linux-kernel - Re: [PATCH 2/2] perf_counter: optimize context switch between identical inherited contexts

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Date:	Fri, 22 May 2009 12:08:17 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 2/2] perf_counter: optimize context switch between
 identical inherited contexts

On Fri, 2009-05-22 at 19:56 +1000, Paul Mackerras wrote:
> Peter Zijlstra writes:
> 
> > On Fri, 2009-05-22 at 14:27 +1000, Paul Mackerras wrote:
> > > Since we don't have individual fds for the counters in a cloned
> > > context, the only thing that can make two clones of a given parent
> > > different after they have been cloned is enabling or disabling all
> > > counters with prctl.  To account for this, we keep a count of the
> > > number of enabled counters in each context.  Two contexts must have
> > > the same number of enabled counters to be considered equivalent.
> > 
> > Curious point that.. so prctl() can disable counters it doesn't own.
> > 
> > Shouldn't we instead fix that?
> 
> Well, prctl enables/disables the counters that are counting on the
> current process, regardless of who or what created them.  I always
> thought that was a little strange; maybe it is useful to be able to
> disable all the counters that any other process might have put on to
> you, but I can't think of a scenario where you'd really need to do
> that, particularly since the disable is a one-shot operation, and
> doesn't prevent new (enabled) counters being attached to you.
> 
> On the other hand, what does "all the counters I own" mean?  Does it
> mean all the ones that I have fds open for?  Or does it mean all the
> ones that I created?  Either way we don't have a good way to enumerate
> them.

I'm for all counters you created (ie have a fd for). Being able to
disable counters others created on you just sounds wrong.

If we can settle on a semantic, I'm sure we can implement it :-)

Ingo, Corey, any opinions?

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