Re: [PATCH] perf_events: improve x86 event scheduling (v5)

[Peter Zijlstra <peterz@infradead.org>]

On Tue, 2010-01-19 at 16:55 +0100, Frederic Weisbecker wrote:
> > Also, I see you set an ->unthrottle, but then don't implement it, but
> > comment it as todo, which is strange because that implies its broken. If
> > there's an ->unthrottle method it will throttle, so if its todo, the
> > safest thing is to not set it.
> 
> 
> Yeah, that's because I have a too vague idea on what is the purpose
> of the unthrottle() callback.
> 
> I've read the concerned codes that call this, several times, and I still
> can't figure out what happens there, not sure what is meant by throttle
> or unthrottle there :-/ 

OK, so not setting it is relatively safe.

As to what it does, it has to undo everything you do when
perf_event_overflow() returns true, which happens when ->unthrottle is
set and we get more than sysctl_perf_event_sample_rate/HZ events in a
jiffy.

If you look at the x86 implementation, you'll see that we simply disable
the hardware counter when the overflow call returns true, so the
unthrottle() callback simply enables it again.