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 10:14:04 +0800
From:   Xuewen Yan <xuewen.yan94@...il.com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Qais Yousef <qais.yousef@....com>,
        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>,
        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

On Sat, Jun 5, 2021 at 12:22 AM Dietmar Eggemann
<dietmar.eggemann@....com> wrote:
>
> On 04/06/2021 18:08, Qais Yousef wrote:
> > On 06/03/21 10:24, Xuewen Yan wrote:
> >> +CC Qais
> >
> > Thanks for the CC :)
> >
> >>
> >>
> >> Hi Quentin
> >>
> >> On Wed, Jun 2, 2021 at 9:22 PM Quentin Perret <qperret@...gle.com> wrote:
> >>>
> >>> +CC Patrick and Tejun
> >>>
> >>> On Wednesday 02 Jun 2021 at 20:38:03 (+0800), Xuewen Yan wrote:
> >>>> From: Xuewen Yan <xuewen.yan@...soc.com>
> >>>>
> >>>> When setting cpu.uclamp.min/max in cgroup, there is no validating
> >>>> like uclamp_validate() in __sched_setscheduler(). It may cause the
> >>>> cpu.uclamp.min is bigger than cpu.uclamp.max.
> >>>
> >>> ISTR this was intentional. We also allow child groups to ask for
> >>> whatever clamps they want, but that is always limited by the parent, and
> >>> reflected in the 'effective' values, as per the cgroup delegation model.
> >
> > As Quentin said. This intentional to comply with cgroup model.
> >
> > See Limits and Protections sections in Documentation/admin-guide/cgroup-v2.rst
> >
> > Specifically
> >
> >       "all configuration combinations are valid"
> >
> > So user can set cpu.uclamp.min higher than cpu.uclamp.max. But when we apply
> > the setting, cpu.uclamp.min will be capped by cpu.uclamp.max. I can see you
> > found the cpu_util_update_eff() logic.
>
> To support this:
>
> Patrick had appropriate checks in his `[PATCH v10 12/16] sched/core:
> uclamp: Extend CPU's cgroup controller`.
>
> https://lkml.kernel.org/r/20190621084217.8167-13-patrick.bellasi@arm.com
>
> But is was discussed that cgroup v2 `resource distribution model`
> configurations (here protection/limit: uclamp_min/uclamp_max) should not
> be restricted.
>
> Further down in this thread:
>
> "... Limits always trump protection in effect of course but please don't
> limit what can be configured..."

Okay, I have got it. Thanks a lot!

BR
xuewen.yan