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Date: Mon, 02 Feb 2009 12:03:37 -0800
From: Corey Ashford <cjashfor@...ux.vnet.ibm.com>
To: Andi Kleen <andi@...stfloor.org>
CC: Ingo Molnar <mingo@...e.hu>, linux-kernel@...r.kernel.org,
Thomas Gleixner <tglx@...utronix.de>,
Andrew Morton <akpm@...ux-foundation.org>,
Stephane Eranian <eranian@...glemail.com>,
Eric Dumazet <dada1@...mosbay.com>,
Robert Richter <robert.richter@....com>,
Arjan van de Veen <arjan@...radead.org>,
Peter Anvin <hpa@...or.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Paul Mackerras <paulus@...ba.org>,
"David S. Miller" <davem@...emloft.net>,
Maynard Johnson <mpjohn@...ibm.com>, carll@...ibm.com
Subject: Re: Performance counter API review was [patch] Performance Counters
for Linux, v3
Andi Kleen wrote:
[snip]
> - Global tracing. Right now there seem to be two modi: per task and
> per CPU. But a common variant is global tracing of all CPUs. While this
> could be in theory done right now by attaching to each CPU
> this has the problem that it doesn't interact very well with CPU
> hot plug. The application would need to poll for additional/lost
> CPUs somehow and then re-attach to them (or detach). This would
> likely be quite clumsy and slow. It would be better if the kernel supported
> that better.
>
> Or alternative here is to do nothing and keep oprofile for that job
> (which it doesn't do that badly)
>
This issue is of particular interest to us, from the IBM Power toolchain
perspective.
Ingo, do you think it would be feasible to add an ability to open a
single file descriptor that could give global counting (and sampling) on
all CPU's? I realize this would entail creating a context per cpu in
the kernel.
How to present the count data back to user space is another issue. For
example, do you sum the counts of a particular event type across all
CPUs or do you keep them separate, and have the user space app read them
up per-cpu (perhaps not knowing exactly which cpu they come from)?
I realize that perfmon doesn't have this ability either, it's currently
per-cpu as well for global counting.
But it seems as long as you are going so far as providing a thread
inheritance feature (which I assume uses a summing approach for
providing counts back to user space), that this "pan-cpu" counting
feature might not be too difficult to implement. It sure would simplify
the life of user space apps, as Andi said.
--
Regards,
- Corey
Corey Ashford
Software Engineer
IBM Linux Technology Center, Linux Toolchain
Beaverton, OR
503-578-3507
cjashfor@...ibm.com
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