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 09:37:52 -0700
From:	Jason Low <jason.low2@...com>
To:	Mike Galbraith <umgwanakikbuti@...il.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 Thu, 2014-04-24 at 04:51 +0200, Mike Galbraith wrote:
> 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.

Are you thinking of something along the lines of this:

@@ -6658,7 +6658,8 @@ static int idle_balance(struct rq *this_rq)
         */
        this_rq->idle_stamp = rq_clock(this_rq);
 
-       if (this_rq->avg_idle < sysctl_sched_migration_cost)
+       if (this_rq->avg_idle < sysctl_sched_migration_cost ||
+           spin_is_contended(&this_rq->lock))
                goto out;

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