linux-kernel - Re: [PATCH 0/4] perf: Precise task / softirq / hardirq filtered stats/profiles

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Date:	Fri, 21 May 2010 20:36:49 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Frederic Weisbecker <fweisbec@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Arnaldo Carvalho de Melo <acme@...hat.com>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH 0/4] perf: Precise task / softirq / hardirq filtered
 stats/profiles

* Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, 2010-05-21 at 17:12 +0200, Ingo Molnar wrote:
> > 'exclusion' is the ABI detail. The feature your patches 
> > implement are to allow 'softirq limited' or 'task-context 
> > limited' or 'hardirq profiling' - which is way cool.
> > 
> > One thing i'd like to see in this feature is for it to 
> > work on pure event counting - i.e. 'perf stat' as well. 
> 
> Its not really exclusion, all it does is discard samples 
> when in the wrong context (which happens to work 
> reasonably well for all the swevents, except for the 
> timer ones).
> 
> If you really want to do exclusion you have to 
> disable/enable on *IRQ entry/exit, but I guess that gets 
> to be prohibitive on costs.

Yeah, i know - this is what i tried to allude to in my 
other part of my reply:

> > If we extended your feature to perf stat, we might be 
> > able to get a lot more precise measurements in terms 
> > of kernel optimizations (and kernel bloat).

Right, so there's two ways to do it, one is the 
disable/enable what you mention, the other would be to 
save the count and then read again and subtract the delta. 

( the RDPMC based delta method can be made to work for 
  sampling as well, even if the NMI hits in the middle of 
  the softirq or hardirq. )

Two reads might be cheaper than a disable+enable. 
Especially if it's done using RDPMC.

We should do it like that, not by discarding samples, and 
overhead should be OK as long as we dont do the 
disable/enable (or delta read) if the feature is off.

If a simple enable/disable or read/read costs too much 
then we need to prod hw makers about it. But it should be 
OK i think.

	Ingo
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