linux-kernel - Re: EEVDF and NUMA balancing

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Message-ID: <edbd8ecd-148c-b366-fd46-3531dec39d49@inria.fr>
Date: Thu, 28 Dec 2023 19:34:48 +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

> > > > > > I'm surprised that you have mainly CPU_NEWLY_IDLE. Do you know the reason ?
> > > > >
> > > > > No.  They come from do_idle calling the scheduler.  I will look into why
> > > > > this happens so often.
> > > >
> > > > Hmm, the CPU was idle and received a need resched which triggered the
> > > > scheduler but there was nothing to schedule so it goes back to idle
> > > > after running a newly_idle _load_balance.
> > >
> > > I spent quite some time thinking the same until I saw the following code
> > > in do_idle:
> > >
> > > preempt_set_need_resched();
> > >
> > > So I have the impression that do_idle sets need resched itself.
> >
> > But of course that code is only executed if need_resched is true.  But I
>
> Yes, that is your root cause. something, most probably in interrupt
> context, wakes up your CPU and expect to wake up a thread
>
> > don't know who would be setting need resched on each clock tick.
>
> that can be a timer, interrupt, ipi, rcu ...
> a trace should give you some hints

I have the impression that it is the goal of calling nohz_csd_func on each
clock tick that causes the calls to need_resched.  If the idle process is
polling, call_function_single_prep_ipi just sets need_resched to get the
idle process to stop polling.  But there is no actual task that the idle
process should schedule.  The need_resched then prevents the idle process
from stealing, due to the CPU_NEWLY_IDLE flag, contradicting the whole
purpose of calling nohz_csd_func in the first place.

julia