Re: Why is PERF_FORMAT_GROUP incompatible with inherited events?

[Peter Zijlstra <peterz@infradead.org>]

On Mon, 2010-02-15 at 15:56 +1100, Paul Mackerras wrote:
> On Sun, Feb 14, 2010 at 01:38:27PM +0100, Peter Zijlstra wrote:
> > On Sun, 2010-02-14 at 22:33 +1100, Paul Mackerras wrote:
> > > 
> > > But we don't go and collect the count delta from children without
> > > PERF_FORMAT_GROUP, so why would we with it? 
> > 
> > Yes we do, see perf_event_read_value().
> 
> Ah, true, I should have read the code more carefully.
> 
> > But now that I look at it we don't seem to do so in
> > perf_output_read_one()... I guess we should fix that.
> 
> I suppose it should give the same value as read() would, but the
> possibly unbounded interrupt latency is a bit of a worry.  I can't
> think of a way to avoid it, though (other than not using
> PERF_SAMPLE_READ with inherited sampling events :).
> 
> > There is of course the lock inversion in the .read() code reported by
> > stephane, but other than that is seems to actually support inherited &&
> > group just fine.
> > 
> > So I think if we fix that lock inversion and make the PERF_SAMPLE_READ
> > code look like the .read() code it should all work out.

I now realize that this is going to be very complicated because it
involves sending IPIs from NMI context, which is rather involved.

So I might have meant:
  attr->inherit && (attr->sample_format & PERF_SAMPLE_READ)

to be mutually exclusive.