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

[Peter Zijlstra]

On Mon, Apr 10, 2023 at 01:43:48PM -0700, kan.liang@xxxxxxxxxxxxxxx wrote:
> From: Kan Liang <kan.liang@xxxxxxxxxxxxxxx>
> 
> 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?