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

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Message-ID: <7c45c1a8-24cb-6798-5b6f-3b5dfc9b490d@arm.com>
Date:   Wed, 15 Aug 2018 11:37:13 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Patrick Bellasi <patrick.bellasi@....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 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()

You find CONFIG_SCHED_DEBUG=y in production kernels as well.

[...]

> Here you right, I would say that it should always be:
> 
>      clamp_value <= uc_cpu->value[clamp_id]
> 
> since this matches the update done at the end of uclamp_cpu_get_id():
> 
>     if (uc_cpu->value[clamp_id] < clamp_value)
>          uc_cpu->value[clamp_id] = clamp_value;
> 
> Perhaps we can add another safety check here, similar to the one
> above, to have something like:
> 
>      clamp_value = uc_grp[group_id].value;
> #ifdef SCHED_DEBUG
>      if (unlikely(clamp_value > uc_cpu->value[clamp_id])) {
>          WARN(1, "invalid CPU[%d] clamp group [%d:%d] value\n",
>                  cpu_of(rq), clamp_id, group_id);
> #endif
>      if (clamp_value == uc_cpu->value[clamp_id])
>          uclamp_cpu_update(rq, clamp_id);

Yes, but I would prefer a BUG_ON() one liner.