linux-kernel - Re: Perf events/ARM

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Date:	Tue, 01 Dec 2009 15:40:44 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Jamie Iles <jamie@...ieiles.com>,
	Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org
Subject: Re: Perf events/ARM

On Tue, 2009-12-01 at 15:31 +0100, Ingo Molnar wrote:
> * Jamie Iles <jamie@...ieiles.com> wrote:
> 
> > Hi,
> > 
> > I'm looking at adding support for the hardware performance counters in ARMv6
> > using the new perf events framework. I have a simple setup that uses the
> > counters on their own, but wrt the perf events framework:
> > 
> > 	- what are the requirements of set_perf_event_pending() and
> > 	perf_event_do_pending()? As far as I can tell from sparc/x86/powerpc,
> > 	set_perf_event_pending() triggers an interrupt that then calls
> > 	perf_event_do_pending(). Does perf_event_do_pending need to run in
> > 	interrupt context or could I use a soft IRQ if platforms don't have a
> > 	spare IRQ?
> 
> softirq would be fine too i suspect - but then you need to increase the 
> buffering of perf_pending_head, as multiple hardirqs could hit before 
> the softirq processing has finished.
> 
> As that gets complex quick, an acceptable first-order approach would be 
> to just ignore those lost events and run it from a softirq - i _think_ 
> everything should be OK.

Things like wakeups and ->event_limit might get delayed.

Delayed wakeups can be mitigated by larger buffers, delayed disable on
->event_limit is not something you can fix up.

Does your PMU generate regular interrupts or actual NMIs? If its normal
interrupts you can simply call perf_event_do_pending() at the
pmu-interrupt tail.

x86 does a self-ipi to get from NMI context into IRQ context as fast as
possible, simply because you cannot do very much from NMI context.

> > 	- ARM does not have proper support for atomic64's. Other than
> > 	performance, would there be any known problems with using the generic
> > 	spinlocked atomic64's?
> 
> Not a problem at all. Even performance-wise they are pretty nice - Paul 
> has done a nice job hashing it along 16 spinlocks - so for small SMP 
> systems there should be no global cacheline bounce.

Depends, again if your PMU generates NMIs a spinlock'ed version won't
work.

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