linux-kernel - Re: EEVDF and NUMA balancing

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Date: Fri, 5 Jan 2024 15:51:50 +0100 (CET)
From: Julia Lawall <julia.lawall@...ia.fr>
To: Vincent Guittot <vincent.guittot@...aro.org>
cc: 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

> 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().

x86 has TIF_POLLING_NRFLAG defined to be a non zero value, which I think
is sufficient to cause the issue.

julia