linux-kernel - Re: [patch 0/3] [Announcement] Performance Counters for Linux

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Date:	Fri, 5 Dec 2008 09:24:31 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	David Miller <davem@...emloft.net>
Cc:	a.p.zijlstra@...llo.nl, paulus@...ba.org, tglx@...utronix.de,
	linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
	akpm@...ux-foundation.org, eranian@...glemail.com,
	dada1@...mosbay.com, robert.richter@....com, arjan@...radead.org,
	hpa@...or.com, rostedt@...dmis.org
Subject: Re: [patch 0/3] [Announcement] Performance Counters for Linux

* David Miller <davem@...emloft.net> wrote:

> > > These things aren't measuring time, or even just cycles, they are 
> > > measuring things like L2 cache misses, cpu cycles, and other 
> > > similar kinds of events.
> > > 
> > > So these counters are going to measure all of the damn crap 
> > > assosciated with doing the read() call as well as the real work the 
> > > task does.
> > 
> > that's wrong, look at the example we posted - see it pasted below.
> 
> It's still too simple to be useful.
> 
> There are so many aspects other than the immediate PC that monitoring 
> tasks want to inspect when a counter overflows.

fully agreed.

While most of the flat profilers like oprofile will be happy with the PC 
alone, i do think we want a couple of extended notification types.

Right now we begun with the most trivial ones:

  enum perf_record_type {
          PERF_RECORD_SIMPLE,
          PERF_RECORD_IRQ,
  };

... but it would be natural to do a PERF_RECORD_GP_REGISTERS as well. 
Perhaps even a PERF_RECORD_STACKTRACE using the sysprof facilities, to do 
a hierarchic multi-dimension profile that sysprof does so nicely.

Note that the record type is an independent attribute of a counter. It 
can be set regardless of the even type - and it can be set independently 
for each counter. So you can have say 3 'simple' counters with no irqs 
plus one 'all registers' counter which generates an IRQ: and then you can 
read out the simple counters at the same type.

We could also perhaps do a PERF_RECORD_ALL: it represents a snapshot of 
all active counter values in the task. This is _far_ better than forcibly 
scheduling the monitored task.

	Ingo
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