linux-kernel - Re: [PATCH V8 2/2] perf/x86/intel: Support PEBS counters snapshotting

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Message-ID: <20250114103015.GN5388@noisy.programming.kicks-ass.net>
Date: Tue, 14 Jan 2025 11:30:15 +0100
From: Peter Zijlstra <peterz@...radead.org>
To: kan.liang@...ux.intel.com
Cc: mingo@...hat.com, acme@...nel.org, namhyung@...nel.org,
	irogers@...gle.com, adrian.hunter@...el.com,
	linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org,
	ak@...ux.intel.com, eranian@...gle.com, dapeng1.mi@...ux.intel.com
Subject: Re: [PATCH V8 2/2] perf/x86/intel: Support PEBS counters snapshotting

On Mon, Jan 06, 2025 at 06:21:03AM -0800, kan.liang@...ux.intel.com wrote:

> The current Linux perf sample read feature intends to read the counters
> of other member events when the leader event is overflowing.

This doesn't sound right. Any event in the group that has 'read_format &
FORMAT_GROUP' will read the entire group. There is absolutely nothing
leader specific here. Nor sampling for that matter, read() also works.

You can have 3 sampling events, all with FORMAT_GROUP on, with a
non-sampling leader.

Anyway, let me go read the code...