linux-kernel - Re: [PATCH 1/1] sched/uclamp: release per-task uclamp control if user set to default value

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Message-ID: <20201007150013.GA219885@ubuntu>
Date:   Wed, 7 Oct 2020 23:00:13 +0800
From:   Yun Hsiang <hsiang023167@...il.com>
To:     Qais Yousef <qais.yousef@....com>
Cc:     Patrick Bellasi <patrick.bellasi@...bug.net>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        peterz@...radead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] sched/uclamp: release per-task uclamp control if
 user set to default value

On Mon, Oct 05, 2020 at 06:15:00PM +0100, Qais Yousef wrote:
> On 10/05/20 18:58, Patrick Bellasi wrote:
> 
> [...]
> 
> > >> it can not go back to the initial state to let the module(group) control.
> > >
> > > In case A changes its values e.g. from 3a to 3b it will go back to be
> > > controlled by /TG again (like it was when it had no user defined
> > > values).
> > 
> > True, however it's also true that strictly speaking once a task has
> > defined a per-task value, we will always aggregate/clamp that value wrt
> > to TG and SystemWide value.
> > 
> > >> But the other tasks in the group will be affected by the group.
> > 
> > This is not clear to me.
> > 
> > All tasks in a group will be treated independently. All the tasks are
> > subject to the same _individual_ aggregation/clamping policy.
> 
> I think the confusing bit is this check in uclamp_tg_restrict()
> 
> 1085         uc_max = task_group(p)->uclamp[clamp_id];
> 1086         if (uc_req.value > uc_max.value || !uc_req.user_defined)
> 1087                 return uc_max;
> 
> If a task is !user_defined then it'll *inherit* the TG value. So you can end
> up with 2 different behaviors based on that flag. I.e: if 2 tasks have their
> util_min=0, but one is user_defined while the other isn't, the effective
> uclamp value will look different for the 2 tasks.
> 
> IIUC, Yun wants to be able to reset this user_defined flag to re-enable this
> inheritance behavior for a task. Which I agree with you, seems a sensible thing
> to allow (via new sched_setattr() flag of course).
>

Yes, this is what I want. As Dietmar and Pavan said, use 0 and 1024 to
reset user_defined is problematic. I'll send a V2 patch that use
SCHED_FLAG_UTIL_CLAMP_RESET to reset the user_defined bit.
Thank for the suggestion!

> 
> Thanks
> 
> --
> Qais Yousef
>

Thanks,
Yun