Re: [PATCH v3] cpuidle: poll_state: Add time limit to poll_idle()

[Rik van Riel <riel@surriel.com>]

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On Sun, 2018-03-25 at 23:34 +0200, Rafael J. Wysocki wrote:
> On Sunday, March 25, 2018 10:15:52 PM CEST Rik van Riel wrote:
> > 
> > I plan to try two more things:
> > 
> > 1) Disable polling on SMT systems, with
> >    the idea that putting one thread to
> >    sleep with monitor/mwait in C1 will
> >    allow the other thread to run faster.
> 
> Sounds plausible.

Plausible, but wrong. Tests showed that CPU use
during the peak load of this test increased from
about 71% to about 78% with this change, or just
under 10% increase relative to the baseline.

Coincidentally, that is the same CPU use increase
I have seen with the poll_idle() changes. Not sure
if that means anything...

> > 2) Insert more cpu_relax() calls into the
> >    main loop, so the CPU core spends more
> >    of its time in cpu_relax() and less
> >    time doing other things:
> 
> Well, maybe it's a matter of doing cpu_relax() between any other bits
> of
> significant computation in there:

I tried that, as well, and some other variations.

Every single change to poll_idle() that I tried
seems to result in a 9-10% relative increase in
CPU use during the peak load of the test.

During the busiest parts of the load, every CPU
sees on the order of 20k context switches a second.

kind regards,

Rik
-- 
All Rights Reversed.

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