RE: [tip:sched/urgent] sched: Fix select_idle_sibling() regressionin selecting an idle SMT sibling

[Shi, Alex]

> >
> > 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.

In fact, since L1 cache, D-TLB is shared between SMT siblings, it is not every application can benefit from wakeup on different Cores. 
> 
> 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.

Testing started on impacted NHM and SND machines for benchmark netperf hackbench thread and our small case. 
> 
> One experiment with p-states turned off and the second experiment with
> c-states turned off.
> 
> I suspect mostly deeper core c-states might be contributing to the
> behavior that you are seeing.

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