Re: [tip:sched/urgent] sched: Fix select_idle_sibling() regression in selecting an idle SMT sibling

["Alex,Shi" <alex.shi@intel.com>]

On Fri, 2011-12-23 at 10:43 -0800, Suresh Siddha wrote: 
> On Wed, 2011-12-21 at 18:16 -0800, Shi, Alex wrote:
> > On Thu, 2011-12-22 at 10:03 +0800, Siddha, Suresh B wrote:
> > > On Wed, 2011-12-21 at 17:31 -0800, Shi, Alex wrote:
> > > > This patch partly fixed a performance regression that triggered by
> > > > 4dcfe1025b513c2c, but issue still exists.
> > > 
> > > So how much was the regression caused by the commit 4dcfe1025b513c2c and
> > > how much did we recover with this fix I posted. If we are talking about
> > > the regression caused by this single commit 4dcfe1025b513c2c, then I
> > > don't know of any other related fixes other than the recent fix we
> > > pushed upstream (ab2789213d224202237292d78aaa0c386c7b28b2).
> > 
> > A little complex for the whole thing. 
> > on 4 sockets EX machine, 3~5% hackbench thread regression due to 4dcfe
> > can be recovered by ab2789. 
> > 
> > But on 2 sockets SNB machine, 1024 clients loop netperf TCP-RR has about
> > 9% regression. and your patch seem recover 2~3%. 
> > 
> > And on a 2 sockets nhm, one of our private benchmark was impact much 20
> > +% regression. that benchmark just run 4 process, each of process open a
> > thread, and the thread tasks is to locate randomly pages and than read
> > from 4 times/write 1 time data into a page.  The ab2789 commit seems no
> > help our benchmark. 
> 
> Ok. Can you please try couple of experiments with two kernels? Two
> kernels being the base kernel (prior to 4dcfe1025b513c2c) and the second
> kernel with the commit ab2789213d224202237292d78aaa0c386c7b28b2.
> 
> One experiment with p-states turned off and the second experiment with
> c-states turned off.

I did testing on both of kernel with setting 'performance' gov for all
CPU P-states, and disable cpuidle by setting cpuidle.off=1 in cmdline. 

But didn't find measurable impact on performance result.  

> 
> I suspect mostly deeper core c-states might be contributing to the
> behavior that you are seeing.
> 
> thanks,
> suresh
>