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Date: Wed, 20 Dec 2017 15:22:02 +0100
From: Peter Zijlstra <peterz@infradead.org>
To: Brendan Jackman <brendan.jackman@arm.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Ingo Molnar <mingo@kernel.org>, linux-kernel@vger.kernel.org,
        Ingo Molnar <mingo@redhat.com>,
        Morten Rasmussen <morten.rasmussen@arm.com>
Subject: Re: [PATCH v2 2/2] sched/fair: Update blocked load from newly idle
 balance
Message-ID: <20171220142202.2x7yx3w43wxfel7d@hirez.programming.kicks-ass.net>
References: <20171201180157.18937-1-brendan.jackman@arm.com>
 <20171201180157.18937-3-brendan.jackman@arm.com>
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On Fri, Dec 01, 2017 at 06:01:57PM +0000, Brendan Jackman wrote:
> @@ -7913,6 +7928,29 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
>  	if (child && child->flags & SD_PREFER_SIBLING)
>  		prefer_sibling = 1;
>  
> +#ifdef CONFIG_NO_HZ_COMMON
> +	if (env->idle == CPU_NEWLY_IDLE) {
> +		int cpu;
> +
> +		/* Update the stats of NOHZ idle CPUs in the sd */
> +		for_each_cpu_and(cpu, sched_domain_span(env->sd),
> +				 nohz.idle_cpus_mask) {
> +			struct rq *rq = cpu_rq(cpu);
> +
> +			/* ... Unless we've already done since the last tick */
> +			if (time_after(jiffies, rq->last_blocked_load_update_tick))
> +				update_blocked_averages(cpu);
> +		}
> +	}
> +	/*
> +	 * If we've just updated all of the NOHZ idle CPUs, then we can push
> +	 * back the next nohz.next_update, which will prevent an unnecessary
> +	 * wakeup for the nohz stats kick
> +	 */
> +	if (cpumask_subset(nohz.idle_cpus_mask, sched_domain_span(env->sd)))
> +		nohz.next_update = jiffies + LOAD_AVG_PERIOD;
> +#endif
> +
>  	load_idx = get_sd_load_idx(env->sd, env->idle);
>  
>  	do {

We're already going to be iterating all those CPUs through
update_sg_lb_stats(), why not push it all the way down there and avoid
the double iteration?