linux-kernel - Re: [PATCH] perfcounters: Make s/w counters in a group only count when group is on

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Message-ID: <20090313131309.GB10117@elte.hu>
Date:	Fri, 13 Mar 2009 14:13:09 +0100
From:	Ingo Molnar <mingo@...e.hu>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Paul Mackerras <paulus@...ba.org>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH] perfcounters: Make s/w counters in a group only count
	when group is on


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

> On Fri, 2009-03-13 at 13:44 +0100, Peter Zijlstra wrote:
> 
> > The same for task migration, most migrations happen when 
> > they are in fact running, so there too we can account the 
> > migration either before we rip it off the src cpu, or after 
> > we place it on the dst cpu.
> 
> Right, I got confused between being on the cpu and having 
> ->state = R.
> 
> Migrations are the odd one out indeed, but I'd rather fudge a 
> little with the migration counter itself than have this weird 
> asymmetry.

Agreed. There should really be no difference between software 
and hardware counters as far as the generic perfcounters code 
goes. It's a magic "metric" that gets read out somehow, and 
which generates events somehow.

We can have various grades of hardware versus software counters: 

- 'pure hardware counters' where both the count and events come 
  from some hw register

- 'pure software counters' where both the count and events are 
  generated by software

- 'hybride counters' where for example the count might be from a 
  hardware register, but the event is generated by a hrtimer 
  (because the hardware is not capable of generating events).

the is_software_counter() assymetry broke this generally relaxed 
model of counters.

	Ingo
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