linux-kernel - Re: [PATCH 2/5] perf: Destroy event's children on task exit

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Date:	Wed, 16 Jul 2014 14:20:47 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jiri Olsa <jolsa@...nel.org>, linux-kernel@...r.kernel.org,
	Jiri Olsa <jolsa@...hat.com>,
	Alexander Yarygin <yarygin@...ux.vnet.ibm.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH 2/5] perf: Destroy event's children on task exit


* Peter Zijlstra <peterz@...radead.org> wrote:

> On Fri, Jul 11, 2014 at 01:56:19PM +0200, Jiri Olsa wrote:
> > From: Jiri Olsa <jolsa@...hat.com>
> > 
> > When task exits we close:
> >   1) all events that are installed in task
> >   2) all events owned by task (via file descriptor)
> > 
> > But we don't close children events of 2) events. Those children
> > events stay until the child task exits and are useless with the
> > parent being gone, because we have no way to get to values any
> > more.
> > 
> > Plus if the event stays installed in task even with the owner task
> > gone, it runs the perf callback any time the task forks, for no
> > real reason.
> > 
> > Closing all children events events when the owner task of the
> > parent event is closed.
> 
> So I _think_ the reason we didn't do this is because this is potentially
> very expensive and keeping them around isn't too much bother, they'll
> die eventually.
> 
> But I can't really remember. Ingo any recollections / opinions?

As far as I can remember it's just a historic path dependent 
implementation detail, not really a conscious choice: initially we 
didn't remove events (because we couldn't), and then it remained so 
because it broke nothing.

Thanks,

	Ingo
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