linux-kernel - Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks

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Date:	Wed, 10 Feb 2016 02:57:18 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	"Rafael J. Wysocki" <rafael@...nel.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Linux PM list <linux-pm@...r.kernel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Juri Lelli <juri.lelli@....com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 0/3] cpufreq: Replace timers with utilization update callbacks

On Wed, Feb 10, 2016 at 2:02 AM, Steve Muckle <steve.muckle@...aro.org> wrote:
> On 02/09/2016 12:05 PM, Rafael J. Wysocki wrote:
>>>> One concern I had was, given that the lone scheduler update hook is in
>>>> CFS, is it possible for governor updates to be stalled due to RT or DL
>>>> task activity?
>>>
>>> I don't think they may be completely stalled, but I'd prefer Peter to
>>> answer that as he suggested to do it this way.
>>
>> In any case, if that concern turns out to be significant in practice, it may
>> be addressed like in the appended modification of patch [1/3] from the $subject
>> series.
>>
>> With that things look like before from the cpufreq side, but the other sched
>> classes also get a chance to trigger a cpufreq update.  The drawback is the
>> cpu_clock() call instead of passing the time value from update_load_avg(), but
>> I guess we can live with that if necessary.
>>
>> FWIW, this modification doesn't seem to break things on my test machine.
>>
> ...
>> Index: linux-pm/kernel/sched/rt.c
>> ===================================================================
>> --- linux-pm.orig/kernel/sched/rt.c
>> +++ linux-pm/kernel/sched/rt.c
>> @@ -2212,6 +2212,9 @@ static void task_tick_rt(struct rq *rq,
>>
>>       update_curr_rt(rq);
>>
>> +     /* Kick cpufreq to prevent it from stalling. */
>> +     cpufreq_kick();
>> +
>>       watchdog(rq, p);
>>
>>       /*
>> Index: linux-pm/kernel/sched/deadline.c
>> ===================================================================
>> --- linux-pm.orig/kernel/sched/deadline.c
>> +++ linux-pm/kernel/sched/deadline.c
>> @@ -1197,6 +1197,9 @@ static void task_tick_dl(struct rq *rq,
>>  {
>>       update_curr_dl(rq);
>>
>> +     /* Kick cpufreq to prevent it from stalling. */
>> +     cpufreq_kick();
>> +
>>       /*
>>        * Even when we have runtime, update_curr_dl() might have resulted in us
>>        * not being the leftmost task anymore. In that case NEED_RESCHED will
>
> I think additional hooks such as enqueue/dequeue would be needed in
> RT/DL. The task tick callbacks will only run if a task in that class is
> executing at the time of the tick. There could be intermittent RT/DL
> task activity in a frequency domain (the only task activity there, no
> CFS tasks) that doesn't happen to overlap the tick. Worst case the task
> activity could be periodic in such a way that it never overlaps the tick
> and the update is never made.

So if I'm reading this correctly, it would be better to put the hooks
into update_curr_rt/dl()?

Thanks,
Rafael