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Message-ID: <9a4db39373cf4acaf91ef6db92df116b74fef992.camel@redhat.com>
Subject: Re: [PATCH] timers/nohz: Update nohz load even if tick already
 stopped
From:   Scott Wood <swood@redhat.com>
To:     Peter Zijlstra <peterz@infradead.org>
Cc:     Frederic Weisbecker <frederic@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>
Date:   Wed, 11 Dec 2019 14:46:01 -0600
In-Reply-To: <20191030133130.GY4097@hirez.programming.kicks-ass.net>
References: <20191028150716.22890-1-frederic@kernel.org>
         <20191029100506.GJ4114@hirez.programming.kicks-ass.net>
         <52d963553deda810113accd8d69b6dffdb37144f.camel@redhat.com>
         <20191030133130.GY4097@hirez.programming.kicks-ass.net>
Organization: Red Hat
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On Wed, 2019-10-30 at 14:31 +0100, Peter Zijlstra wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index eb42b71faab9..d02d1b8f40af 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3660,21 +3660,17 @@ static void sched_tick_remote(struct work_struct
> *work)
>  	u64 delta;
>  	int os;
>  
> -	/*
> -	 * Handle the tick only if it appears the remote CPU is running in
> full
> -	 * dynticks mode. The check is racy by nature, but missing a tick or
> -	 * having one too much is no big deal because the scheduler tick
> updates
> -	 * statistics and checks timeslices in a time-independent way,
> regardless
> -	 * of when exactly it is running.
> -	 */
> -	if (idle_cpu(cpu) || !tick_nohz_tick_stopped_cpu(cpu))
> +	if (!tick_nohz_tick_stopped_cpu(cpu))
>  		goto out_requeue;
>  
>  	rq_lock_irq(rq, &rf);
> -	curr = rq->curr;
> -	if (is_idle_task(curr) || cpu_is_offline(cpu))
> +	/*
> +	 * We must not call calc_load_nohz_remote() when not in NOHZ mode.
> +	 */
> +	if (cpu_is_offline(cpu) || !tick_nohz_tick_stopped(cpu))
>  		goto out_unlock;

Is it really a problem if calc_load_nohz_remote() gets called in
non-NOHZ?  It won't race due to rq lock -- and we're already mixing
remote and non-remote updates because the normal tick timer can still be
run while "stopped".

-Scott