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

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Date:	Fri, 17 Dec 2010 15:07:38 -0500
From:	Jason Baron <jbaron@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>, 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, Dec 16, 2010 at 09:56:25PM +0100, Peter Zijlstra wrote:
> 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.
> 
> 

Hi,

since atomic_t is just an 'int' from include/linux/types.h, so for all
arches. We can cast any refernces to an atomic_t in
include/linux/jump_label_ref.h

So for when jump labels are disabled case we could have
one struct:

struct jump_label_key {
	int state;
}

and then we could then have (rough c code):

jump_label_enable(struct jump_label_key *key)
{
	key->state = 1;
}

jump_label_disable(struct jump_label_key *key)
{
	key->state = 0;
}

jump_label_inc(struct jump_label_key *key)
{
	atomic_inc((atomic_t *)key)
}

jump_label_dec(struct jump_label_key *key)
{
	atomic_dec((atomic_t *)key)
}

bool unlikely_switch(struct jump_label_key *key)
{
	if (key->state)
		return true;
	return false;
}

bool unlikely_switch_atomic(struct jump_label_key *key)
{
	if (atomic_read((atomic_t *)key)
		return true;
	return false;
}

can we agree on something like this?

thanks,

-Jason	
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