Re: [PATCH 0/4] perf: Precise task / softirq / hardirq filteredstats/profiles

From: Ingo Molnar
Date: Fri May 21 2010 - 14:37:15 EST

* Peter Zijlstra <peterz@xxxxxxxxxxxxx> 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.

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