Re: change in sched cpu_power causing regressions with SCHED_MC

[Peter Zijlstra <peterz@infradead.org>]

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.