Message-ID: <CAJWu+oqD8gW0JuAR_5ewc=BxTnxT-e3fsi7uzifRxSFVtr2PDg@mail.gmail.com>
Date:   Sat, 10 Jun 2017 23:59:35 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Linux PM <linux-pm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
        Len Brown <lenb@...nel.org>,
        "Rafael J . Wysocki" <rjw@...ysocki.net>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <Juri.Lelli@....com>,
        Patrick Bellasi <patrick.bellasi@....com>
Subject: Re: [PATCH v2 1/2] cpufreq: Make iowait boost a policy option

Hi Peter,

On Sat, Jun 10, 2017 at 6:56 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> On Sat, Jun 10, 2017 at 01:08:18AM -0700, Joel Fernandes wrote:
>
>> Adding Juri and Patrick as well to share any thoughts. Replied to
>> Peter in the end of this email.
>
> Oh sorry, I completely missed your earlier reply :-(

No problem. I appreciate you taking time to reply, thanks.

>> >>> Are you trying to boost the CPU frequency so that a process waiting on
>> >>> I/O does its next set of processing quickly enough after iowaiting on
>> >>> the previous I/O transaction, and is ready to feed I/O the next time
>> >>> sooner?
>> >>
>> >> This. So we break the above pattern by boosting the task that wakes from
>> >> IO-wait. Its utilization will never be enough to cause a significant
>> >> bump in frequency on its own, as its constantly blocked on the IO
>> >> device.
>> >
>> > It sounds like this problem can happen with any other use-case where
>> > one task blocks on the other, not just IO. Like a case where 2 tasks
>> > running on different CPUs block on a mutex, then on either task can
>> > wait on the other causing their utilization to be low right?
>
> No, with two tasks bouncing on a mutex this does not happen. For both
> tasks are visible and consume time on the CPU. So, if for example, a
> task A blocks on a task B, then B will still be running, and cpufreq
> will still see B and provide it sufficient resource to keep running.
> That is, if B is cpu bound, and we recognise it as such, it will get
> full CPU.
>
> The difference with the IO is that the IO device is completely
> invisible. This makes sense in that cpufreq cannot affect the devices
> performance, but it does lead to the above issue.

But if task A and B are on different CPUs due to CPU affinity and
these CPUs are on different frequency domains and bouncing on a mutex,
then you would run into the same problem right?

>> >>> The case I'm seeing a lot is a background thread does I/O request and
>> >>> blocks for short period, and wakes up. All this while the CPU
>> >>> frequency is low, but that wake up causes a spike in frequency. So
>> >>> over a period of time, you see these spikes that don't really help
>> >>> anything.
>> >>
>> >> So the background thread is doing some spurious IO but nothing
>> >> consistent?
>> >
>> > Yes, its not a consistent pattern. Its actually a 'kworker' that woke
>> > up to read/write something related to the video being played by the
>> > YouTube app and is asynchronous to the app itself. It could be writing
>> > to the logs or other information. But this definitely not a consistent
>> > pattern as in the use case you described but intermittent spikes. The
>> > frequency boosts don't help the actual activity of playing the video
>> > except increasing power.
>
> Right; so one thing we can try is to ramp-up the boost. Because
> currently its a bit of an asymmetric thing in that we'll instantly boost
> to max and then slowly back off again.
>
> If instead we need to 'earn' full boost by repeatedly blocking on IO
> this might sufficiently damp your spikes.

Cool, that sounds like a great idea.

>> >> Also note that if you set the boost OPP to the lowest OPP you
>> >> effectively do disable it.
>> >>
>> >> Looking at the code, it appears we already have this in
>> >> iowait_boost_max.
>> >
>> > Currently it is set to:
>> >  sg_cpu->iowait_boost_max = policy->cpuinfo.max_freq
>> >
>> > Are you proposing to make this a sysfs tunable so we can override what
>> > the iowait_boost_max value is?
>
> Not sysfs, but maybe cpufreq driver / platform. For example have it be
> the OPP that provides the max Instructions per Watt.
>
>> Peter I didn't hear back from you. Maybe my comment here did not make
>> much sense to you?
>
> Again sorry; I completely missed it :/

No problem, thank you for replying. :)

>> That could be because I was confused what you meant by
>> iowait_boost_max setting to 0. Currently afaik there isn't an upstream
>> way of doing this. Were you suggesting making iowait_boost_max as a
>> tunable and setting it to 0?
>
> Tunable as in exposed to the driver, not userspace.

Got it.

> But I'm hoping an efficient OPP and the ramp-up together would be enough
> for your case and also still work for our desktop/server loads.

Ok. I am trying repro this with a synthetic test and measure
throughput so that I have a predictable usecase.

I was also thinking of another approach where when a p->in_iowait task
wakes up, we don't decay its util_avg. Then we calculate the total
time it was blocked due to I/O and then use that to correct the error
in the rq's util_avg (since the task's contribution to the rq util_avg
could have decayed while it iowait-ing). This will in a sense boost
the util_avg. Do you think that's a workable approach? That way if the
task is waiting very briefly, then the error to correct would be small
and we wouldn't just end up ramping to max frequency.
I think the other way to do it could be to not decay the rq's util_avg
while a task is waiting on I/O (maybe by checking rq->nr_iowait?).

Thanks,
Joel

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