linux-kernel - Re: perf: some questions about perf software events

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Date:	Tue, 30 Nov 2010 23:44:31 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Franck Bui-Huu <vagabon.xyz@...il.com>
Cc:	lkml <linux-kernel@...r.kernel.org>,
	Francis Moreau <francis.moro@...il.com>
Subject: Re: perf: some questions about perf software events

On Sat, 2010-11-27 at 14:28 +0100, Franck Bui-Huu wrote:
> Peter Zijlstra <a.p.zijlstra@...llo.nl> writes:
> 
> > On Wed, 2010-11-24 at 12:35 +0100, Franck Bui-Huu wrote:
> 
> [...]
> 
> >> That is for no 'contiguous' events, setting a sampling frequency doesn't
> >> really make sense since for example you could set a frequency to 1000 HZ
> >> for the software ALIGNMENT_FAULT event and never get any samplings or at
> >> least getting sampling but with a totally different rate. And the
> >> current code doesn't look to handle sample_freq anyway.
> >
> > All the freq bits are in the generic code, it re-computes the rate on
> > the timer-tick as well as on each event occurrence. 
> >
> > Freq driven sampling should work just fine with swevents.
> >
> 
> Yes, but how does it behave with ALIGNMENT_FAULTS for example ?
> 
> Such event may happen at a very disparate rate or it can even never
> happen at all.

Then of course we'll never make the freq target, again, software events
aren't special there.

> >
> >> Also I'm currently not seeing any real differences between cpu-clock and
> >> task-clock events. They both seem to count the time elapsed when the
> >> task is running on a CPU. Am I wrong ?
> >
> > No, Francis already noticed that, I probably wrecked it when I added the
> > multi-pmu stuff, its on my todo list to look at (Francis also handed me
> > a little patchlet), but I keep getting distracted with other stuff :/
> 
> OK.
> 
> Does it make sense to adjust the period for both of them ?
> 
> Also, when creating a task clock event, passing 'pid=-1' to
> sys_perf_event_open() doesn't really make sense, does it ?
> 
> Same with cpu clock and 'pid=n': whatever <n> value, the event measure
> the cpu wall time clock.
> 
> Perhaps proposing only one clock in the API and internally bind this
> clock to the cpu or task clock depending on pid or cpu parameters would
> have been better ?
> 

No, it actually makes sense to count both cpu and task clock on a task
(cpu clock basically being wall-time).


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