linux-kernel - Re: perf: FORMAT_GROUP on attached process stops working in 3.16-rc

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Message-ID: <20140715143745.GK9918@twins.programming.kicks-ass.net>
Date:	Tue, 15 Jul 2014 16:37:45 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Vince Weaver <vincent.weaver@...ne.edu>
Cc:	linux-kernel@...r.kernel.org, Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: perf: FORMAT_GROUP on attached process stops working in 3.16-rc

On Tue, Jul 15, 2014 at 10:36:33AM -0400, Vince Weaver wrote:
> Hello
> 
> I noticed in the 3.16-rc kernels that one of my regression tests was 
> failing, specifically
> 	https://github.com/deater/perf_event_tests/blob/master/tests/bugs/read_group_attached.c
> 
> This test creates an event group with two events and PERF_FORMAT_GROUP
> set.
> 
> It creates a child process, stops it, attaches the event group to it, 
> starts the events and the child, waits for the child to finish, stops the
> events, then reads the value.
> 
> In theory a subsequent read from the group leader should return values for 
> *both* events (due to PERF_FORMAT_GROUP) but this stopped working, it only 
> returns the value for the leader.
> 
> I've bisected it down to change: 15a2d4de0eab5 
> 	"perf: Always destroy groups on exit"
> 
> So is this expected behavior due to this patch, or is it a regression?

Regression, lemme try and wrap my head around it. Thanks!

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