Re: [PATCH 1/1] sched: Consider CPU contention in frequency & load-balance busiest CPU selection

[Peter Zijlstra <peterz@infradead.org>]

On Thu, Apr 06, 2023 at 05:50:30PM +0200, Dietmar Eggemann wrote:
> Use new cpu_boosted_util_cfs() instead of cpu_util_cfs().
> 
> The former returns max(util_avg, runnable_avg) capped by max CPU
> capacity. CPU contention is thereby considered through runnable_avg.
> 
> The change in load-balance only affects migration type `migrate_util`.

But why, and how does it affect? That is, isn't this Changelog a wee bit
sparse?

> Suggested-by: Vincent Guittot <vincent.guittot@linaro.org>
> Signed-off-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
> ---
>  kernel/sched/cpufreq_schedutil.c |  3 ++-
>  kernel/sched/fair.c              |  2 +-
>  kernel/sched/sched.h             | 19 +++++++++++++++++++
>  3 files changed, 22 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sched/cpufreq_schedutil.c b/kernel/sched/cpufreq_schedutil.c
> index e3211455b203..728b186cd367 100644
> --- a/kernel/sched/cpufreq_schedutil.c
> +++ b/kernel/sched/cpufreq_schedutil.c
> @@ -158,7 +158,8 @@ static void sugov_get_util(struct sugov_cpu *sg_cpu)
>  	struct rq *rq = cpu_rq(sg_cpu->cpu);
>  
>  	sg_cpu->bw_dl = cpu_bw_dl(rq);
> -	sg_cpu->util = effective_cpu_util(sg_cpu->cpu, cpu_util_cfs(sg_cpu->cpu),
> +	sg_cpu->util = effective_cpu_util(sg_cpu->cpu,
> +					  cpu_boosted_util_cfs(sg_cpu->cpu),
>  					  FREQUENCY_UTIL, NULL);
>  }
>  
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bc358dc4faeb..5ae36224a1c2 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -10481,7 +10481,7 @@ static struct rq *find_busiest_queue(struct lb_env *env,
>  			break;
>  
>  		case migrate_util:
> -			util = cpu_util_cfs(i);
> +			util = cpu_boosted_util_cfs(i);
>  
>  			/*
>  			 * Don't try to pull utilization from a CPU with one
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 060616944d7a..f42c859579d9 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2994,6 +2994,25 @@ static inline unsigned long cpu_util_cfs(int cpu)
>  	return min(util, capacity_orig_of(cpu));
>  }
>  
> +/*
> + * cpu_boosted_util_cfs() - Estimates the amount of CPU capacity used by
> + *                          CFS tasks.
> + *
> + * Similar to cpu_util_cfs() but also take possible CPU contention into
> + * consideration.
> + */
> +static inline unsigned long cpu_boosted_util_cfs(int cpu)
> +{
> +	unsigned long runnable;
> +	struct cfs_rq *cfs_rq;
> +
> +	cfs_rq = &cpu_rq(cpu)->cfs;
> +	runnable = READ_ONCE(cfs_rq->avg.runnable_avg);
> +	runnable = min(runnable, capacity_orig_of(cpu));
> +
> +	return max(cpu_util_cfs(cpu), runnable);
> +}
> +
>  static inline unsigned long cpu_util_rt(struct rq *rq)
>  {
>  	return READ_ONCE(rq->avg_rt.util_avg);
> -- 
> 2.25.1
>