linux-kernel - Re: [PATCH 3/3] sched, fair: Stop searching for tasks in newidle balance if there are runnable tasks

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Date:	Thu, 24 Apr 2014 04:51:28 +0200
From:	Mike Galbraith <umgwanakikbuti@...il.com>
To:	Jason Low <jason.low2@...com>
Cc:	mingo@...nel.org, peterz@...radead.org,
	linux-kernel@...r.kernel.org, daniel.lezcano@...aro.org,
	alex.shi@...aro.org, preeti@...ux.vnet.ibm.com,
	vincent.guittot@...aro.org, morten.rasmussen@....com, aswin@...com,
	chegu_vinod@...com
Subject: Re: [PATCH 3/3] sched, fair: Stop searching for tasks in newidle
 balance if there are runnable tasks

On Wed, 2014-04-23 at 18:30 -0700, Jason Low wrote: 
> It was found that when running some workloads (such as AIM7) on large systems
> with many cores, CPUs do not remain idle for long. Thus, tasks can
> wake/get enqueued while doing idle balancing.
> 
> In this patch, while traversing the domains in idle balance, in addition to
> checking for pulled_task, we add an extra check for this_rq->nr_running for
> determining if we should stop searching for tasks to pull. If there are
> runnable tasks on this rq, then we will stop traversing the domains. This
> reduces the chance that idle balance delays a task from running.
> 
> This patch resulted in approximately a 6% performance improvement when
> running a Java Server workload on an 8 socket machine.

Checking rq->lock for contention before ever going to idle balancing as
well should give you a bit more.  No need to run around looking for work
that's trying to arrive.  By not going there, perhaps stacking tasks,
you may head off a future bounce as well.

-Mike

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