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

[Ian Rogers <irogers@google.com>]

On Fri, Nov 19, 2021 at 6:47 AM John Garry <john.garry@huawei.com> wrote:
>
> 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.

The change is specific to x86 as the weak symbol override only happens
on x86. For x86 it only applies if the cpu/slots/ event exists. In the
future it may be used for more than just this - Kajol Jain mentioned a
similar problem for powerpc:
https://lore.kernel.org/lkml/6d1fcb97-223d-7d8e-b5cb-0f10dbc62880@linux.ibm.com/

Thanks,
Ian

> Thanks,
> John