linux-kernel - Re: unpredictability in scheduler test results -- still present

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Message-ID: <48DACF55.3070209@nortel.com>
Date:	Wed, 24 Sep 2008 17:37:57 -0600
From:	"Chris Friesen" <cfriesen@...tel.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org
Subject: Re: unpredictability in scheduler test results -- still present

Chris Friesen wrote:

> This behaviour (that load balancing is messed up) is now almost 
> continuous with both current tip/master and current Linus git.  On the 
> first test after booting, it seems to work okay (although there are 
> still issues with fairness).  On every subsequent test, fairness is good 
> but it only uses one of the two cpus.
> 
> Also, building a kernel with "-j10" results in one cpu being mostly idle 
> while the other one is 100% busy. It used to be both 100% busy--if I get 
> time today I may try bisecting it.

It turns out that disabling CONFIG_DYNAMIC_FTRACE makes the load 
balancing problem go away and causes all cpus to be used.

With this option enabled, the problem seems to be present as far back as 
2.6.27-rc2.  (2.6.27-rc1 doesn't compile on my machine, and 2.6.26 
doesn't have ftrace).

I have no idea why turning on dynamic ftrace would affect load balancing 
behaviour, but it's very repeatable.  The very first test run after 
booting works fine, and all successive runs fail to balance properly.

Chris
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