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

[peterz]

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.

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.