linux-kernel - Re: [PATCH v7] sched: Consolidate cpufreq updates

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Message-ID: <CAKfTPtBahrD5L8CbB4BijAvnwq=yG375TWDUuEvNipyTDYGQTA@mail.gmail.com>
Date: Mon, 2 Sep 2024 14:30:46 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Qais Yousef <qyousef@...alina.io>
Cc: Christian Loehle <christian.loehle@....com>, "Rafael J. Wysocki" <rafael@...nel.org>, 
	Viresh Kumar <viresh.kumar@...aro.org>, Ingo Molnar <mingo@...nel.org>, 
	Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>, 
	Steven Rostedt <rostedt@...dmis.org>, Dietmar Eggemann <dietmar.eggemann@....com>, 
	Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, 
	Valentin Schneider <vschneid@...hat.com>, Hongyan Xia <hongyan.xia2@....com>, 
	John Stultz <jstultz@...gle.com>, linux-pm@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v7] sched: Consolidate cpufreq updates

On Sun, 1 Sept 2024 at 19:51, Qais Yousef <qyousef@...alina.io> wrote:
>
> On 08/13/24 10:27, Vincent Guittot wrote:
> > On Tue, 13 Aug 2024 at 10:25, Vincent Guittot
> > <vincent.guittot@...aro.org> wrote:
> > >
> > > On Mon, 5 Aug 2024 at 17:35, Christian Loehle <christian.loehle@....com> wrote:
> > > > Hi Qais,
> > > > the idea of SCHED_CPUFREQ_FORCE_UPDATE and the possiblity of spamming
> > > > freq updates still bothered me so let me share my thoughts even though
> > > > it might be niche enough for us not to care.
> > > >
> > > > 1. On fast_switch systems, assuming they are fine with handling the
> > > > actual updates, we have a bit more work on each context_switch() and
> > > > some synchronisation, too. That should be fine, if anything there's
> > > > some performance regression in a couple of niche cases.
> > > >
> > > > 2. On !fast_switch systems this gets more interesting IMO. So we have
> > > > a sugov DEADLINE task wakeup for every (in a freq-diff resulting)
> > > > update request. This task will preempt whatever and currently will
> > > > pretty much always be running on the CPU it ran last on (so first CPU
> > > > of the PD).
> > >
> > > The !fast_switch is a bit of concern for me too but not for the same
> > > reason and maybe the opposite of yours IIUC your proposal below:
> > >
> > > With fast_switch we have the following sequence:
> > >
> > > sched_switch() to task A
> > > cpufreq_driver_fast_switch -> write new freq target
> > > run task A
> > >
> > > This is pretty straight forward but we have the following sequence
> > > with !fast_switch
> > >
> > > sched_switch() to task A
> > > queue_irq_work -> raise an IPI on local CPU
> > > Handle IPI -> wakeup and queue sugov dl worker on local CPU (always
> > > with 1 CPU per PD)
> > > sched_switch() to sugov dl task
> > > __cpufreq_driver_target() which can possibly block on a lock
> > > sched_switch() to task A
> > > run task A
> > >
> >
> > sent a bit too early
> >
> > > We can possibly have 2 context switch and one IPi for each "normal"
> > > context switch which is not really optimal
> >
> > It would be good to find a way to skip the spurious back and forth
> > between the normal task and sugov
>
> Hmm I think we use affinity to keep the sugov running on policy->related_cpus.
> Relaxing this will make it less of a problem, but won't eliminate it.

yes, but it's not a problem of relaxing affinity here

The problem is that the 1st switch to task A will be preempted by
sugov so the 1st switch is useless. You should call cpufreq_update
before switching to A so that we skip the useless switch to task A and
directly switch to sugov 1st then task A

>
> I'll have a think about it, is this a blocker for now?
>
>