linux-kernel - Re: [RFC PATCH 0/3]perf/core: extend perf_reg and perf_sample_regs

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Message-ID: <1447114872.13889.2.camel@ellerman.id.au>
Date:	Tue, 10 Nov 2015 11:21:12 +1100
From:	Michael Ellerman <mpe@...erman.id.au>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Madhavan Srinivasan <maddy@...ux.vnet.ibm.com>,
	linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, x86@...nel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, Jiri Olsa <jolsa@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Stephane Eranian <eranian@...il.com>,
	Russell King <linux@....linux.org.uk>,
	Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will.deacon@....com>,
	Benjamin Herrenschmidt <benh@...nel.crashing.org>,
	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
Subject: Re: [RFC PATCH 0/3]perf/core: extend perf_reg and
 perf_sample_regs_intr

On Fri, 2015-11-06 at 11:25 +0100, Peter Zijlstra wrote:

> On Fri, Nov 06, 2015 at 09:04:00PM +1100, Michael Ellerman wrote:

> > It's a perrenial request from our hardware PMU folks to be able to see the raw
> > values of the PMU registers.
> > 
> > I think partly it's so that they can verify that perf is doing what they want,
> > and some of it is that they're interested in some of the more obscure info that
> > isn't plumbed out through other perf interfaces.
> > 
> > We've used various internal hacks over the years to keep them happy. This is an
> > attempt to use a somewhat standard mechanism.
> > 
> > It would also be helpful for those of us working on the perf hardware backends,
> > to be able to verify that we're programming things correctly, without resorting
> > to debug printks etc.
> > 
> > Basically we want to sample regs at the time of the perf interrupt, so we
> > though PERF_SAMPLE_REGS_INTR made senes :)
> > 
> > But if you think this is the wrong mechanism within perf, then please let us
> > know.
> > 
> > I know perf's mission is to abstract as much of the arcane hardware details
> > into a generic interface and make PMUs actually useful for normal folks, and we
> > are committed to that, but it would also be useful to be able to get the raw
> > values for a different type of user.
> > 
> > Maddy's patch only exports PMC1-6 and MMCR0/1. I think we also need to export
> > some others, in particular MMCRA has a lot of stuff in it, half of which is not
> > even architected. So that would have to be exported as "POWER8_MMCRA". And then
> > there's the SIAR/SDAR/SIER which contain a bunch of info on sampled
> > instructions that is not currently plumbed out.
> 
> OK, no objections then. But this is useful information and should be
> included in the patch set.

Thanks, yeah definitely needs more explanation in the patch set.

cheers

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