Re: [PATCH RFC v4] cpufreq: schedutil: Make iowait boost more energy efficient

[Joel Fernandes]

On Tue, Jul 11, 2017 at 7:14 AM, Joel Fernandes <joelaf@xxxxxxxxxx> wrote:
[..]
>>> +             }
>>>       } else if (sg_cpu->iowait_boost) {
>>>               s64 delta_ns = time - sg_cpu->last_update;
>>>
>>>               /* Clear iowait_boost if the CPU apprears to have been idle. */
>>>               if (delta_ns > TICK_NSEC)
>>>                       sg_cpu->iowait_boost = 0;
>>> +
>>> +             /*
>>> +              * Since we don't decay iowait_boost when its consumed during
>>> +              * the previous SCHED_CPUFREQ_IOWAIT update, decay it now.
>>> +              */
>>> +             if (sg_cpu->prev_iowait_boost) {
>>
>> SCHED_CPUFREQ_IOWAIT flag is set only by CFS from the enqueue_task() and in many
>> cases we call the util hook twice from the same enqueue_task() instance before
>> returning (2nd one after updating util). And in such cases we will set
>> iowait_boost as 0 on the second call.
>>
>> Have you ever seen two consecutive calls to sugov_set_iowait_boost() with IOWAIT
>> flag set ? Can we get the ratio of that against the other case where we have
>> IOWAIT flag set in first call, followed by one or more non-IOWAIT calls and then
>> IOWAIT again ?
>>
>> I am asking because if the calls with IOWAIT flag aren't consecutive then we
>> will make iowait_boost as 0 in the next non-IOWAIT call.
>
> Yes, I've seen that happen in my testing (consecutive iowait). I
> haven't seen the other case where you have IOWAIT followed by
> non-IOWAIT for a repeated set of IOWAIT requests. Would you more
> comfortable if we moved sugov_set_iowait_boost() after the
> sugov_should_update_freq() ? That way if there are consecutive
> requests in the same path, then it most likely rate-limiting will
> prevent such updates. I will also try to collect some stats as you
> suggested to see if how often if at all this can happen.

Just to be more clear, I was saying that I've only seen repeated
IOWAIT requests in the update path, not IOWAIT followed by non-IOWAIT
cpufreq updates for a repeated sequence of IOWAIT wakeups.

thanks,

-Joel