linux-kernel - Re: [PATCH 3/3] events: Ensure that timers are updated without requiring read() call

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Date:	Mon, 27 Jun 2011 11:34:08 +0200
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	Eric B Munson <emunson@...bm.net>
Cc:	mingo@...e.hu, borislav.petkov@....com, bblum@...rew.cmu.edu,
	linux-kernel@...r.kernel.org, mhack@...ibm.com, eranian@...gle.com
Subject: Re: [PATCH 3/3] events: Ensure that timers are updated without
 requiring read() call

On Fri, 2011-06-24 at 08:49 -0400, Eric B Munson wrote:

> Now that I think about it, this will just mask the problem.  I have a test
> program uses the mmap'd user space page to access event counters (it never
> calls read()).  In this case, the timer values in the event will never be
> updated.  It will display "properly" but the structure won't ever be correct.
> Given that, how can we keep the event values current?

Well the idea is that you do a userspace read of the counter, on x86
that would be using rdpmc and use the provided event count as base. See
the comment in struct perf_event_mmap_page.

Currently we don't have rdpmc support for x86, but it shouldn't be hard.
We should poke at CR4 in our CPU_STARTING callback, and fix up the mess
called perf_event_index() to deal with the strange and wonderful
encoding rdpmc needs (might want an event->pmu->index callback or so).

Currently all this only works on PowerPC.
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