linux-kernel - Re: [PATCH 2/6] perf: Support branch events logging

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Date:   Mon, 17 Apr 2023 13:55:16 +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
Subject: Re: [PATCH 2/6] perf: Support branch events logging

On Fri, Apr 14, 2023 at 03:47:29PM -0700, Andi Kleen wrote:

> > You really *must* make then a group such that perf knows what events to
> > associated with the LBR event and constain them to the 0-3 range of
> > PMCs.
> > 
> > If you want multiplexing, simply create multiple groups with an LBR
> > event in them.
> 
> 
> Well if you force groups then you require user space or a user which
> understands all the constraints

The LBR feature is naturally a group read, very similar to
PERF_SAMPLE_READ+PERF_FORMAT_GROUP.

> to create groups. I thought one of the basic ideas of perf was to be able to
> abstract those things.

Yeah, the abstraction at hand is a group.