Re: Why is PERF_FORMAT_GROUP incompatible with inherited events?

[Peter Zijlstra <peterz@infradead.org>]

On Fri, 2010-02-12 at 14:02 +1100, Paul Mackerras wrote:
> We currently have this code in perf_event_alloc() in kernel/perf_event.c:
> 
> 	/*
> 	 * we currently do not support PERF_FORMAT_GROUP on inherited events
> 	 */
> 	if (attr->inherit && (attr->read_format & PERF_FORMAT_GROUP))
> 		goto done;
> 
> plus there is a comment "XXX PERF_FORMAT_GROUP vs inherited events
> seems difficult" next to perf_output_read_group() (but there isn't a
> similar comment on perf_read_hw()).
> 
> First, what is the difficulty referred to here?

IIRC its the fact that we have to go collect the count delta from all
the child counters, which can be quite a lot of work depending on the
number of cpus and children around.

> Secondly, if the difficulty is just to do with the intersection of
> sampling counters, inheritance, and group readout (as seems to be the
> case), could we please allow group readout on ordinary counting
> (non-sampling) counters?  That is, change the test above to something
> like:
> 
> 	if (attr->inherit && attr->sample_period &&
> 	    (attr->read_format & PERF_FORMAT_GROUP))
> 		goto done;
> 
> Any objections to that change?  If it's OK, could we get it into .33
> and .32-stable?

Yeah, that's still broken, you can't do a read without collecting all
the child counts.