Re: [PATCH V2 1/6] perf,core: allow invalid context events to be part of sw/hw groups

[Andi Kleen <andi@firstfloor.org>]

> ... which would give you arbitrary skew, because one counter is
> free-running and the other is not (when scheduling a context in or out we stop
> the PMU)

Everyone just reads the counter and subtracts it from
the last value they've seen.

That's the same how any other shared free running counter work,
such as the standard timer.

The only thing that perf needs to enforce is that the counters
are running with the same event. 

It also wouldn't work for sampling, but the uncore doesn't do
sampling anyways.

> From my PoV that violates group semantics, because now the events aren't
> always counting at the same time (which would be the reason I grouped
> them in the first place).

You never use the absolute value, just differences. The differences 
effectively count only when the group runs.

> However, it is the case that you cannot offer group semantics.

I don't think that's true.

-Andi