Date: Sat, 13 Jun 2009 12:09:16 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Paul Mackerras <paulus@samba.org>
Cc: Corey Ashford <cjashfor@linux.vnet.ibm.com>,
       LKML <linux-kernel@vger.kernel.org>,
       Peter Zijlstra <a.p.zijlstra@chello.nl>
Subject: Re: perf_counters: page fault trace record
Message-ID: <20090613100916.GA9959@elte.hu>
References: <4A330022.3070105@linux.vnet.ibm.com> <18995.8319.322002.917595@cargo.ozlabs.ibm.com> <4A336700.3060309@linux.vnet.ibm.com> <18995.28357.164575.925080@cargo.ozlabs.ibm.com>
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* Paul Mackerras <paulus@samba.org> wrote:

> Corey Ashford writes:
> 
> > Paul Mackerras wrote:
> > > Can't you do what you need just using a page fault software 
> > > counter with sample_type = PERF_SAMPLE_IP | PERF_SAMPLE_ADDR 
> > > and sample_period = 1?
> > 
> > I thought about that, but I was under the (incorrect?) 
> > impression that on Power, the PERF_SAMPLE_ADDR would be set by 
> > the value of the SDAR register, which wouldn't be correct for 
> > the case of a page fault.
> 
> No, the PERF_SAMPLE_ADDR value only comes from SDAR for a hardware 
> counter overflow event.  For the page-fault software counter the 
> PERF_SAMPLE_ADDR value will always be the faulting address.

Corey, could you please add support for it in 'perf'? We dont want 
such sw-counter features to be in the kernel code without matching 
support in tools/perf/.

While user data symbols wont be resolved, if we have a 
--target-address switch in perf record we could see the faulting 
frequency (and the fault coverage - and ordering as well) of shared 
libraries, in perf report and perf annotate.

This would be a very useful facility.

Thanks,

	Ingo
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