From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Eric B Munson <emunson@mgebm.net>
Cc: mingo@elte.hu, borislav.petkov@amd.com, bblum@andrew.cmu.edu,
linux-kernel@vger.kernel.org, mhack@us.ibm.com,
eranian@google.com
Subject: Re: [PATCH 3/3] events: Ensure that timers are updated without requiring read() call
Date: Mon, 27 Jun 2011 11:34:08 +0200 [thread overview]
Message-ID: <1309167248.6701.10.camel@twins> (raw)
In-Reply-To: <20110624124921.GB3727@mgebm.net>
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.
next prev parent reply other threads:[~2011-06-27 9:37 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-06-23 20:34 [PATCH 1/3] events: Add note to update_event_times comment about holding ctx->lock Eric B Munson
2011-06-23 20:34 ` [PATCH 2/3] events: Move lockless timer calculation into helper function Eric B Munson
2011-07-01 15:20 ` [tip:perf/core] " tip-bot for Eric B Munson
2011-06-23 20:34 ` [PATCH 3/3] events: Ensure that timers are updated without requiring read() call Eric B Munson
2011-06-24 9:44 ` Peter Zijlstra
2011-06-24 9:46 ` Peter Zijlstra
2011-06-24 12:44 ` Eric B Munson
2011-06-24 12:49 ` Eric B Munson
2011-06-27 9:34 ` Peter Zijlstra [this message]
2011-07-01 15:19 ` [tip:perf/core] events: Add note to update_event_times comment about holding ctx->lock tip-bot for Eric B Munson
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