linux-kernel - Re: [PATCH] sched/uclamp: Avoid setting cpu.uclamp.min bigger than cpu.uclamp.max

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Date:   Sat, 5 Jun 2021 21:24:20 +0800
From:   Xuewen Yan <xuewen.yan94@...il.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Quentin Perret <qperret@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Benjamin Segall <bsegall@...gle.com>,
        Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Chunyan Zhang <zhang.lyra@...il.com>,
        Ryan Y <xuewyan@...mail.com>,
        Patrick Bellasi <patrick.bellasi@...bug.net>, tj@...nel.org
Subject: Re: [PATCH] sched/uclamp: Avoid setting cpu.uclamp.min bigger than cpu.uclamp.max

Hi Qais

On Sat, Jun 5, 2021 at 7:49 PM Qais Yousef <qais.yousef@....com> wrote:
> > >
> > In addition，In your patch:
> > 6938840392c89 ("sched/uclamp: Fix wrong implementation of cpu.uclamp.min")
> > https://lkml.kernel.org/r/20210510145032.1934078-2-qais.yousef@arm.com
> >
> > + switch (clamp_id) {
> > + case UCLAMP_MIN: {
> > + struct uclamp_se uc_min = task_group(p)->uclamp[clamp_id];
> > + if (uc_req.value < uc_min.value)
> > + return uc_min;
> > + break;
> >
> > When the clamp_id = UCLAMP_MIN, why not judge the uc_req.value is
> > bigger than task_group(p)->uclamp[UCLAMP_MAX] ?
>
> Because of the requirement I pointed you to in cgroup-v2.rst. We must allow any
> value to be requested.
>
> Ultimately if we had
>
>         cpu.uclamp.min = 80
>         cpu.uclamp.max = 50
>
> then we want to remember the original request but make sure the effective value
> is capped.
>
> For the user in the future modifies the values such that
>
>         cpu.uclamp.max = max
>
> Then we want to remember cpu.uclamp.min = 80 and apply it since now the
> cpu.uclamp.max was relaxed to allow the boost value.
>
> > Because when the p->uclamp_req[UCLAMP_MIN] >  task_group(p)->uclamp[UCLAMP_MAX],
> > the patch can not clamp the p->uclamp_req[UCLAMP_MIN/MAX] into
> > [ task_group(p)->uclamp[UCLAMP_MAX],  task_group(p)->uclamp[UCLAMP_MAX] ].
> >
> > Is it necessary to fix it here？
>
> Nope. We must allow any combination values to be accepted and remember them so
> if one changes we ensure the new effective value is updated accordingly.
> This is how cgroups API works.

Sorry. I may not have expressed it clearly. In your patch (which has
not yet merged into the mainline):

6938840392c89 ("sched/uclamp: Fix wrong implementation of cpu.uclamp.min")
 https://lkml.kernel.org/r/20210510145032.1934078-2-qais.yousef@arm.com

This patch will not affect p->uclamp_req, but consider the following situation:

tg->cpu.uclamp.min = 0
tg->cpu.uclamp.max = 50%

p->uclamp_req[UCLAMP_MIN] = 60%
p->uclamp_req[UCLAMP_MIN] = 80%

The function call process is as follows：
uclamp_eff_value() -> uclamp_eff_get() ->uclamp_tg_restrict()

with your patch, the result is:

p->effective_uclamp_min = 60%
p->effective_uclamp_max = 50%

It would not affect the uclamp_task_util(p), but affect the rq:
when p enqueued:
rq->uclamp[UCLAMP_MIN] = 60%
rq->uclamp[UCLAMP_MIN] = 50%

futher more,  in uclamp_rq_util_with() {
...

min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value); //60%
max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);//50%
...
if (unlikely(min_util >= max_util))
return min_util;

return clamp(util, min_util, max_util);
...
}
as a result, it would return 60%.

Thanks!
xuewen