linux-kernel - Re: [PATCH v3 1/2] sched/fair: Introduce short duration task check

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Date:   Tue, 13 Dec 2022 20:24:27 +0800
From:   Chen Yu <yu.c.chen@...el.com>
To:     Honglei Wang <wanghonglei@...ichuxing.com>
CC:     Josh Don <joshdon@...gle.com>, Yicong Yang <yangyicong@...wei.com>,
        <yangyicong@...ilicon.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Joel Fernandes <joel@...lfernandes.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...el.com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Juri Lelli <juri.lelli@...hat.com>,
        "Rik van Riel" <riel@...riel.com>, Aaron Lu <aaron.lu@...el.com>,
        Abel Wu <wuyun.abel@...edance.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        "Steven Rostedt" <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>,
        "Daniel Bristot de Oliveira" <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Hillf Danton <hdanton@...a.com>,
        Len Brown <len.brown@...el.com>,
        Chen Yu <yu.chen.surf@...il.com>,
        Tianchen Ding <dtcccc@...ux.alibaba.com>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v3 1/2] sched/fair: Introduce short duration task check

On 2022-12-13 at 18:06:50 +0800, Honglei Wang wrote:
> 
> 
> On 2022/12/13 13:46, Chen Yu wrote:
> > On 2022-12-12 at 10:17:35 -0800, Josh Don wrote:
> > > > BTW, I've changed the threshold to (sysctl_sched_min_granularity / 8) in my next
> > > > version, as this is the value that fit my previous test case and also not to break
> > > > the case Josh mentioned.
> > > 
> > > Do you mean a hardcoded value of some number of micros, or literally
> > > sched_min_granularity / 8?
> > The latter. According to the test, the average task duration when system
> > is under heavy load:
> > 6 ~ 9 us for netperf
> > 7 ~ 70 us for hackbench
> > 7 ~ 8 us for tbench
> > 13 ~ 20 ms for schbench
> > Overall the duration of the micros are quite small(except for schbench).
> > The default sysctl_sched_min_granularity is 750 us in kernel if no user
> > has changed it. Then 750 / 8 = 93 us, which is close to what you suggested(100us).
> > On the other hand, if someone changes sysctl_sched_min_granularity,
> > then '8' can be viewed as log2(256). That is, if there are 256 CPUs online,
> > and the sysctl_sched_min_granularity is changed to 750 us * log2(256) by
> > the user, we can devide the sysctl_sched_min_granularity by 8 in case the
> > sysctl_sched_min_granularity is too large.
> > 
> 
> Hi Yu,
> 
> Seems there is a min_t() call in get_update_sysctl_factor(). In most cases,
> we'll get 750 us * (1+log2(8)) = 3000 us in default due to
> sysctl_sched_tunable_scaling is set as '1' default. (Correct me if I
> misunderstand).
>
Thanks for pointing this out! I overlooked this part previously.
So (sysctl_sched_min_granularity / 8) is 375 us by default.
> For the value in production environment, I've seen 10 ms and 3 ms in
> different place, FYI. Hope this help.
I see. 10 ms would generate a short duration threshold of 1.25 ms. From
my understanding if the user increases the sysctl_sched_min_granularity
then it is expected to have longer average duration tasks in the system.
And the short duration threshold should also be raised.

thanks,
Chenyu