linux-kernel - Re: [PATCH v5 03/15] sched/core: uclamp: map TASK's clamp values into CPU's clamp groups

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Date:   Wed, 7 Nov 2018 15:55:27 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Paul Turner <pjt@...gle.com>,
        Quentin Perret <quentin.perret@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Todd Kjos <tkjos@...gle.com>,
        Joel Fernandes <joelaf@...gle.com>,
        Steve Muckle <smuckle@...gle.com>,
        Suren Baghdasaryan <surenb@...gle.com>
Subject: Re: [PATCH v5 03/15] sched/core: uclamp: map TASK's clamp values
 into CPU's clamp groups

On Wed, Nov 07, 2018 at 02:48:09PM +0000, Patrick Bellasi wrote:
> On 07-Nov 14:35, Peter Zijlstra wrote:
> You mean se_count overflow ?

Yah..

> > And I'm not really a fan of hiding that error in a define like you keep
> > doing.
> 
> The #define is not there to mask an overflow, it's there to catch the

+#define UCLAMP_MAPERR "clamp value [%u] mapping to clamp group failed\n"

Is what I was talking about.

> > What's wrong with something like:
> > 
> > 	if (SCHED_WARN(free_group_id == UCLAMP_GROUPS))
> > 		return;
> 
> Right, the flow should be:
> 
>   1. try to find a valid clamp group
>   2. if you don't find one, the data structures are corrupted
>      warn once for data corruption
>      do not map this scheduling entity and return
>   3. map the scheduling entity
> 
> Is that ok ?

That's what the proposed does.

> > and
> > 
> > > +	uc_map_new.se_count = uc_map_old.se_count + 1;
> > 
> > 	if (SCHED_WARN(!new.se_count))
> > 		new.se_count = -1;
> 
> Mmm... not sure we can recover from a corrupted refcount or an
> overflow.
> 
> What should we do on these cases, disable uclamp completely ?

You can teach put to never decrement -1 (aka. all 1s).

But its all SCHED_DEBUG stuff anyway, so who really cares.