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

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Message-ID: <20230414103832.GD83892@hirez.programming.kicks-ass.net>
Date:   Fri, 14 Apr 2023 12:38:32 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     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
Subject: Re: [PATCH 2/6] perf: Support branch events logging

On Mon, Apr 10, 2023 at 01:43:48PM -0700, kan.liang@...ux.intel.com wrote:
> From: Kan Liang <kan.liang@...ux.intel.com>
> 
> With the cycle time information between branches, stalls can be easily
> observed. But it's difficult to explain what causes the long delay.
> 
> Add a new field to collect the occurrences of events since the last
> branch entry, which can be used to provide some causality information
> for the cycle time values currently recorded in branches.
> 
> Add a new branch sample type to indicate whether include occurrences of
> events in branch info.
> 
> Only support up to 4 events with saturating at value 3.
> In the current kernel, the events are ordered by either the counter
> index or the enabling sequence. But none of the order information is
> available to the user space tool.
> Add a new PERF_SAMPLE format, PERF_SAMPLE_BRANCH_EVENT_IDS, and generic
> support to dump the event IDs of the branch events.
> Add a helper function to detect the branch event flag.
> These will be used in the following patch.

I'm having trouble reverse engineering this. Can you more coherently
explain this feature and how you've implemented it?