linux-kernel - Re: [PATCH v3 3/3] sched/rt: Add support for SD_PREFER_SIBLING on find_lowest

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Message-ID: <20170323092123.GP11100@X58A-UD3R>
Date:   Thu, 23 Mar 2017 18:21:23 +0900
From:   Byungchul Park <byungchul.park@....com>
To:     peterz@...radead.org, mingo@...nel.org
Cc:     linux-kernel@...r.kernel.org, juri.lelli@...il.com,
        rostedt@...dmis.org, kernel-team@....com, srostedt@...hat.com,
        ghaskins@...ell.com
Subject: Re: [PATCH v3 3/3] sched/rt: Add support for SD_PREFER_SIBLING on
 find_lowest_rq()

On Thu, Mar 23, 2017 at 11:12:49AM +0900, Byungchul Park wrote:
> It would be better to avoid pushing tasks to other cpu within
> a SD_PREFER_SIBLING domain, instead, get more chances to check other
> siblings.

+cc ghaskins@...ell.com
+cc srostedt@...hat.com

> 
> Signed-off-by: Byungchul Park <byungchul.park@....com>
> ---
>  kernel/sched/rt.c | 17 +++++++++++++++++
>  1 file changed, 17 insertions(+)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 979b734..6332b2ad 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1624,6 +1624,7 @@ static int find_lowest_rq(struct task_struct *task)
>  	struct cpumask *lowest_mask = this_cpu_cpumask_var_ptr(local_cpu_mask);
>  	int this_cpu = smp_processor_id();
>  	int cpu      = task_cpu(task);
> +	int fallback_cpu = -1;
>  
>  	/* Make sure the mask is initialized first */
>  	if (unlikely(!lowest_mask))
> @@ -1671,6 +1672,15 @@ static int find_lowest_rq(struct task_struct *task)
>  			best_cpu = cpumask_first_and(lowest_mask,
>  						     sched_domain_span(sd));
>  			if (best_cpu < nr_cpu_ids) {
> +				/*
> +				 * If current domain is SD_PREFER_SIBLING
> +				 * flaged, we have to get more chances to
> +				 * check other siblings.
> +				 */
> +				if (sd->flags & SD_PREFER_SIBLING) {
> +					fallback_cpu = best_cpu;
> +					continue;
> +				}
>  				rcu_read_unlock();
>  				return best_cpu;
>  			}
> @@ -1679,6 +1689,13 @@ static int find_lowest_rq(struct task_struct *task)
>  	rcu_read_unlock();
>  
>  	/*
> +	 * If fallback_cpu is valid, all our quesses failed *except* for
> +	 * SD_PREFER_SIBLING domain. Now, we can return the fallback cpu.
> +	 */
> +	if (fallback_cpu != -1)
> +		return fallback_cpu;
> +
> +	/*
>  	 * And finally, if there were no matches within the domains
>  	 * just give the caller *something* to work with from the compatible
>  	 * locations.
> -- 
> 1.9.1