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

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Message-ID: <20180815105428.GA7388@e110439-lin>
Date:   Wed, 15 Aug 2018 11:54:28 +0100
From:   Patrick Bellasi <patrick.bellasi@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     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 15-Aug 11:37, Dietmar Eggemann wrote:
> On 08/14/2018 06:49 PM, Patrick Bellasi wrote:
> >Hi Dietmar!
> >
> >On 14-Aug 17:44, Dietmar Eggemann wrote:
> >>On 08/06/2018 06:39 PM, Patrick Bellasi wrote:
> 
> [...]
> 
> >>This one indicates that there are some holes in your ref-counting.
> >
> >Not really, this has been added not because I've detected a refcount
> >issue... but because it was suggested as a possible safety check in a
> >previous code review comment:
> >
> >    https://lore.kernel.org/lkml/20180720151156.GA31421@e110439-lin/
> >
> >>It's probably easier to debug that there is still a task but the
> >>uc_grp[group_id].tasks value == 0 (A). I assume the other problem exists as
> >>well, i.e. last task and uc_grp[group_id].tasks > 1 (B)?
> >>
> >>You have uclamp_cpu_[get/put](_id)() in [enqueue/dequeue]_task.
> >>
> >>Patch 04/14 introduces its use in uclamp_task_update_active().
> >>
> >>Do you know why (A) (and (B)) are happening?
> >
> >I've never saw that warning in my tests so far so, again, the warning
> >is there just to support testing/debugging when refcounting code
> >is/will be touched in the future. That's also the reason why is
> >SCHED_DEBUG protected.
> 
> Ah, OK, I thought you really see it more often and that it also relate to
> Pavan's comment on 02/14 about the missing treatment of exiting tasks.
> 
> If this is only for testing/debugging, I would suggest a simple one line
> BUG_ON()

These are (eventually) considered as recoverable errors... thus,
AFAIK, using BUG_ON is overkilling and discouraged:
  https://elixir.bootlin.com/linux/latest/source/include/asm-generic/bug.h#L42

> You find CONFIG_SCHED_DEBUG=y in production kernels as well.

AFAIK, that setting is discouraged for production kernels...
Moreover, it's still better to WARN sometimes on a production kernel
the crash the device, isnt't it?

-- 
#include <best/regards.h>

Patrick Bellasi