linux-kernel - Re: EEVDF and NUMA balancing

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Date: Fri, 5 Jan 2024 18:27:19 +0100 (CET)
From: Julia Lawall <julia.lawall@...ia.fr>
To: Julia Lawall <julia.lawall@...ia.fr>
cc: Vincent Guittot <vincent.guittot@...aro.org>, 
    Peter Zijlstra <peterz@...radead.org>, Ingo Molnar <mingo@...hat.com>, 
    Dietmar Eggemann <dietmar.eggemann@....com>, Mel Gorman <mgorman@...e.de>, 
    linux-kernel@...r.kernel.org
Subject: Re: EEVDF and NUMA balancing



On Fri, 5 Jan 2024, Julia Lawall wrote:

>
>
> On Fri, 5 Jan 2024, Vincent Guittot wrote:
>
> > On Fri, 5 Jan 2024 at 15:51, Julia Lawall <julia.lawall@...ia.fr> wrote:
> > >
> > > > Your system is calling the polling mode and not the default
> > > > cpuidle_idle_call() ? This could explain why I don't see such problem
> > > > on my system which doesn't have polling
> > > >
> > > > Are you forcing the use of polling mode ?
> > > > If yes, could you check that this problem disappears without forcing
> > > > polling mode ?
> > >
> > > I expanded the code in do_idle to:
> > >
> > >                 if (cpu_idle_force_poll) { c1++;
> > >                         tick_nohz_idle_restart_tick();
> > >                         cpu_idle_poll();
> > >                 } else if (tick_check_broadcast_expired()) { c2++;
> > >                         tick_nohz_idle_restart_tick();
> > >                         cpu_idle_poll();
> > >                 } else { c3++;
> > >                         cpuidle_idle_call();
> > >                 }
> > >
> > > Later, I have:
> > >
> > >         trace_printk("force poll: %d: c1: %d, c2: %d, c3: %d\n",cpu_idle_force_poll, c1, c2, c3);
> > >         flush_smp_call_function_queue();
> > >         schedule_idle();
> > >
> > > force poll, c1 and c2 are always 0, and c3 is always some non-zero value.
> > > Sometimes small (often 1), and sometimes large (304 or 305).
> > >
> > > So I don't think it's calling cpu_idle_poll().
> >
> > I agree that something else
> >
> > >
> > > x86 has TIF_POLLING_NRFLAG defined to be a non zero value, which I think
> > > is sufficient to cause the issue.
> >
> > Could you trace trace_sched_wake_idle_without_ipi() ans csd traces as well ?
> > I don't understand what set need_resched() in your case; having in
> > mind that I don't see the problem on my Arm systems and IIRC Peter
> > said that he didn't face the problem on his x86 system.
>
> TIF_POLLING_NRFLAG doesn't seem to be defined on Arm.
>
> Peter said that he didn't see the problem, but perhaps that was just
> random.  It requires a NUMA move to occur.  I make 20 runs to be sure to
> see the problem at least once.  But another machine might behave
> differently.
>
> I believe the call chain is:
>
> scheduler_tick
>   trigger_load_balance
>     nohz_balancer_kick
>       kick_ilb
>         smp_call_function_single_async
>           generic_exec_single
>             __smp_call_single_queue
>               send_call_function_single_ipi
>                 call_function_single_prep_ipi
>                   set_nr_if_polling <====== sets need_resched
>
> I'll make a trace to reverify that.

This is what I see at a tick, which corresponds to the call chain shown
above:

          bt.B.x-4184  [046]   466.410605: bputs:                scheduler_tick: calling trigger_load_balance
          bt.B.x-4184  [046]   466.410605: bputs:                trigger_load_balance: calling nohz_balancer_kick
          bt.B.x-4184  [046]   466.410605: bputs:                trigger_load_balance: calling kick_ilb
          bt.B.x-4184  [046]   466.410607: bprint:               trigger_load_balance: calling smp_call_function_single_async 22
          bt.B.x-4184  [046]   466.410607: bputs:                smp_call_function_single_async: calling generic_exec_single
          bt.B.x-4184  [046]   466.410607: bputs:                generic_exec_single: calling __smp_call_single_queue
          bt.B.x-4184  [046]   466.410608: bputs:                __smp_call_single_queue: calling send_call_function_single_ipi
          bt.B.x-4184  [046]   466.410608: bputs:                __smp_call_single_queue: calling call_function_single_prep_ipi
          bt.B.x-4184  [046]   466.410608: bputs:                call_function_single_prep_ipi: calling set_nr_if_polling
          bt.B.x-4184  [046]   466.410609: sched_wake_idle_without_ipi: cpu=22

julia