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Date: Thu, 18 Jul 2013 14:35:31 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Cc: Rik van Riel <riel@redhat.com>, Jason Low <jason.low2@hp.com>,
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Subject: Re: [RFC] sched: Limit idle_balance() when it is being used too
 frequently
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On Thu, Jul 18, 2013 at 05:45:46PM +0530, Srikar Dronamraju wrote:
> We take locks if and only if we see imbalance and want to pull the
> tasks.
> However if the newly idle balance is not finding an imbalance then this
> may not be an issue.
> 
> Probably /proc/schedstats will give a better picture.

Right, so we're interested in move_tasks() calls that fail to 'deliver'.
There's a few conditions in there that can cause us to not move a task,
most of them not counted.

The few that are; are from can_mirgrate_task():

  se.statistics.nr_failed_migrations_affine
  se.statistics.nr_failed_migrations_running
  se.statistics.nr_failed_migrations_hot

If we see significant increments on those we'll be taking locks. 

The only one I can see a good way around is the hot one, we could ignore
hotness in favour of newidle -- although I could see that being
detrimental, we'll just have to try or so ;-)

_running shouldn't be much of a problem since we don't bother if
nr_running <= 1. And _affine is out of our reach anyway.