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

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Date:   Tue, 3 Oct 2023 12:27:33 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     "Liang, Kan" <kan.liang@...ux.intel.com>
Cc:     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, ak@...ux.intel.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 Mon, Oct 02, 2023 at 08:57:57PM -0400, Liang, Kan wrote:

> > Did I already say that the ISE document raises more questions than it
> > provides answers?
> 
> Yes. Would an improved CPUID enumeration can address the questions? For
> example, the CPUID enumeration can give the maximum number of counters
> and supported width? I think we can discuss it with the architect.

So.. no. Suppose another arch goes and does the same, but with a
different number and width of counters. They won't have CPUID.

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.

So then we end up with:

         *      { u64                   nr;
         *        { u64 hw_idx; } && PERF_SAMPLE_BRANCH_HW_INDEX
         *        { u64 from, to, flags } lbr[nr];
+        *        { u64 counters; } cntr[nr] && PERF_SAMPLE_BRANCH_COUNTERS
         *      } && PERF_SAMPLE_BRANCH_STACK

Have it explicitly named counters, have only the one flag and have sysfs
files describe how to decode it.

Then for this Intel thing we have 4 counters of 2 bits, but if someone
else were to do something different (both Power and ARM64 have this
branch stack stuff now) they can describe it.

It is a bit wasteful on bits... but at least its clear I suppose.