Date:   Sat, 10 Jun 2017 01:08:18 -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

Adding Juri and Patrick as well to share any thoughts. Replied to
Peter in the end of this email.

On Wed, May 24, 2017 at 1:17 PM, Joel Fernandes <joelaf@...gle.com> wrote:
> Hi Peter,
>
> On Mon, May 22, 2017 at 1:21 AM, Peter Zijlstra <peterz@...radead.org> wrote:
> [..]
>>> On Fri, May 19, 2017 at 2:42 AM, Peter Zijlstra <peterz@...radead.org> wrote:
>>> > On Thu, May 18, 2017 at 11:23:43PM -0700, Joel Fernandes wrote:
>>> >> Make iowait boost a cpufreq policy option and enable it for intel_pstate
>>> >> cpufreq driver. Governors like schedutil can use it to determine if
>>> >> boosting for tasks that wake up with p->in_iowait set is needed.
>>> >
>>> > Rather than just flat out disabling the option, is there something
>>> > better we can do on ARM?
>>> >
>>> > The reason for the IO-wait boost is to ensure we feed our external
>>> > devices data ASAP, this reduces wait times, increases throughput and
>>> > decreases the duration the devices have to operate.
>>>
>>> Can you help understand how CPU frequency can affect I/O? The ASAP
>>> makes me think of it as a latency thing than a throughput in which
>>> case there should a scheduling priority increase? Also, to me it
>>> sounds more like memory instead of CPU frequency should be boosted
>>> instead so that DMA transfers happen quicker to feed devices data
>>> faster.
>>
>> Suppose your (I/O) device has the task waiting for a completion for 1ms
>> for each request. Further suppose that feeding it the next request takes
>> .1ms at full speed (1 GHz).
>>
>> Then we get, without contending tasks, a cycle of:
>>
>>
>>  R----------R----------                                 (1 GHz)
>>
>>
>> Which comes at 1/11-th utilization, which would then select something
>> like 100 MHz as being sufficient. But then the R part becomes 10x longer
>> and we end up with:
>>
>>
>>  RRRRRRRRRR----------RRRRRRRRRR----------               (100 MHz)
>>
>>
>> And since there's still plenty idle time, and the effective utilization
>> is still the same 1/11th, we'll not ramp up at all and continue in this
>> cycle.
>>
>> Note however that the total time of the cycle increased from 1.1ms
>> to 2ms, for an ~80% decrease in throughput.
>
> Got it, thanks for the explanation.
>
>>> 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?
>
>>> 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.
>
>>> > I realize max freq/volt might not be the best option for you, but is
>>> > there another spot that would make sense? I can imagine you want to
>>> > return your MMC to low power state ASAP as well.
>>> >
>>> >
>>> > So rather than a disable flag, I would really rather see an IO-wait OPP
>>> > state selector or something.
>>>
>>> We never had this in older kernels and I don't think we ever had an
>>> issue where I/O was slow because of CPU frequency. If a task is busy a
>>> lot, then its load tracking signal should be high and take care of
>>> keeping CPU frequency high right?
>>
>> As per the above, no. If the device completion takes long enough to
>> inject enough idle time, the utilization signal will never be high
>> enough to break out of that pattern.
>>
>>> If PELT is decaying the load
>>> tracking of iowaiting tasks too much, then I think that it should be
>>> fixed there (probably decay an iowaiting task lesser?).
>>
>> For the above to work, we'd have to completely discard IO-wait time on
>> the utilization signal. But that would then give the task u=1, which
>> would be incorrect for placement decisions and wreck EAS.
>
> Not completely discard but cap the decay of the signal during IO wait.
>
>>
>>> Considering
>>> that it makes power worse on newer kernels, it'd probably be best to
>>> disable it in my opinion for those who don't need it.
>>
>> You have yet to convince me you don't need it. Sure Android might not
>> have much IO heavy workloads, but that's not to say nothing on ARM ever
>> does.
>>
>> 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?
>

Peter I didn't hear back from you. Maybe my comment here did not make
much sense to you? 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?

Or did you mean to have us carry an out of tree patch that sets it to
0? One of the reason I am pushing this patch is to not have to carry
an out of tree patch that disables it. Looking forward to your reply.

Thanks a lot,
Joel

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