linux-kernel - Re: [PATCH v2 1/2] sched: force update of blocked load of idle cpus

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Message-ID: <20171220140306.egxub7sog4il2plm@hirez.programming.kicks-ass.net>
Date:   Wed, 20 Dec 2017 15:03:06 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Brendan Jackman <brendan.jackman@....com>
Cc:     Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...nel.org>, linux-kernel@...r.kernel.org,
        Ingo Molnar <mingo@...hat.com>,
        Morten Rasmussen <morten.rasmussen@....com>
Subject: Re: [PATCH v2 1/2] sched: force update of blocked load of idle cpus

On Fri, Dec 01, 2017 at 06:01:56PM +0000, Brendan Jackman wrote:
> @@ -8955,8 +8964,20 @@ static void nohz_balancer_kick(void)
>  	if (ilb_cpu >= nr_cpu_ids)
>  		return;
>  
> -	if (test_and_set_bit(NOHZ_BALANCE_KICK, nohz_flags(ilb_cpu)))
> +	if (test_and_set_bit(NOHZ_BALANCE_KICK, nohz_flags(ilb_cpu))) {
> +		if (!only_update) {
> +			/*
> +			 * There's a pending/ongoing nohz kick/balance. If it's
> +			 * just for stats, convert it to a proper load balance.
> +			 */
> +			clear_bit(NOHZ_STATS_KICK, nohz_flags(ilb_cpu));
> +		}
>  		return;
> +	}
> +
> +	if (only_update)
> +		set_bit(NOHZ_STATS_KICK, nohz_flags(ilb_cpu));
> +
>  	/*
>  	 * Use smp_send_reschedule() instead of resched_cpu().
>  	 * This way we generate a sched IPI on the target cpu which

This looks racy.. if its not we don't need atomic ops, if it is but is
still fine it needs a comment.