linux-kernel - Re: [PATCH/RFC 1/2] jump label: make enable/disable o(1)

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Date:	Thu, 16 Dec 2010 21:56:25 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
Cc:	Jason Baron <jbaron@...hat.com>, hpa@...or.com,
	rostedt@...dmis.org, mingo@...e.hu, tglx@...utronix.de,
	andi@...stfloor.org, roland@...hat.com, rth@...hat.com,
	masami.hiramatsu.pt@...achi.com, fweisbec@...il.com,
	avi@...hat.com, davem@...emloft.net, sam@...nborg.org,
	ddaney@...iumnetworks.com, michael@...erman.id.au,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH/RFC 1/2] jump label: make enable/disable o(1)

On Thu, 2010-12-16 at 15:50 -0500, Mathieu Desnoyers wrote:
> * Peter Zijlstra (peterz@...radead.org) wrote:
> > On Thu, 2010-12-16 at 15:36 -0500, Mathieu Desnoyers wrote:
> > > Tracepoints keep their own reference counts for enable/disable, so a
> > > simple "enable/disable" is fine as far as tracepoints are concerned. Why
> > > does perf need that refcounting done by the static jumps ?
> > 
> > Because the refcount is all we have... Why not replace that tracepoint
> > refcount with the jumplabel thing?
> 
> The reason why tracepoints need to keep their own refcount is because
> they support dynamically loadable modules, and hence the refcount must
> be kept outside of the modules, in a table internal to tracepoints,
> so we can attach a probe to a yet unloaded module. Therefore, relying on
> this lower level jump label to keep the refcount is not appropriate for
> tracepoints, because the refcount only exists when the module is live.

That's not a logical conclusion, you can keep these jump_label keys
outside of the module just fine.

> I know that your point of view is "let users of modules suffer", but
> this represents a very large portion of Linux users I am not willing to
> let suffer knowingly.

Feh, I'd argue to remove this special tracepoint crap, the only
in-kernel user (ftrace) doesn't even make use of it. This weird ass
tracepoint semantic being different from the ftrace trace_event
semantics has caused trouble before.


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