linux-kernel - Re: [PATCH 1/5] perf: Disallow mmap() on per-task inherited events

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Date:	Tue, 25 May 2010 10:19:41 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Paul Mackerras <paulus@...ba.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/5] perf: Disallow mmap() on per-task inherited events

On Tue, 2010-05-25 at 10:55 +1000, Paul Mackerras wrote:
> On Tue, May 18, 2010 at 03:32:59PM +0200, Peter Zijlstra wrote:
> 
> > Since we now have working per-task-per-cpu events for a while,
> > disallow mmap() on per-task inherited events. Those things were
> > a performance problem anyway, and doing away with it allows
> > us to optimize the buffer somewhat by assuming there is only a single
> > writer.
> 
> This also disallows user-space access to hardware counter for this
> event -- which is arguably OK, since doing the userspace read would
> give a different answer to read() on the event fd, as the read() sums
> up all the child counters for us.  Nevertheless, I think this
> side-effect is worth mentioning.

Right, using rdpmc (and similar) should be strictly limited to strict
self-monitoring.
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