linux-kernel - Re: [PATCH v3 03/14] sched/core: uclamp: add CPU's clamp groups accounting

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Date:   Thu, 16 Aug 2018 15:21:17 +0100
From:   Quentin Perret <quentin.perret@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     Patrick Bellasi <patrick.bellasi@....com>,
        linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tejun Heo <tj@...nel.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Paul Turner <pjt@...gle.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 v3 03/14] sched/core: uclamp: add CPU's clamp groups
 accounting

On Thursday 16 Aug 2018 at 15:45:45 (+0200), Dietmar Eggemann wrote:
> On 08/16/2018 03:37 PM, Quentin Perret wrote:
> > > > IMHO, if this is something which should not happen at all, a BUG_ON() is the
> > > > right thing to do here.
> > > 
> > > I don't agree on that. I agree it should not happen but since it's a
> > > recoverable error it think we should not panic.
> > 
> > FWIW, if this is a recoverable error, I think Linus will agree with
> > Patrick on this one :-)
> > 
> > https://lkml.org/lkml/2016/10/4/1
> 
> Yeah, not really agreeing here that this is a recoverable error.

A non-recoverable scenario could be, for example, if you corrupt your
stack and there is absolutely _nothing_ you can do to keep the system up
and running, because it's just too broken. I don't feel like we're
talking about such an extreme case here ...

> Besides, we
> only consider under-run here, what about over-run?
> 
> Currently this warning doesn't hit and if the code will be changed and it
> hits, I still find a BUG_ON more appealing here ...
> 
> So this error scenario can happen over and over again and we always recover
> from ? The important thing is that we find the culprit for this behaviour as
> fast as possible ...

Agreed, we want to debug that ASAP, but WARN should let us do that just
fine, I think.

Quentin