linux-kernel - Re: [tip:sched/core] sched/rt: Reduce rq lock contention by eliminating locking of non-feasible target

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Date:	Tue, 24 Feb 2015 11:12:47 -0500
From:	Steven Rostedt <rostedt@...dmis.org>
To:	tip-bot for Tim Chen <tipbot@...or.com>
Cc:	suruchi.a.kadu@...el.com, linux-kernel@...r.kernel.org,
	sbohrer@...advisors.com, peterz@...radead.org, mingo@...nel.org,
	tglx@...utronix.de, doug.nelson@...el.com,
	torvalds@...ux-foundation.org, tim.c.chen@...ux.intel.com,
	hpa@...or.com, stable@...r.kernel.org,
	Clark Williams <williams@...hat.com>
Subject: Re: [tip:sched/core] sched/rt:  Reduce rq lock contention by
 eliminating locking of non-feasible target

On Sun, 1 Feb 2015 09:52:24 -0800
tip-bot for Tim Chen <tipbot@...or.com> wrote:

> Commit-ID:  80e3d87b2c5582db0ab5e39610ce3707d97ba409
> Gitweb:     http://git.kernel.org/tip/80e3d87b2c5582db0ab5e39610ce3707d97ba409
> Author:     Tim Chen <tim.c.chen@...ux.intel.com>
> AuthorDate: Fri, 12 Dec 2014 15:38:12 -0800
> Committer:  Ingo Molnar <mingo@...nel.org>
> CommitDate: Fri, 30 Jan 2015 19:38:49 +0100
> 
> sched/rt: Reduce rq lock contention by eliminating locking of non-feasible target
> 
> This patch adds checks that prevens futile attempts to move rt tasks
> to a CPU with active tasks of equal or higher priority.
> 
> This reduces run queue lock contention and improves the performance of
> a well known OLTP benchmark by 0.7%.
> 
> Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Cc: Shawn Bohrer <sbohrer@...advisors.com>
> Cc: Suruchi Kadu <suruchi.a.kadu@...el.com>
> Cc: Doug Nelson<doug.nelson@...el.com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Link: http://lkml.kernel.org/r/1421430374.2399.27.camel@schen9-desk2.jf.intel.com
> Signed-off-by: Ingo Molnar <mingo@...nel.org>

I see this has been added to mainline already. I think this should go
to stable as well. As far back as it applies. I'll even add this to the
stable-rt if need be. But I rather pull it into stable-rt via the
stable branches.

Thanks,

-- Steve

> ---
>  kernel/sched/rt.c | 17 ++++++++++++++++-
>  1 file changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
> index 6725e3c..f4d4b07 100644
> --- a/kernel/sched/rt.c
> +++ b/kernel/sched/rt.c
> @@ -1340,7 +1340,12 @@ select_task_rq_rt(struct task_struct *p, int cpu, int sd_flag, int flags)
>  	     curr->prio <= p->prio)) {
>  		int target = find_lowest_rq(p);
>  
> -		if (target != -1)
> +		/*
> +		 * Don't bother moving it if the destination CPU is
> +		 * not running a lower priority task.
> +		 */
> +		if (target != -1 &&
> +		    p->prio < cpu_rq(target)->rt.highest_prio.curr)
>  			cpu = target;
>  	}
>  	rcu_read_unlock();
> @@ -1617,6 +1622,16 @@ static struct rq *find_lock_lowest_rq(struct task_struct *task, struct rq *rq)
>  
>  		lowest_rq = cpu_rq(cpu);
>  
> +		if (lowest_rq->rt.highest_prio.curr <= task->prio) {
> +			/*
> +			 * Target rq has tasks of equal or higher priority,
> +			 * retrying does not release any lock and is unlikely
> +			 * to yield a different result.
> +			 */
> +			lowest_rq = NULL;
> +			break;
> +		}
> +
>  		/* if the prio of this runqueue changed, try again */
>  		if (double_lock_balance(rq, lowest_rq)) {
>  			/*

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