Re: [PATCH V7 07/14] perf/core: Add a new PERF_EV_CAP_COEXIST event capability

[Liang, Kan]

On 7/24/2020 12:43 PM, peterz@xxxxxxxxxxxxx wrote:
> On Fri, Jul 24, 2020 at 04:59:34PM +0200, Peter Zijlstra wrote:
> > On Fri, Jul 24, 2020 at 07:46:32AM -0700, Andi Kleen wrote:
> > > > Something that seems to 'work' is:
> > > > '{cycles,cpu/instructions,period=50000/}', so maybe you can make the
> > > > group modifier :S use any sampling event if there is one, and otherwise
> > > > designate the leader.
> > > >
> > > > Then you can write things like:
> > > >
> > > >    '{slots, metric1, metric2, cpu/cycles,freq=50000/}:S'
> > > >
> > > > and then since cycles is specified as a sampling event, it will use
> > > > that.
> > >
> > > Okay possible, but it makes things more complicated
> > > for the user to understand and requires special documentation.
> > > Hopefully it's worth it the internal simplification.
> >
> > You already require special documentation for this metrics stuff. We
> > already need to state that SLOTS cannot be a sampling event, so you
> > already need to pay attention to this anyway.
> >
> > A shortcut could be a :s event modifier, then you can write:
> >
> >   '{slots, metric1, metric2, cycles:s}:S'
> >
> > and have the tool select the :s tagged one.

It looks like PT encountered a similar issue as us.
They use the 2nd event of the group as the
"leader". I think we can simply extend the function to check the slots 
event in perf tool.
https://lore.kernel.org/lkml/20200401101613.6201-17-adrian.hunter@xxxxxxxxx/


> Having slots as leader also would allow doing something like
> FORMAT_METRIC, where we return sibling/leader in some fashion.
>
> That also makes sense for instructions, because, IIRC,
> instructions/slots is the better IPC.
>
> And we should probably consider FORMAT_RESET.

What's FORMAT_RESET for?

Thanks,
Kan