Re: [PATCH v6] cpufreq: schedutil: Make iowait boost more energy efficient

[Joel Fernandes]

On Sat, Jul 22, 2017 at 2:44 PM, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote:
> On Saturday, July 22, 2017 12:47:53 AM Joel Fernandes wrote:
>> Currently the iowait_boost feature in schedutil makes the frequency go to max
>> on iowait wakeups.  This feature was added to handle a case that Peter
>> described where the throughput of operations involving continuous I/O requests
>> [1] is reduced due to running at a lower frequency, however the lower
>> throughput itself causes utilization to be low and hence causing frequency to
>> be low hence its "stuck".
>>
>> Instead of going to max, its also possible to achieve the same effect by
>> ramping up to max if there are repeated in_iowait wakeups happening. This patch
>> is an attempt to do that. We start from a lower frequency (policy->min)
>> and double the boost for every consecutive iowait update until we reach the
>> maximum iowait boost frequency (iowait_boost_max).
>>
>> I ran a synthetic test (continuous O_DIRECT writes in a loop) on an x86 machine
>> with intel_pstate in passive mode using schedutil. In this test the iowait_boost
>> value ramped from 800MHz to 4GHz in 60ms. The patch achieves the desired improved
>> throughput as the existing behavior.
>>
>> Also while at it, make iowait_boost and iowait_boost_max as unsigned int since
>> its unit is kHz and this is consistent with struct cpufreq_policy.
>>
>> [1] https://patchwork.kernel.org/patch/9735885/
>>
>> Cc: Srinivas Pandruvada <srinivas.pandruvada@xxxxxxxxxxxxxxx>
>> Cc: Len Brown <lenb@xxxxxxxxxx>
>> Cc: Rafael J. Wysocki <rjw@xxxxxxxxxxxxx>
>> Cc: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
>> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
>> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
>> Suggested-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
>> Signed-off-by: Joel Fernandes <joelaf@xxxxxxxxxx>
>> ---
>> Viresh, made slight modifications to the last approach we agreed on using, but
>> nothing we didn't already discuss. I also dropped the RFC tag since I think
>> this is increasingly now becoming final (or has become final if no one else has
>> any other objection).
>>
>>  kernel/sched/cpufreq_schedutil.c | 37 +++++++++++++++++++++++++++++++------
>>  1 file changed, 31 insertions(+), 6 deletions(-)
>>
>> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
>> index 622eed1b7658..0c0b6c8c15fc 100644
>> --- a/kernel/sched/cpufreq_schedutil.c
>> +++ b/kernel/sched/cpufreq_schedutil.c
>> @@ -53,6 +53,7 @@ struct sugov_cpu {
>>       struct update_util_data update_util;
>>       struct sugov_policy *sg_policy;
>>
>> +     bool iowait_boost_pending;
>>       unsigned long iowait_boost;
>>       unsigned long iowait_boost_max;
>>       u64 last_update;
>> @@ -172,30 +173,53 @@ static void sugov_set_iowait_boost(struct sugov_cpu *sg_cpu, u64 time,
>>                                  unsigned int flags)
>>  {
>>       if (flags & SCHED_CPUFREQ_IOWAIT) {
>> -             sg_cpu->iowait_boost = sg_cpu->iowait_boost_max;
>> +             if (sg_cpu->iowait_boost_pending)
>> +                     return;
>> +
>> +             sg_cpu->iowait_boost_pending = true;
>> +
>> +             if (sg_cpu->iowait_boost) {
>> +                     sg_cpu->iowait_boost = min(sg_cpu->iowait_boost << 1,
>> +                                                sg_cpu->iowait_boost_max);
>
> I would do
>
>                         sg_cpu->iowait_boost <<= 1;
>                         if (sg_cpu->iowait_boost > sg_cpu->iowait_boost_max)
>                                 sg_cpu->iowait_boost = sg_cpu->iowait_boost_max;
>
> as that's easeir to read.
>
> The rest of the patch is fine by me.

Done, and resent patches. Also added one more to change the
iowait_boost and iowait_boost_max to unsigned it.

thanks,

-Joel