Re: sched: tweak select_idle_sibling to look for idle threads

[Chris Mason]

On Sun, Apr 10, 2016 at 12:04:21PM +0200, Mike Galbraith wrote:
> On Sat, 2016-04-09 at 15:05 -0400, Chris Mason wrote:
> 
> > This does preserve the existing logic to prefer idle cores over idle
> > CPU threads, and includes some tests to try and avoid the idle scan when we're
> > actually better off sharing a non-idle CPU with someone else.
> 
> My box says the "oh nevermind" checks aren't selective enough, tbench
> dropped 4% at clients=cores, and 2% at clients=threads.

I knew this part would need more experimentation, so I kept v1 as simple
as possible.  On my box, tbench clients=cores is 5% faster,
clients=threads is 4% faster.  bounce_to_target() is a small version of
task_hot(), I did get more accurate decisions by using the full
task_hot(), so I can try that again.

I'm testing on one of these:

Intel(R) Xeon(R) CPU E5-2660 v2 @ 2.20GHz, which has two sockets and 10
cores per socket.

What are you testing with?  If it's two sockets or less I may be able to
find one to reproduce with.

> 
> > Benchmarks in production show overall capacity going up between 2-5%
> > depending on the metric.
> 
> Latency rules all loads certainly exist, and clearly want some love,
> but the bigger the socket, and the more threads/core, the more that
> traverse is gonna hurt the others, so seems either we need a better
> filter, or a (yeah yeah, yet another damn) tweakable.
> 
> Oh, and bounce_to_target() seems an odd way to say full_traverse.

Sure, I can rename it.

-chris