linux-kernel - Re: [PATCH v2] sched: Reduce contention in update_cfs_rq_blocked

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Date:	Mon, 1 Sep 2014 14:55:05 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Low <jason.low2@...com>
Cc:	Tim Chen <tim.c.chen@...ux.intel.com>,
	Paul Turner <pjt@...gle.com>, Ingo Molnar <mingo@...nel.org>,
	Ben Segall <bsegall@...gle.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] sched: Reduce contention in update_cfs_rq_blocked_load

On Thu, Aug 28, 2014 at 12:46:36PM -0700, Jason Low wrote:
> On Wed, 2014-08-27 at 16:32 -0700, Tim Chen wrote:

> > If there are multiple non-forced updates, option 1's error seems to
> > accumulate and non-bounded as we do not actually update?  
> > Is this a concern?
> 
> It should be fine. Once the delta is large enough, we will end up doing
> the update anyway.

Well, the thing is you can have nr_cpus * 12.5% of outstanding delta;
that might be a lot, esp on the large machines.

Now there's two problems with all this; the first is the relative
threshold, typically such per-cpu things have a fixed update threshold,
this makes it much easier to qualify the actual error.

Secondly the indeed the nr_cpus in the error bound. Some things; like
the proportion code scale the threshold by log2(nr_cpus) in an attempt
to do something sensible there.

But yes, unbounded errors here are a problem, sure relaxing the updates
makes things go fast, they also make things go skew.
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