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Date:   Thu, 25 Apr 2019 13:56:20 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Nicholas Piggin <npiggin@gmail.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Frederic Weisbecker <fweisbec@gmail.com>,
        Ingo Molnar <mingo@redhat.com>, linux-kernel@vger.kernel.org
Subject: Re: [PATCH] kernel/sched: run nohz idle load balancer on
 HK_FLAG_MISC CPUs
Message-ID: <20190425115620.GQ4038@hirez.programming.kicks-ass.net>
References: <20190412042613.28930-1-npiggin@gmail.com>
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On Fri, Apr 12, 2019 at 02:26:13PM +1000, Nicholas Piggin wrote:
> The nohz idle balancer runs on the lowest idle CPU. This can
> interfere with isolated CPUs, so confine it to HK_FLAG_MISC
> housekeeping CPUs.
> 
> HK_FLAG_SCHED is not used for this because it is not set anywhere
> at the moment. This could be folded into HK_FLAG_SCHED once that
> option is fixed.

Frederic? Anyway, I thnk I'll take this patch as is.

Thanks!

> The problem was observed with increased jitter on an application
> running on CPU0, caused by nohz idle load balancing being run on
> CPU1 (an SMT sibling).
> 
> Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
> ---
>  kernel/sched/fair.c | 16 ++++++++++------
>  1 file changed, 10 insertions(+), 6 deletions(-)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index fdab7eb6f351..d29ca323214d 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -9522,22 +9522,26 @@ static inline int on_null_domain(struct rq *rq)
>   * - When one of the busy CPUs notice that there may be an idle rebalancing
>   *   needed, they will kick the idle load balancer, which then does idle
>   *   load balancing for all the idle CPUs.
> + * - HK_FLAG_MISC CPUs are used for this task, because HK_FLAG_SCHED not set
> + *   anywhere yet.
>   */
>  
>  static inline int find_new_ilb(void)
>  {
> -	int ilb = cpumask_first(nohz.idle_cpus_mask);
> +	int ilb;
>  
> -	if (ilb < nr_cpu_ids && idle_cpu(ilb))
> -		return ilb;
> +	for_each_cpu_and(ilb, nohz.idle_cpus_mask,
> +			      housekeeping_cpumask(HK_FLAG_MISC)) {
> +		if (idle_cpu(ilb))
> +			return ilb;
> +	}
>  
>  	return nr_cpu_ids;
>  }
>  
>  /*
> - * Kick a CPU to do the nohz balancing, if it is time for it. We pick the
> - * nohz_load_balancer CPU (if there is one) otherwise fallback to any idle
> - * CPU (if there is one).
> + * Kick a CPU to do the nohz balancing, if it is time for it. We pick any
> + * idle CPU in the HK_FLAG_MISC housekeeping set (if there is one).
>   */
>  static void kick_ilb(unsigned int flags)
>  {
> -- 
> 2.20.1
>