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

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Date:	Sat, 23 May 2009 15:06:52 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	Paul Mackerras <paulus@...ba.org>, 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 Sat, 2009-05-23 at 14:38 +0200, Ingo Molnar wrote:
> > 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?
> 
> It indeed doesnt sound correct that we can disable counters others 
> created on us - especially if they are in a different (higher 
> privileged) security context than us.
> 
> OTOH, enabling/disabling counters in specific functions of a library 
> might be a valid use-case. So perhaps make this an attribute: 
> ..transparent or so, with perf stat defaulting on it to be 
> transparent (i.e. not child context disable-able).

I'm not sure that's something we want to do. Furthermore, if we do want
it, the current implementation is not sufficient, because, as Paul
noted, we can attach a new counter right after the disable.

I really think such limitations should come from whatever security
policy there is on attaching counters. Eg. using selinux, label gnupg as
non-countable so that you simply cannot attach (incl inherit) counters
to it.

Allowing such hooks into libraries will destroy transparency for
developers and I don't think that's something we'd want to promote.

I'll implement my suggestion so we can take it from there.

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