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

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Date:	Wed, 10 Feb 2016 04:09:33 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	"Rafael J. Wysocki" <rafael@...nel.org>
Cc:	Steve Muckle <steve.muckle@...aro.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	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 2:57 AM, Rafael J. Wysocki <rafael@...nel.org> wrote:
> 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()?

If done this way, I guess we may pass rq_clock_task(rq) as the time
arg to cpufreq_update_util() from there and then the cpu_lock() call
I've added to this prototype won't be necessary any more.

Thanks,
Rafael