linux-kernel - Re: perf: child events not killed on release paths, survive indefinitely

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Date:	Fri, 18 Jul 2014 16:42:48 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Mark Rutland <mark.rutland@....com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Paul Mackerras <paulus@...ba.org>,
	Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	"Sahasrabudhe, Sheetal" <sheetals@...cinc.com>,
	Will Deacon <Will.Deacon@....com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: perf: child events not killed on release paths, survive
 indefinitely

On Fri, Jul 18, 2014 at 03:31:57PM +0100, Mark Rutland wrote:

SNIP

> > > I'm not sure what the best way of handling this is. We need to clean up
> > > the children when the last possible user of the event is gone, but it
> > > looks to me like we'd need to have a separate child_refcount or
> > > reader_refcount to be able to tell when the events are still useful and
> > > when the only references which remain are internal.
> > > 
> > > Any ideas?
> > 
> > Jiri was recently poking at that:
> > 
> > lkml.kernel.org/r/1405079782-8139-3-git-send-email-jolsa@...nel.org
> 
> Ah. I hadn't spotted that, thanks for the link.
> 
> That approach (closing child events when the owner exits) doesn't seem
> to fix the general case, as long running tasks (think interactive
> debugger/profiler) could open and close many events before exiting, if
> nothing else wasting memory until it does so.
> 
> My test case triggers with said patch applied (before hanging, probably
> due to the AB-BA deadlock).

yep, peter already found that
http://marc.info/?l=linux-kernel&m=140541548218652&w=2

> 
> Jiri, could you add me to Cc for future versions of that series?
> 
> I'll have a look and see if I can come up with something; otherwise I'm
> happy to test/review. :)

sure, and vice versa ;-)

jirka
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