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

[Rafael J. Wysocki]

On Wed, Feb 10, 2016 at 8:47 PM, Steve Muckle <steve.muckle@xxxxxxxxxx> 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