Messages in this thread | | | From | Valentin Schneider <> | Subject | Re: [PATCH] sched/fair: Fix detection of per-CPU kthreads waking a task | Date | Thu, 25 Nov 2021 15:30:05 +0000 |
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On 25/11/21 14:23, Vincent Guittot wrote: > On Thu, 25 Nov 2021 at 12:16, Valentin Schneider > <Valentin.Schneider@arm.com> wrote: >> I think you can still hit this on a symmetric system; let me try to >> reformulate my other email. >> >> If this (non-patched) condition evaluates to true, it means the previous >> condition >> >> (available_idle_cpu(target) || sched_idle_cpu(target)) && >> asym_fits_capacity(task_util, target) >> >> evaluated to false, so for a symmetric system target sure isn't idle. >> >> prev == smp_processor_id() implies prev == target, IOW prev isn't >> idle. Now, consider: >> >> p0.prev = CPU1 >> p1.prev = CPU1 >> >> CPU0 CPU1 >> current = don't care current = swapper/1 >> >> ttwu(p1) >> ttwu_queue(p1, CPU1) >> // or >> ttwu_queue_wakelist(p1, CPU1) >> >> hrtimer_wakeup() >> wake_up_process() >> ttwu() >> idle_cpu(CPU1)? no >> >> is_per_cpu_kthread(current)? yes >> prev == smp_processor_id()? yes >> this_rq()->nr_running <= 1? yes >> => self enqueue >> >> ... >> schedule_idle() >> >> This works if CPU0 does either a full enqueue (rq->nr_running == 1) or just >> a wakelist enqueue (rq->ttwu_pending > 0). If there was an idle CPU3 >> around, we'd still be stacking p0 and p1 onto CPU1. >> >> IOW this opens a window between a remote ttwu() and the idle task invoking >> schedule_idle() where the idle task can stack more tasks onto its CPU. > > Your use case above is out of the scope of this patch and has always > been there, even for other per cpu kthreads. In such case, the wake up > is not triggered by current (idle or another per cpu kthread) but by > an interrupt (hrtimer in your case).
Technically the idle task didn't pass is_per_cpu_kthread(p) when that condition was added, this is somewhat of a "new development" - but you're right on the hardirq side of things.
> If we want to filter wakeup > generated by interrupt context while a per cpu kthread is running, it > would be better to fix all cases and test the running context like > this >
I think that could make sense - though can the idle task issue wakeups in process context? If so that won't be sufficient. A quick audit tells me:
o rcu_nocb_flush_deferred_wakeup() happens before calling into cpuidle o I didn't see any wakeup issued from the cpu_pm_notifier call chain o I'm not entirely sure about flush_smp_call_function_from_idle(). I found this thing in RCU:
smp_call_function_single(cpu, rcu_exp_handler)
rcu_exp_handler() rcu_report_exp_rdp() rcu_report_exp_cpu_mult() __rcu_report_exp_rnp() swake_up_one()
IIUC if set_nr_if_polling() then the smp_call won't send an IPI and should be handled in that flush_foo_from_idle() call.
I'd be tempted to stick your VincentD's conditions together, just to be safe...
> --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -6397,7 +6397,8 @@ static int select_idle_sibling(struct > task_struct *p, int prev, int target) > * essentially a sync wakeup. An obvious example of this > * pattern is IO completions. > */ > - if (is_per_cpu_kthread(current) && > + if (!in_interrupt() && > + is_per_cpu_kthread(current) && > prev == smp_processor_id() && > this_rq()->nr_running <= 1) { > return prev; > >> >> > >> >> -- >> >> 2.25.1 >> >>
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