == smp_processor_id() == > target). > > I couldn't prove such wakeups can happen right now, but if/when they do > (AIUI it would just take someone to add a wake_up_process() down some > smp_call_function() callback) then we'll need the above. If you're still > not convinced by now, I won't push it further.
From a quick experiment, even with the asym_fits_capacity(), I can trigger the following:
[ 0.118855] select_idle_sibling: wakee=kthreadd:2 nr_cpus_allowed=8 current=swapper/0:1 in_task=1 [ 0.128214] select_idle_sibling: wakee=rcu_gp:3 nr_cpus_allowed=8 current=swapper/0:1 in_task=1 [ 0.137327] select_idle_sibling: wakee=rcu_par_gp:4 nr_cpus_allowed=8 current=swapper/0:1 in_task=1 [ 0.147221] select_idle_sibling: wakee=kworker/u16:0:7 nr_cpus_allowed=8 current=swapper/0:1 in_task=1 [ 0.156994] select_idle_sibling: wakee=mm_percpu_wq:8 nr_cpus_allowed=8 current=swapper/0:1 in_task=1 [ 0.171943] select_idle_sibling: wakee=rcu_sched:10 nr_cpus_allowed=8 current=swapper/0:1 in_task=1
So the in_task() condition doesn't appear to be enough to filter wakeups while we have the swapper as a current.
> > > > >> + in_task() && > >> prev == smp_processor_id() && > >> this_rq()->nr_running <= 1) { > >> return prev; > >>
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