linux-kernel - Re: [PATCH 5/6] cpufreq: governor: replace per-cpu delayed work with timers

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Date:	Tue, 3 Nov 2015 14:01:57 -0500
From:	Ashwin Chaugule <ashwin.chaugule@...aro.org>
To:	Viresh Kumar <viresh.kumar@...aro.org>
Cc:	Rafael Wysocki <rjw@...ysocki.net>,
	Linaro Kernel Mailman List <linaro-kernel@...ts.linaro.org>,
	"linux-pm@...r.kernel.org" <linux-pm@...r.kernel.org>,
	"Rafael J. Wysocki" <rafael.j.wysocki@...el.com>,
	open list <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] cpufreq: governor: replace per-cpu delayed work with timers

Hi Viresh,

On 30 October 2015 at 22:36, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> Hi Ashwin,
>
> On 30-10-15, 16:46, Ashwin Chaugule wrote:
>> On 29 October 2015 at 08:27, Viresh Kumar <viresh.kumar@...aro.org> wrote:
>> > This could be made lightweight by keeping per-cpu deferred timers with a
>> > single work item, which is scheduled by the first timer that expires.
>>
>> Single shared work item - would perhaps make it a bit more clear.
>
> Okay, in case that I need to repost this, I will reword it.

Thanks.

>
>> > +void gov_cancel_work(struct cpu_common_dbs_info *shared)
>> > +{
>> > +       unsigned long flags;
>> > +
>> > +       /*
>> > +        * No work will be queued from timer handlers after skip_work is
>> > +        * updated. And so we can safely cancel the work first and then the
>> > +        * timers.
>> > +        */
>> > +       spin_lock_irqsave(&shared->timer_lock, flags);
>> > +       shared->skip_work++;
>> > +       spin_unlock_irqrestore(&shared->timer_lock, flags);
>> > +
>> > +       cancel_work_sync(&shared->work);
>> > +
>> > +       gov_cancel_timers(shared->policy);
>> > +
>> > +       shared->skip_work = 0;
>>
>> Why doesnt this require the spin_lock protection?
>
> Because there is no race here. We have already removed all
> queued-timers and the shared work.

I see.

>
>> > -static void dbs_timer(struct work_struct *work)
>> > +static void dbs_work_handler(struct work_struct *work)
>> >  {
>
>> > +       mutex_lock(&shared->timer_mutex);
>> > +       delay = dbs_data->cdata->gov_dbs_timer(policy, eval_load);
>> > +       mutex_unlock(&shared->timer_mutex);
>> > +
>> > +       shared->skip_work--;
>>
>> Ditto.
>
> Again, there is no race here. We have already removed the
> queued-timers for the entire policy.

Makes sense.

> The only other user is the
> gov_cancel_work() thread (which is called while stopping the governor
> or updating the sampling rate), which doesn't depend on this being
> decremented as that will wait for the work to finish.
>
>> > +       gov_add_timers(policy, delay);
>> > +}
>> > +
>> > +static void dbs_timer_handler(unsigned long data)
>> > +{
>> > +       struct cpu_dbs_info *cdbs = (struct cpu_dbs_info *)data;
>> > +       struct cpu_common_dbs_info *shared = cdbs->shared;
>> > +       struct cpufreq_policy *policy;
>> > +       unsigned long flags;
>> > +
>> > +       spin_lock_irqsave(&shared->timer_lock, flags);
>> > +       policy = shared->policy;
>> > +
>> > +       /*
>> > +        * Timer handler isn't allowed to queue work at the moment, because:
>> > +        * - Another timer handler has done that
>> > +        * - We are stopping the governor
>> > +        * - Or we are updating the sampling rate of ondemand governor
>> > +        */
>> > +       if (shared->skip_work)
>> > +               goto unlock;
>> > +
>> > +       shared->skip_work++;
>> > +       queue_work(system_wq, &shared->work);
>> >
>>
>> So, IIUC, in the event that this function gets called back to back and
>> the first Work hasn't dequeued yet, then this queue_work() will not
>> really enqueue, since queue_work_on() will return False?
>
> In that case we wouldn't reach queue_work() in the first place as
> skip_work will be incremented on the first call and the second call
> will simply return early.
>
>> If so, then
>> does it mean we're skipping more recent CPU freq requests? Should we
>> cancel past Work if it hasn't been serviced?
>
> It doesn't matter. Its only the work handler that is going to do some
> useful work, and there is no difference in the first or the second
> request.

Yea, thanks for clarifying. I think I missed the del_timer_sync()
which had raised my doubts.

Reviewed-by: Ashwin Chaugule <ashwin.chaugule@...aro.org>

Regards,
Ashwin.
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