linux-kernel - Re: [PATCH] sched/fair: Rate limit calls to update_blocked

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Message-ID: <8db5ebd3-4e5c-cd7e-e0cb-dc86c3cafb5c@arm.com>
Date:   Mon, 25 Jan 2021 18:53:45 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Joel Fernandes <joel@...lfernandes.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Paul McKenney <paulmck@...nel.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Qais Yousef <qais.yousef@....com>,
        Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>
Subject: Re: [PATCH] sched/fair: Rate limit calls to update_blocked_averages()
 for NOHZ

On 25/01/2021 18:30, Vincent Guittot wrote:
> On Mon, 25 Jan 2021 at 11:45, Dietmar Eggemann <dietmar.eggemann@....com> wrote:
>>
>> On 22/01/2021 20:10, Joel Fernandes wrote:
>>> Hi Vincent,
>>>
>>> Thanks for reply. Please see the replies below:
>>>
>>> On Fri, Jan 22, 2021 at 05:56:22PM +0100, Vincent Guittot wrote:
>>>> On Fri, 22 Jan 2021 at 16:46, Joel Fernandes (Google)
>>>> <joel@...lfernandes.org> wrote:

[...]

>> If I understood you correctly, you want to avoid these frequent calls
>> to update_blocked_averages() here to further avoid invoking sched_util
>> via update_blocked_averages() -> cpufreq_update_util() (since 'decayed'
>> is set) very often in your setup.
> 
> So It's not clear if the problem that joel wants to raise, is about:
> - the running time of  update_blocked_averages
> - the running time of the cpufreq_update_util which is called because
> utilization has decayed during the update of blocked load
> - the wake up latency because of newly_idle lb

Pretty much so.

IIRC his interest is driven by the fact that he saw much less activity
in newly_idle lb and therefore cpufreq_update_util on a system with the
same kernel and userspace but with less CPUs (i.e. also smaller
frequency domains) and less cgroups (with blocked load) and started
wondering why.

I assume that since he understands this environment now much better, he
should be able to come up with better test numbers to show if there is a
performance issue on his 2+6 DynamIQ system and if yes, where exactly in
this code path.