Re: [PATCH 1/3] sched/cpufreq: always consider blocked FAIR utilization

[Viresh Kumar]

On 10-05-18, 16:05, Patrick Bellasi wrote:
> Since the refactoring introduced by:
> 
>    commit 8f111bc357aa ("cpufreq/schedutil: Rewrite CPUFREQ_RT support")
> 
> we aggregate FAIR utilization only if this class has runnable tasks.
> This was mainly due to avoid the risk to stay on an high frequency just
> because of the blocked utilization of a CPU not being properly decayed
> while the CPU was idle.
> 
> However, since:
> 
>    commit 31e77c93e432 ("sched/fair: Update blocked load when newly idle")
> 
> the FAIR blocked utilization is properly decayed also for IDLE CPUs.
> 
> This allows us to use the FAIR blocked utilization as a safe mechanism
> to gracefully reduce the frequency only if no FAIR tasks show up on a
> CPU for a reasonable period of time.
> 
> Moreover, we also reduce the frequency drops of CPUs running periodic
> tasks which, depending on the task periodicity and the time required
> for a frequency switch, was increasing the chances to introduce some
> undesirable performance variations.
> 
> Reported-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Signed-off-by: Patrick Bellasi <patrick.bellasi@xxxxxxx>
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Rafael J. Wysocki <rafael.j.wysocki@xxxxxxxxx>
> Cc: Vincent Guittot <vincent.guittot@xxxxxxxxxx>
> Cc: Viresh Kumar <viresh.kumar@xxxxxxxxxx>
> Cc: Joel Fernandes <joelaf@xxxxxxxxxx>
> Cc: linux-kernel@xxxxxxxxxxxxxxx
> Cc: linux-pm@xxxxxxxxxxxxxxx
> ---
>  kernel/sched/cpufreq_schedutil.c | 17 ++++++++---------
>  1 file changed, 8 insertions(+), 9 deletions(-)

Do we need a Fixes tag and Cc stable ?

> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index d2c6083304b4..a74d05160e66 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -183,22 +183,21 @@ static void sugov_get_util(struct sugov_cpu *sg_cpu)
>  static unsigned long sugov_aggregate_util(struct sugov_cpu *sg_cpu)
>  {
>  	struct rq *rq = cpu_rq(sg_cpu->cpu);
> -	unsigned long util;
>  
> -	if (rq->rt.rt_nr_running) {
> -		util = sg_cpu->max;
> -	} else {
> -		util = sg_cpu->util_dl;
> -		if (rq->cfs.h_nr_running)
> -			util += sg_cpu->util_cfs;
> -	}
> +	if (rq->rt.rt_nr_running)
> +		return sg_cpu->max;
>  
>  	/*
> +	 * Utilization required by DEADLINE must always be granted while, for
> +	 * FAIR, we use blocked utilization of IDLE CPUs as a mechanism to
> +	 * gracefully reduce the frequency when no tasks show up for longer
> +	 * periods of time.
> +	 *
>  	 * Ideally we would like to set util_dl as min/guaranteed freq and
>  	 * util_cfs + util_dl as requested freq. However, cpufreq is not yet
>  	 * ready for such an interface. So, we only do the latter for now.
>  	 */
> -	return min(util, sg_cpu->max);
> +	return min(sg_cpu->max, (sg_cpu->util_dl + sg_cpu->util_cfs));
>  }
>  
>  static void sugov_set_iowait_boost(struct sugov_cpu *sg_cpu, u64 time, unsigned int flags)

Acked-by: Viresh Kumar <viresh.kumar@xxxxxxxxxx>

-- 
viresh