linux-kernel - Re: [sched/numa] 096aa33863a: -21.4% hackbench.throughput, -20.2% netperf.Throughput

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Date:	Fri, 8 Aug 2014 10:38:53 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Rik van Riel <riel@...hat.com>
Cc:	Fengguang Wu <fengguang.wu@...el.com>,
	Dave Hansen <dave.hansen@...el.com>,
	Ingo Molnar <mingo@...e.hu>,
	LKML <linux-kernel@...r.kernel.org>, lkp@...org,
	Peter Zijlstra <peterz@...radead.org>
Subject: Re: [sched/numa] 096aa33863a: -21.4% hackbench.throughput, -20.2%
 netperf.Throughput_Mbps


* Rik van Riel <riel@...hat.com> wrote:

> On 08/07/2014 06:53 AM, Fengguang Wu wrote:
> > Hi Rik,
> > 
> > We noticed the below performance regression in commit
> > 096aa33863a5e48de52d2ff30e0801b7487944f4 ("sched/numa: Decay
> > ->wakee_flips instead of zeroing")
> > 
> > b1ad065e65f5610  096aa33863a5e48de52d2ff30  testbox/testcase/testparams
> > ---------------  -------------------------  ---------------------------
> >     122361 ± 0%     -21.4%      96140 ± 0%  lkp-snb01/hackbench/50%-process-pipe
> >     122361 ± 0%     -21.4%      96140 ± 0%  TOTAL hackbench.throughput
> 
> I guess the performance of that benchmark depends on it
> "slipping under the wire" after each time the kernel
> zeroes out ->wakee_flips.
> 
> Depending on repeatedly pulling the wakee back to the same
> node as the waker suggests something else in the kernel may
> be pulling the wakee to another place in the system repeatedly,
> as well, just at a lower frequency (load balancer?).
> 
> I have also noticed that select_idle_sibling often fails to
> find an idle sibling within the LLC domain, even when it
> exists. Fixing that bug sometimes results in lower performance.
> 
> It appears that some of the performance results of the scheduler
> appear on the code acting in an opposite way to its documented
> intention.
> 
> It may be best to revert 096aa33863a for now...

Mind sending a revert patch, with an explanation, a Reported-by, etc?

Thanks,

	Ingo
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