Subject: Re: change in sched cpu_power causing regressions with SCHED_MC
From: Peter Zijlstra <peterz@infradead.org>
To: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Ingo Molnar <mingo@redhat.com>, LKML <linux-kernel@vger.kernel.org>,
       "Ma, Ling" <ling.ma@intel.com>,
       "Zhang, Yanmin" <yanmin_zhang@linux.intel.com>,
       "ego@in.ibm.com" <ego@in.ibm.com>,
       "svaidy@linux.vnet.ibm.com" <svaidy@linux.vnet.ibm.com>
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Date: Sat, 13 Feb 2010 11:42:04 +0100
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On Sat, 2010-02-13 at 11:36 +0100, Peter Zijlstra wrote:
> On Fri, 2010-02-12 at 17:31 -0800, Suresh Siddha wrote:
> > 
> > We have one more problem that Yanmin and Ling Ma reported. On a dual
> > socket quad-core platforms (for example platforms based on NHM-EP), we
> > are seeing scenarios where one socket is completely busy (with all the 4
> > cores running with 4 tasks) and another socket is completely idle.
> > 
> > This causes performance issues as those 4 tasks share the memory
> > controller, last-level cache bandwidth etc. Also we won't be taking
> > advantage of turbo-mode as much as we like. We will have all these
> > benefits if we move two of those tasks to the other socket. Now both the
> > sockets can potentially go to turbo etc and improve performance.
> > 
> > In short, your recent change (shown below) broke this behavior. In the
> > kernel summit you mentioned you made this change with out affecting the
> > behavior of SMT/MC. And my testing immediately after kernel-summit also
> > didn't show the problem (perhaps my test didn't hit this specific
> > change). But apparently we are having performance issues with this patch
> > (Ling Ma's bisect pointed to this patch). I will look more detailed into
> > this after the long weekend (to see if we can catch this scenario in
> > fix_small_imbalance() etc). But wanted to give you a quick heads up.
> > Thanks.
> 
> Right, so the behaviour we want should be provided by SD_PREFER_SIBLING,
> it provides the capacity==1 thing the cpu_power games used to provide.
> 
> Not saying it's not broken, but that's where the we should be looking to
> fix it.


BTW, do you think its possible to automate such test cases and put them
in a test-suite?
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