linux-kernel - Re: [PATCH v2 1/2] perf cs-etm: Fix decoding for sparse CPU maps

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Message-ID: <20260119135907.GA2732125@e132581.arm.com>
Date: Mon, 19 Jan 2026 13:59:07 +0000
From: Leo Yan <leo.yan@....com>
To: Adrian Hunter <adrian.hunter@...el.com>
Cc: James Clark <james.clark@...aro.org>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Mike Leach <mike.leach@...aro.org>,
	John Garry <john.g.garry@...cle.com>, Will Deacon <will@...nel.org>,
	Leo Yan <leo.yan@...ux.dev>, Peter Zijlstra <peterz@...radead.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Namhyung Kim <namhyung@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
	Jiri Olsa <jolsa@...nel.org>, Ian Rogers <irogers@...gle.com>,
	Thomas Falcon <thomas.falcon@...el.com>, coresight@...ts.linaro.org,
	linux-arm-kernel@...ts.infradead.org,
	linux-perf-users@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 1/2] perf cs-etm: Fix decoding for sparse CPU maps

On Mon, Jan 19, 2026 at 02:11:43PM +0200, Adrian Hunter wrote:

[...]

> >>   0 0 0x200 [0x90]: PERF_RECORD_ID_INDEX nr: 4
> >>   ... id: 771  idx: 0  cpu: 2  tid: -1
> >>   ... id: 772  idx: 1  cpu: 3  tid: -1
> >>   ... id: 773  idx: 0  cpu: 2  tid: -1
> >>   ... id: 774  idx: 1  cpu: 3  tid: -1
> > 
> > Seems to me that this patch works around the issue by using the CPU ID
> > instead, but event->auxtrace.idx is broken.
> > 
> > Should we store the correct index in event->auxtrace.idx (e.g., in the
> > __perf_event__synthesize_id_index()) ?
>
> The idx value represents a perf events ring buffer.  Events on the same
> CPU can share the same ring buffer.  But in the case of per-thread
> recording, different threads have different ring buffers and therefore
> different idx values.
> 
> So I don't think the idx value is wrong.  It is just not the same thing
> as CPU number.

Thanks a lot for the explanation, this makes sense to me.

Leo