linux-kernel - RE: [PATCH v2 5/5] perf: Enable applicable siblings when group leader is enable-on-exec

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Date:	Tue, 22 Nov 2011 14:45:59 +0100
From:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
To:	"Zhu, DengCheng" <dczhu@...s.com>
Cc:	"Barzilay, Eyal" <eyal@...s.com>,
	"Fortuna, Zenon" <zenon@...s.com>,
	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...e.hu>,
	Arnaldo Carvalho de Melo <acme@...stprotocols.net>,
	"ralf@...ux-mips.org" <ralf@...ux-mips.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH v2 5/5] perf: Enable applicable siblings when group
 leader is enable-on-exec

On Tue, 2011-11-22 at 13:24 +0000, Zhu, DengCheng wrote:
> > ________________________________________
> > From: Peter Zijlstra [a.p.zijlstra@...llo.nl]
> > Sent: Tuesday, November 22, 2011 6:51 PM
> > To: Zhu, DengCheng
> > Cc: Barzilay, Eyal; Fortuna, Zenon; Paul Mackerras; Ingo Molnar; Arnaldo Carvalho de Melo; ralf@...ux-mips.org; LKML
> > Subject: Re: [PATCH v2 5/5] perf: Enable applicable siblings when group leader is enable-on-exec
> >
> > On Tue, 2011-11-22 at 11:30 +0800, Deng-Cheng Zhu wrote:
> >> Currently, when grouped events are created disabled and enable-on-exec, the
> >> siblings won't be enabled on exec in fact. The problem looks like this:
> >
> > Arguably that's a daft thing to do, since if the leader is disabled the
> > group won't get scheduled anyway. But I guess we should at least try to
> > deal with it when people do do it.
> 
> Well, by "grouped events" I mean "all of the grouped events", not only the
> group leader. In fact the leader (and only the leader) will be enabled by
> going through ctx->flexible_groups in perf_event_enable_on_exec().

Right, I understood that. What I said was daft was to tag the
non-leaders as enabled_on_exec,disabled. They wouldn't get scheduled
anyway for as long as the leader is off.

> > Seems perf-stat is a bit daft this way.
> >
> >> This patch fixes it.
> >
> > I guess it does, but its not pretty, event_enable_on_exec() already
> > calls __perf_event_mark_enable(), now this recursion is limited because
> > siblings can't have a sibling list of their own, but still.
> 
> I did it like this just by reading the code comment of
> __perf_event_mark_enabled(): "Enabling the leader of a group effectively
> enables all the group members that aren't explicitly disabled ... Note:
> this works for group members as well as group leaders since the non-leader
> members' sibling_lists will be empty."
> 
> So I suppose dealing with siblings' state in this traversal is the right
> thing to do and introduces minimal code turmoil, although the latter is by
> no means critical.

Yeah, I just don't really like the recursion thing... Also, there's more
ways to get to __perf_event_mark_enabled() and not all those want to
actually do enable_on_exec().

> > The below is a somewhat larger patch that avoids the recursion (and does
> > a small cleanup by eradicating all those useless ctx arguments). Quick
> > testing seems to indicate it works, but please confirm.
> 
> I have no objection of deleting the redundant ctx arguments, but that's
> another topic.

Yeah, I should probably split that into a separate patch.
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