Re: [RFC PATCH 0/3]perf/core: extend perf_reg and perf_sample_regs_intr

From: Madhavan Srinivasan <maddy@linux.vnet.ibm.com>
To: Michael Ellerman <mpe@ellerman.id.au>,
	Peter Zijlstra <peterz@infradead.org>
Cc: linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org,
	linux-arm-kernel@lists.infradead.org, x86@kernel.org,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@kernel.org>, Jiri Olsa <jolsa@kernel.org>,
	Arnaldo Carvalho de Melo <acme@kernel.org>,
	Stephane Eranian <eranian@gmail.com>,
	Russell King <linux@arm.linux.org.uk>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will.deacon@arm.com>,
	Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Subject: Re: [RFC PATCH 0/3]perf/core: extend perf_reg and perf_sample_regs_intr
Date: Sat, 7 Nov 2015 09:58:36 +0530	[thread overview]
Message-ID: <563D7DF4.4020100@linux.vnet.ibm.com> (raw)
In-Reply-To: <1446804240.21859.4.camel@ellerman.id.au>

On Friday 06 November 2015 03:34 PM, Michael Ellerman wrote:
> On Fri, 2015-11-06 at 10:24 +0100, Peter Zijlstra wrote:
>> On Fri, Nov 06, 2015 at 12:57:17PM +0530, Madhavan Srinivasan wrote:
>>> On Thursday 05 November 2015 06:37 PM, Peter Zijlstra wrote:
>>>> On Thu, Nov 05, 2015 at 02:16:15AM +0530, Madhavan Srinivasan wrote:
>>>>> Second patch updates struct arch_misc_reg for arch/powerpc with pmu registers
>>>>> and adds offsetof macro for the same. It extends perf_reg_value()
>>>>> to use reg idx to decide on struct to return value from.
>>>> Why; what's in those regs?
>>> Was out and did not have access to mail, so missed to respond in time.
>>>
>>> In current implementation of patch 2, have added 
>>> few pmu control/status and counter registers,
>>> which give additional information about the PMU context
>>> for the sample.
>> Yes, I saw that, you still haven't answered the question though. What is
>> in those regs? Why is exposing that information like this the best
>> option.
> 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.

Sure. I will rework the patch to include the other regs also.

Maddy

>
> cheers
>