Re: [PATCH 7/8] sched, net: Fixup busy_loop_us_clock()

[Peter Zijlstra]

On Thu, Nov 28, 2013 at 06:49:00PM +0200, Eliezer Tamir wrote:
> I have tested this patch and I see a performance regression of about
> 1.5%.

Cute, can you qualify your metric? Since this is a poll loop the only
metric that would be interesting is the response latency. Is that what's
increased by 1.5%? Also, what's the standard deviation of your result?

Also, can you provide relevant perf results for this? Is it really the
sti;cli pair that's degrading your latency?

Better yet, can you provide us with a simple test-case that we can run
locally (preferably single machine setup, using localnet or somesuch).

> Maybe it would be better, rather then testing in the fast path, to
> simply disallow busy polling altogether when sched_clock_stable is
> not true?

Sadly that doesn't work; sched_clock_stable can become false at any time
after boot (and does, even on recent machines).

That said; let me see if I can come up with a few patches to optimize
the entire thing; that'd be something we all benefit from.
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