linux-kernel - Re: [PATCH v2 2/2] sched: Make iowait

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Date:   Fri, 19 May 2017 09:10:20 -0700
From:   Joel Fernandes <joelaf@...gle.com>
To:     Viresh Kumar <viresh.kumar@...aro.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>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH v2 2/2] sched: Make iowait_boost optional in schedutil

Hi Viresh,

On Thu, May 18, 2017 at 11:50 PM, Viresh Kumar <viresh.kumar@...aro.org> wrote:
> On 18-05-17, 23:23, Joel Fernandes wrote:
>> We should apply the iowait boost only if cpufreq policy has iowait boost
>> enabled. Also make it a schedutil configuration from sysfs so it can be turned
>> on/off if needed (by default initialize it to the policy value).
>>
>> For systems that don't need/want it enabled, such as those on arm64 based
>> mobile devices that are battery operated, it saves energy when the cpufreq
>> driver policy doesn't have it enabled (details below):
>>
>> Here are some results for energy measurements collected running a YouTube video
>> for 30 seconds:
>> Before: 8.042533 mWh
>> After: 7.948377 mWh
>> Energy savings is ~1.2%
>>
>> Cc: Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>
>> Cc: Len Brown <lenb@...nel.org>
>> Cc: Rafael J. Wysocki <rjw@...ysocki.net>
>> Cc: Viresh Kumar <viresh.kumar@...aro.org>
>> Cc: Ingo Molnar <mingo@...hat.com>
>> Cc: Peter Zijlstra <peterz@...radead.org>
>> Signed-off-by: Joel Fernandes <joelaf@...gle.com>
>> ---
>>  kernel/sched/cpufreq_schedutil.c | 32 ++++++++++++++++++++++++++++++++
>>  1 file changed, 32 insertions(+)
>>
>> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
>> index 76877a62b5fa..0e392b58b9b3 100644
>> --- a/kernel/sched/cpufreq_schedutil.c
>> +++ b/kernel/sched/cpufreq_schedutil.c
>> @@ -24,6 +24,7 @@
>>  struct sugov_tunables {
>>       struct gov_attr_set attr_set;
>>       unsigned int rate_limit_us;
>> +     bool iowait_boost_enable;
>
> I suggested s/iowait_boost_enable/iowait_boost/ and you said okay for
> that change.

Yes, I somehow only picked up 'bool' from your comment.  I'll drop the
'_enable' in the next version. Sorry and thanks.

>>  };
>>
>>  struct sugov_policy {
>> @@ -171,6 +172,11 @@ static void sugov_get_util(unsigned long *util, unsigned long *max)
>>  static void sugov_set_iowait_boost(struct sugov_cpu *sg_cpu, u64 time,
>>                                  unsigned int flags)
>>  {
>> +     struct sugov_policy *sg_policy = sg_cpu->sg_policy;
>> +
>> +     if (!sg_policy->tunables->iowait_boost_enable)
>> +             return;
>> +
>>       if (flags & SCHED_CPUFREQ_IOWAIT) {
>>               sg_cpu->iowait_boost = sg_cpu->iowait_boost_max;
>>       } else if (sg_cpu->iowait_boost) {
>> @@ -386,10 +392,34 @@ static ssize_t rate_limit_us_store(struct gov_attr_set *attr_set, const char *bu
>>       return count;
>>  }
>>
>> +static ssize_t iowait_boost_enable_show(struct gov_attr_set *attr_set,
>> +                                     char *buf)
>> +{
>> +     struct sugov_tunables *tunables = to_sugov_tunables(attr_set);
>> +
>> +     return sprintf(buf, "%u\n", tunables->iowait_boost_enable);
>> +}
>> +
>> +static ssize_t iowait_boost_enable_store(struct gov_attr_set *attr_set,
>> +                                      const char *buf, size_t count)
>> +{
>> +     struct sugov_tunables *tunables = to_sugov_tunables(attr_set);
>> +     bool enable;
>> +
>> +     if (kstrtobool(buf, &enable))
>> +             return -EINVAL;
>> +
>> +     tunables->iowait_boost_enable = enable;
>> +
>> +     return count;
>> +}
>> +
>>  static struct governor_attr rate_limit_us = __ATTR_RW(rate_limit_us);
>> +static struct governor_attr iowait_boost_enable = __ATTR_RW(iowait_boost_enable);
>>
>>  static struct attribute *sugov_attributes[] = {
>>       &rate_limit_us.attr,
>> +     &iowait_boost_enable.attr,
>>       NULL
>>  };
>
> Do we really need this right now? I mean, are you going to use it this
> way? It may never get used eventually and may be better to leave the
> sysfs option for now.

I felt it is good to leave it to the system designer and have the
policy set a 'default' value, so incase it isn't touched by the
designer from userspace, then the policy default is fine, and if the
designer chooses to change it then he has the option to. This is also
how we currently set the rate limits for schedutil in android. I don't
feel strongly about one way or the other and if the general consensus
is to drop this part then I'm fine. I'm curious to know what others
think as well though.

thanks,

-Joel