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Date:   Thu, 8 Feb 2018 15:00:05 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Vincent Guittot <vincent.guittot@linaro.org>
Cc:     mingo@kernel.org, linux-kernel@vger.kernel.org,
        valentin.schneider@arm.com, morten.rasmussen@foss.arm.com,
        brendan.jackman@arm.com, dietmar.eggemann@arm.com,
        Will Deacon <will.deacon@arm.com>
Subject: Re: [PATCH v2 1/3] sched: Stop nohz stats when decayed
Message-ID: <20180208140005.GH25201@hirez.programming.kicks-ass.net>
References: <1517944987-343-1-git-send-email-vincent.guittot@linaro.org>
 <1517944987-343-2-git-send-email-vincent.guittot@linaro.org>
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On Tue, Feb 06, 2018 at 08:23:05PM +0100, Vincent Guittot wrote:

> @@ -9207,13 +9231,15 @@ void nohz_balance_enter_idle(int cpu)
>  	if (!housekeeping_cpu(cpu, HK_FLAG_SCHED))
>  		return;
>  
> +	rq->has_blocked_load = 1;
> +
>  	if (rq->nohz_tick_stopped)
> -		return;

this case is difficult... needs thinking

> +		goto out;
>  
>  	/*
>  	 * If we're a completely isolated CPU, we don't play.
>  	 */
> -	if (on_null_domain(cpu_rq(cpu)))
> +	if (on_null_domain(rq))
>  		return;
>  
>  	rq->nohz_tick_stopped = 1;
> @@ -9222,6 +9248,13 @@ void nohz_balance_enter_idle(int cpu)
>  	atomic_inc(&nohz.nr_cpus);
>  
>  	set_cpu_sd_state_idle(cpu);

	/*
	 * Ensures that if nohz_idle_balance() fails to observe our
	 * @idle_cpus_mask store, it must observe the @has_blocked
	 * store.
	 */
	smp_mb__after_atomic();

> +
> +out:
> +	/*
> +	 * Each time a cpu enter idle, we assume that it has blocked load and
> +	 * enable the periodic update of the load of idle cpus
> +	 */
> +	WRITE_ONCE(nohz.has_blocked, 1);
>  }


> @@ -9374,6 +9407,16 @@ static bool nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>  
>  	SCHED_WARN_ON((flags & NOHZ_KICK_MASK) == NOHZ_BALANCE_KICK);
>  
> +	/*
> +	 * We assume there will be no idle load after this update and clear
> +	 * the has_blocked flag. If a cpu enters idle in the mean time, it will
> +	 * set the has_blocked flag and trig another update of idle load.
> +	 * Because a cpu that becomes idle, is added to idle_cpus_mask before
> +	 * setting the flag, we are sure to not clear the state and not
> +	 * check the load of an idle cpu.
> +	 */
> +	WRITE_ONCE(nohz.has_blocked, 0);

	/*
	 * Ensures that if we miss the CPU, we must see the has_blocked
	 * store from nohz_balance_enter_idle().
	 */
	smp_mb();

>  	for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
>  		if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
>  			continue;


I _think_, but my brain isn't quite willing to turn on today.

Without this ordering I think it would be possible to loose has_blocked
and not observe the CPU either.