linux-kernel - Re: [RFC] sched: Limit idle_balance() when it is being used too frequently

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Date:	Thu, 18 Jul 2013 12:03:47 -0700
From:	Jason Low <jason.low2@...com>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Rik van Riel <riel@...hat.com>,
	Ingo Molnar <mingo@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Mike Galbraith <efault@....de>,
	Thomas Gleixner <tglx@...utronix.de>,
	Paul Turner <pjt@...gle.com>, Alex Shi <alex.shi@...el.com>,
	Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Morten Rasmussen <morten.rasmussen@....com>,
	Namhyung Kim <namhyung@...nel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Kees Cook <keescook@...omium.org>,
	Mel Gorman <mgorman@...e.de>, aswin@...com,
	scott.norton@...com, chegu_vinod@...com
Subject: Re: [RFC] sched: Limit idle_balance() when it is being used too
 frequently

On Thu, 2013-07-18 at 17:42 +0530, Srikar Dronamraju wrote:
> > > 
> > > idle_balance(u64 idle_duration)
> > > {
> > >   u64 cost = 0;
> > > 
> > >   for_each_domain(sd) {
> > >     if (cost + sd->cost > idle_duration/N)
> > >       break;
> > > 
> > >     ...
> > > 
> > >     sd->cost = (sd->cost + this_cost) / 2;
> > >     cost += this_cost;
> > >   }
> > > }
> > > 
> > > I would've initially suggested using something like N=2 since we're dealing
> > > with averages and half should ensure we don't run over except for the worst
> > > peaks. But we could easily use a bigger N.
> > 
> > I ran a few AIM7 workloads for the 8 socket HT enabled case and I needed
> > to set N to more than 20 in order to get the big performance gains.
> > 
> 
> As per your observation, newly idle balancing isn't picking tasks and
> mostly finding the domains to be balanced. find_busiest_queue() is
> under rcu. So where and how are we getting these performance gains?

I actually just ran fserver on 8 sockets (which idle balance lowers the
performance in this workload at this socket count), and for this
workload, idle balancing is finding tasks to move fairly often on a
per-cpu basis. So I guess it is not always the case that idle balancing
isn't moving tasks on this box.

Jason

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