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

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Message-ID: <CAJZ5v0jBB8-q5DOakUoMrPrjBSttZgDuNBEivR5WfdWR4s1mKA@mail.gmail.com>
Date:	Wed, 10 Feb 2016 22:49:24 +0100
From:	"Rafael J. Wysocki" <rafael@...nel.org>
To:	Steve Muckle <steve.muckle@...aro.org>
Cc:	"Rafael J. Wysocki" <rafael@...nel.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 8:47 PM, Steve Muckle <steve.muckle@...aro.org> wrote:
> On 02/09/2016 07:09 PM, Rafael J. Wysocki wrote:
>>>> >> 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()?
>
> That should AFAICS be sufficient to avoid stalling. It may be more than
> is required as that covers more than just enqueue/dequeue but I'm not
> sure offhand.
>
>>
>> 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.
>
> Is it rq_clock_task() or rq_clock()? The former can omit irq time so may
> gradually fall behind wall clock time, delaying callbacks in cpufreq.

What matters to us is the difference between the current time and the
time we previously took a sample and there shouldn't be too much
difference between the two in that respect.

Both are good enough IMO, but I can update the patch to use rq_clock()
if that's preferred.

Thanks,
Rafael