linux-kernel - Re: [PATCH 00/20] perf vendors events arm64: Multiple Arm CPUs

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Date:   Mon, 16 May 2022 19:05:27 +0100
From:   Nick Forrington <nick.forrington@....com>
To:     Ian Rogers <irogers@...gle.com>
Cc:     John Garry <john.garry@...wei.com>, linux-kernel@...r.kernel.org,
        linux-perf-users@...r.kernel.org, Will Deacon <will@...nel.org>,
        Mathieu Poirier <mathieu.poirier@...aro.org>,
        Leo Yan <leo.yan@...aro.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...nel.org>,
        Namhyung Kim <namhyung@...nel.org>,
        Andi Kleen <ak@...ux.intel.com>,
        Kajol Jain <kjain@...ux.ibm.com>,
        James Clark <james.clark@....com>,
        Andrew Kilroy <andrew.kilroy@....com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>
Subject: Re: [PATCH 00/20] perf vendors events arm64: Multiple Arm CPUs

On 15/05/2022 23:03, Ian Rogers wrote:
> On Thu, May 12, 2022 at 8:53 AM John Garry <john.garry@...wei.com> wrote:
>
> Generally this looks ok:
>
> Reviewed-by: John Garry <john.garry@...wei.com>
>
> If you are feeling particularly helpful then you can add support for any
> events missing to pre-existing core support, like a57-a72.
>
> I'll raise John's "ok" and say this looks great! :-D Some thoughts:

Thanks Ian!

> The mapfile.csv cpuid values don't directly align with:
> https://github.com/ARM-software/data/blob/master/cpus.json
> but this definitely looks deliberate.

Correct - they use different formats.

mapfile.csv uses the MIDR format

https://developer.arm.com/documentation/100442/0100/register-descriptions/aarch64-system-registers/midr-el1--main-id-register--el1

The cpus.json "cpuid" is the implementer and part number from the MIDR 
(the other fields are always fixed in mapfile.csv)
> The new events lack the PMU "Unit" value. The current perf json is
> pretty free form and leads to problems if two PMUs are present.
> Context is here:
> https://lore.kernel.org/lkml/CAP-5=fWRRZsyJZ-gky-FOFz79zW_3r78d_0APpj5sf66HqTpLw@mail.gmail.com/
>
> My idea to rationalize this is to mirror what is already done in
> sysfs, that is the event data is specific to a PMU. As a lot of "Unit"
> values are missing from events on x86 a reasonable guess if the "Unit"
> is missing is to use "cpu". Poking a Google Pixel 4a, I see that all
> PMU data is in "armv8_pmuv3". So for ARM I could guess this is always
> the case, ie all events should belong to armv8_pmuv3. This may not be
> right and could lead to confusion like an event BR_COND_MIS_PRED
> having an alias of "armv8_pmuv3/BR_COND_MIS_PRED/" but it really
> should have some other PMU name in there. I just raise this in case
> there is a fix for this we could incorporate into this patch series,
> maybe "armv8_pmuv3" is always the PMU and my life is easy.

My understanding is that all JSON events under arm64/arm apply to the 
CPU PMU, although there could be 2 (or more) armv8_pmuv3 devices in a 
herterogeneous system (armv8_pmuv3_0, armv8_pmuv3_1, ...) - each with 
different events.

So I don't think static "Unit" data would be helpful, but it should be 
possible to map JSON events to appropriate CPUs with existing data.

e.g. /sys/bus/event_source/devices/armv8_pmuv3_0/cpus shows the CPUs 
associated with a PMU device, and each CPU can be mapped to JSON events 
via the MIDR (as is done already)

Thanks,
Nick