linux-kernel - Re: [PATCH v4 1/2] sched/uclamp: Fix initialization of struct uclamp

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Date:   Sat, 27 Jun 2020 00:17:20 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Patrick Bellasi <patrick.bellasi@...bug.net>
Cc:     Qais Yousef <qais.yousef@....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>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Chris Redpath <chris.redpath@....com>,
        Lukasz Luba <lukasz.luba@....com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v4 1/2] sched/uclamp: Fix initialization of struct uclamp_rq


On 26/06/20 13:32, Patrick Bellasi wrote:
> On Thu, Jun 25, 2020 at 17:43:51 +0200, Qais Yousef <qais.yousef@....com> wrote...
>
>> struct uclamp_rq was zeroed out entirely in assumption that in the first
>> call to uclamp_rq_inc() they'd be initialized correctly in accordance to
>> default settings.
>
> Perhaps I was not clear in my previous comment:
>
>    https://lore.kernel.org/lkml/87sgekorfq.derkling@matbug.net/
>
> when I did say:
>
>    Does not this means the problem is more likely with
>    uclamp_rq_util_with(), which should be guarded?
>
> I did not mean that we have to guard the calls to that function but
> instead that we should just make that function aware of uclamp being
> opted in or not.
>
>> But when next patch introduces a static key to skip
>> uclamp_rq_{inc,dec}() until userspace opts in to use uclamp, schedutil
>> will fail to perform any frequency changes because the
>> rq->uclamp[UCLAMP_MAX].value is zeroed at init and stays as such. Which
>> means all rqs are capped to 0 by default.
>
> The initialization you wants to do here it's needed because with the
> current approach you keep calling the same uclamp_rq_util_with() and
> keep doing min/max aggregations even when uclamp is not opted in.
> But this means also that we have min/max aggregation _when not really
> required_.
>
>> Fix it by making sure we do proper initialization at init without
>> relying on uclamp_rq_inc() doing it later.
>
> My proposal was as simple as:
>
> ---8<---
>   static __always_inline
>   unsigned long uclamp_rq_util_with(struct rq *rq, unsigned long util,
>                                 struct task_struct *p)
>   {
>       unsigned long min_util = READ_ONCE(rq->uclamp[UCLAMP_MIN].value);
>       unsigned long max_util = READ_ONCE(rq->uclamp[UCLAMP_MAX].value);
>
> +       if (!static_branch_unlikely(&sched_uclamp_used))
> +               return rt_task(p) ? uclamp_none(UCLAMP_MAX) : util
>
>       if (p) {
>               min_util = max(min_util, uclamp_eff_value(p, UCLAMP_MIN));
>               max_util = max(max_util, uclamp_eff_value(p, UCLAMP_MAX));
>       }
>
>       /*
>        * Since CPU's {min,max}_util clamps are MAX aggregated considering
>        * RUNNABLE tasks with _different_ clamps, we can end up with an
>        * inversion. Fix it now when the clamps are applied.
>        */
>       if (unlikely(min_util >= max_util))
>               return min_util;
>
>       return clamp(util, min_util, max_util);
>   }
> ---8<---
>
> Such small change is more self-contained IMHO and does not remove
> an existing optimizations like this lazy RQ's initialization at first
> usage.
>
> Moreover, it can folded in the following patch, with all the other
> static keys shortcuts.

I'd have to think some more over it, but I like this in that we wouldn't
have to molest schedutil anymore.