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

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Message-ID: <20140908113958.GT3588@twins.programming.kicks-ass.net>
Date:	Mon, 8 Sep 2014 13:39:58 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jiri Olsa <jolsa@...hat.com>
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 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?

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