Message-ID: <1398307888.26653.18.camel@marge.simpson.net>
Subject: Re: [PATCH 3/3] sched, fair: Stop searching for tasks in newidle
 balance if there are runnable tasks
From: Mike Galbraith <umgwanakikbuti@gmail.com>
To: Jason Low <jason.low2@hp.com>
Cc: mingo@kernel.org, peterz@infradead.org, linux-kernel@vger.kernel.org,
        daniel.lezcano@linaro.org, alex.shi@linaro.org,
        preeti@linux.vnet.ibm.com, vincent.guittot@linaro.org,
        morten.rasmussen@arm.com, aswin@hp.com, chegu_vinod@hp.com
Date: Thu, 24 Apr 2014 04:51:28 +0200
In-Reply-To: <1398303035-18255-4-git-send-email-jason.low2@hp.com>
References: <1398303035-18255-1-git-send-email-jason.low2@hp.com>
	 <1398303035-18255-4-git-send-email-jason.low2@hp.com>
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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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