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 10:12:29 +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 12:08 AM Qais Yousef <qais.yousef@....com> 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.
>

Thanks a lot for your patience to explain, sorry for my ignorance of
Documentation/admin-guide/cgroup-v2.rst.

> >
> > It does not affect the 'effective' value. That because there is
> > protection in cpu_util_update_eff():
> > /* Ensure protection is always capped by limit */
> > eff[UCLAMP_MIN] = min(eff[UCLAMP_MIN], eff[UCLAMP_MAX]);
> >
> > When users set the cpu.uclamp.min > cpu.uclamp.max:
> > cpu.uclamp.max = 50;
> > to set : cpu.uclamp.min = 60;
> > That would make the uclamp_req[UCLAMP_MIN].value = 1024* 60% = 614,
> > uclamp_req[UCLAMP_MAX].value = 1024* 50% = 512;
> > But finally, the  uclamp[UCLAMP_MIN].value = uclamp[UCLAMP_MAX].value
> > = 1024* 50% = 512;
> >
> > Is it deliberately set not to validate because of the above?
>
> Sorry I'm not following you here. What code paths were you trying to explain
> here?
>
> Did you actually hit any problem here?

I just gave an example of the difference of uclamp_req and uclamp
without my patch, and can ignore it.

>
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 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？

Thanks
xuewen