linux-kernel - Re: [RESEND PATCH V3 1/6] perf: Add branch stack extra

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Date:   Tue, 3 Oct 2023 18:33:00 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Andi Kleen <ak@...ux.intel.com>
Cc:     "Liang, Kan" <kan.liang@...ux.intel.com>, mingo@...hat.com,
        acme@...nel.org, linux-kernel@...r.kernel.org,
        mark.rutland@....com, alexander.shishkin@...ux.intel.com,
        jolsa@...nel.org, namhyung@...nel.org, irogers@...gle.com,
        adrian.hunter@...el.com, eranian@...gle.com,
        alexey.v.bayduraev@...ux.intel.com, tinghao.zhang@...el.com,
        Sandipan Das <sandipan.das@....com>,
        Ravi Bangoria <ravi.bangoria@....com>,
        Athira Rajeev <atrajeev@...ux.vnet.ibm.com>
Subject: Re: [RESEND PATCH V3 1/6] perf: Add branch stack extra

On Tue, Oct 03, 2023 at 08:06:59AM -0700, Andi Kleen wrote:
> > I'm thinking we should do something like expose branch_counter_nr and
> > branch_counter_width in the sysfs node, and then rename this extra field
> > to counters.
> > 
> > Then userspace can do something like:
> > 
> > 	for (i = 0; i < branch_counter_nr; i++) {
> > 		counter[i] = counters & ((1 << branch_counter_width) - 1);
> > 		counters >>= branch_counter_width;
> > 	}
> > 
> > to extract the actual counter values.
> 
> perf script/report won't necessarily have access to the sysfs
> values if they run on a different system
> 
> It would need extra PT style metadata written by perf record to
> perf.data and read by the user tools.
> 
> Seems complicated. It would be better if it just parsed on its own.

Well, you really don't want to repeat the 4,2 thing in every event, that
seems daft (and a waste of space, because how large do we want those
fields to be etc..).

We don't really have a better place for these sorts of things. And yeah,
the PT thing sets precedent here.