linux-kernel - Re: [PATCH 1/9] perf: Remove redundant parent context check from context

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Message-ID: <20140908123200.GC17728@krava.brq.redhat.com>
Date:	Mon, 8 Sep 2014 14:32:00 +0200
From:	Jiri Olsa <jolsa@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Jiri Olsa <jolsa@...nel.org>, linux-kernel@...r.kernel.org,
	Andi Kleen <andi@...stfloor.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Corey Ashford <cjashfor@...ux.vnet.ibm.com>,
	David Ahern <dsahern@...il.com>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...nel.org>,
	"Jen-Cheng(Tommy) Huang" <tommy24@...ech.edu>,
	Namhyung Kim <namhyung@...nel.org>,
	Paul Mackerras <paulus@...ba.org>,
	Stephane Eranian <eranian@...gle.com>
Subject: Re: [PATCH 1/9] perf: Remove redundant parent context check from
 context_equiv

On Mon, Sep 08, 2014 at 01:39:58PM +0200, Peter Zijlstra wrote:
> On Mon, Sep 08, 2014 at 12:01:22PM +0200, Peter Zijlstra wrote:
> > On Mon, Sep 08, 2014 at 11:48:55AM +0200, Peter Zijlstra wrote:
> > 
> > > > The thing is; I don't understand those reasons. That commit log doesn't
> > > > explain.
> > > 
> > > Ah wait, I finally see. I think we want to fix that exit path, not
> > > disallow the cloning.
> > > 
> > > The thing is, by not allowing this optimization simple things like eg.
> > > pipe-test say very expensive.
> > 
> > So its 179033b3e064 ("perf: Add PERF_EVENT_STATE_EXIT state for events
> > with exited task") that introduces the problem. Before that things would
> > work correctly afaict.
> > 
> > The exit would remove from the context but leave the event in existence.
> > Both the fd and the inherited events would have references to it, only
> > once those are gone do we destroy the actual event.
> 
> I have another 'problem' with 179033b3e064. What if you 'want' to
> continue monitoring after the initial task died? Eg. if you want to
> monitor crap that unconditionally daemonizes.
> 
> So at this point I would suggest we revert both 179033b3e064 and
> 1f9a7268c67f so that the context switch optimization works again for
> simple 2 task things (pipe-test).
> 
> If we re-introduce 179033b3e064 (which I think makes sense) we need to
> make sure it works with the context switch optimization (we could add
> perf_event::task I think, it would clean up a few other things iirc) and
> we'd have to make it conditional so we can still monitor daemons.
> 
> Or am I totally missing things again?

as I said in previous email I dont think the 179033b3e064 introduced
the issue and I think we should keep 1f9a7268c67f to have correct
numbers ;-)

I'll think about the fix to keep non-cloned context in optimized switch

jirka
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