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

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Message-ID: <20130717180156.GS23818@dyad.programming.kicks-ass.net>
Date:	Wed, 17 Jul 2013 20:01:56 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Jason Low <jason.low2@...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 Wed, Jul 17, 2013 at 01:51:51PM -0400, Rik van Riel wrote:
> On 07/17/2013 12:18 PM, Peter Zijlstra wrote:

> >So the way I see things is that the only way newidle balance can slow down
> >things is if it runs when we could have ran something useful.
> 
> Due to contention on the runqueue locks of other CPUs,
> newidle also has the potential to keep _others_ from
> running something useful.

Right, although that should only happen when we do have an imbalance and want
to go move something. Which in Jason's case is 'rare'. But yes, I suppose
there's other scenarios where this is far more likely.

> Could we prevent that downside by measuring both the
> time spent idle, and the time spent in idle balancing,
> and making sure the idle balancing time never exceeds
> more than N% of the idle time?

Sure:

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.
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