linux-kernel - Re: [PATCH v3 1/2] cpufreq: schedutil: Fix iowait boost reset

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Date:   Mon, 21 May 2018 10:33:56 -0700
From:   Joel Fernandes <joel@...lfernandes.org>
To:     Patrick Bellasi <patrick.bellasi@....com>
Cc:     linux-kernel@...r.kernel.org, linux-pm@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        "Rafael J . Wysocki" <rafael.j.wysocki@...el.com>,
        Viresh Kumar <viresh.kumar@...aro.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <juri.lelli@...hat.com>, kernel-team@...roid.com
Subject: Re: [PATCH v3 1/2] cpufreq: schedutil: Fix iowait boost reset

On Mon, May 21, 2018 at 09:51:19AM +0100, Patrick Bellasi wrote:
> A more energy efficient update of the IO wait boosting mechanism has
> been introduced in:
> 
>    commit a5a0809bc58e ("cpufreq: schedutil: Make iowait boost more energy efficient")
> 
> where the boost value is expected to be:
> 
>  - doubled at each successive wakeup from IO
>    staring from the minimum frequency supported by a CPU
> 
>  - reset when a CPU is not updated for more then one tick
>    by either disabling the IO wait boost or resetting its value to the
>    minimum frequency if this new update requires an IO boost.
> 
> This approach is supposed to "ignore" boosting for sporadic wakeups from
> IO, while still getting the frequency boosted to the maximum to benefit
> long sequence of wakeup from IO operations.
> 
> However, these assumptions are not always satisfied.
> For example, when an IO boosted CPU enters idle for more the one tick
> and then wakes up after an IO wait, since in sugov_set_iowait_boost() we
> first check the IOWAIT flag, we keep doubling the iowait boost instead
> of restarting from the minimum frequency value.
> 
> This misbehavior could happen mainly on non-shared frequency domains,
> thus defeating the energy efficiency optimization, but it can also
> happen on shared frequency domain systems.
> 
> Let fix this issue in sugov_set_iowait_boost() by:
>  - first check the IO wait boost reset conditions
>    to eventually reset the boost value
>  - then applying the correct IO boost value
>    if required by the caller
> 
> Reported-by: Viresh Kumar <viresh.kumar@...aro.org>
> Signed-off-by: Patrick Bellasi <patrick.bellasi@....com>
> Cc: Ingo Molnar <mingo@...hat.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@...el.com>
> Cc: Viresh Kumar <viresh.kumar@...aro.org>
> Cc: Joel Fernandes <joelaf@...gle.com>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: linux-kernel@...r.kernel.org
> Cc: linux-pm@...r.kernel.org
> Fixes: a5a0809bc58e ("cpufreq: schedutil: Make iowait boost more energy efficient")
> 
> ---
> Changes in v3:
>  - split the fix into a separated patch (this one)
>  - added "Fixes" tag (Viresh)
> ---
>  kernel/sched/cpufreq_schedutil.c | 18 ++++++++++--------
>  1 file changed, 10 insertions(+), 8 deletions(-)
> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index e13df951aca7..c4063e578e4d 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -203,6 +203,16 @@ static unsigned long sugov_aggregate_util(struct sugov_cpu *sg_cpu)
>  
>  static void sugov_set_iowait_boost(struct sugov_cpu *sg_cpu, u64 time, unsigned int flags)
>  {
> +	/* Clear iowait_boost if the CPU apprears to have been idle. */
> +	if (sg_cpu->iowait_boost) {
> +		s64 delta_ns = time - sg_cpu->last_update;
> +
> +		if (delta_ns > TICK_NSEC) {
> +			sg_cpu->iowait_boost = 0;
> +			sg_cpu->iowait_boost_pending = false;
> +		}
> +	}
> +

Yes, I guess this effects single policies only. For shared, we are resetting
the iowait boost on the wake-up-from-idle update anyway. So even if it was
doubled, it would be reset.

Looks good to me! thanks,

Reviewed-by: Joel Fernandes (Google) <joel@...lfernandes.org>