linux-kernel - Re: [RFC PATCH 2/5] sched: Add NOHZ_STATS

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Date:   Thu, 1 Feb 2018 19:10:42 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Morten Rasmussen <morten.rasmussen@...s.arm.com>,
        Ingo Molnar <mingo@...nel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Brendan Jackman <brendan.jackman@....com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [RFC PATCH 2/5] sched: Add NOHZ_STATS_KICK

On Wed, Jan 24, 2018 at 09:25:36AM +0100, Vincent Guittot wrote:
> @@ -8861,7 +8875,14 @@ static int idle_balance(struct rq *this_rq, struct rq_flags *rf)
>  			update_next_balance(sd, &next_balance);
>  		rcu_read_unlock();
>  
> -		if (time_after(jiffies, next) && atomic_read(&nohz.stats_state))
> +		/*
> +		 * Update blocked idle load if it has not been done for a
> +		 * while. Try to do it locally before entering idle but kick a
> +		 * ilb if it takes too much time and might delay next local
> +		 * wake up
> +		 */
> +		if (time_after(jiffies, next) && atomic_read(&nohz.stats_state) &&
> +				!_nohz_idle_balance(this_rq, NOHZ_STATS_KICK, CPU_NEWLY_IDLE))
>  			kick_ilb(NOHZ_STATS_KICK);
>  
>  		goto out;

This I really dislike. We're here because avg_idle is _really_ low, we
really should not then call _nohz_idle_balance().