Re: [PATCH v2 2/2] perf parse-events: Architecture specific leader override

From: John Garry
Date: Fri Nov 19 2021 - 09:47:52 EST

On 18/11/2021 22:06, Ian Rogers wrote:
Currently topdown events must appear after a slots event:

$ perf stat -e '{slots,topdown-fe-bound}' /bin/true

Performance counter stats for '/bin/true':

3,183,090 slots
986,133 topdown-fe-bound

Reversing the events yields:

$ perf stat -e '{topdown-fe-bound,slots}' /bin/true
The sys_perf_event_open() syscall returned with 22 (Invalid argument) for event (topdown-fe-bound).

For metrics the order of events is determined by iterating over a
hashmap, and so slots isn't guaranteed to be first which can yield this

Change the set_leader in parse-events, called when a group is closed, so
that rather than always making the first event the leader, if the slots
event exists then it is made the leader. It is then moved to the head of
the evlist otherwise it won't be opened in the correct order.

The result is:

$ perf stat -e '{topdown-fe-bound,slots}' /bin/true

Just curious - does this just affect topdown events? I think x86 is the only arch which has them.