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

[John Garry <john.garry@huawei.com>]

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
> Error:
> 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
> error.
> 
> 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.

Thanks,
John