linux-kernel - Re: [PATCH 0/2] jump label: 2.6.38 updates

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Date:	Fri, 11 Feb 2011 16:13:56 -0500
From:	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>
To:	Peter Zijlstra <peterz@...radead.org>
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 0/2] jump label: 2.6.38 updates

* Peter Zijlstra (peterz@...radead.org) wrote:
> On Wed, 2011-01-05 at 10:43 -0500, Jason Baron wrote:
> > Hi,
> > 
> > The first patch uses the storage space of the jump label key address
> > as a pointer into the update table. In this way, we can find all
> > the addresses that need to be updated without hashing.
> > 
> > The second patch introduces:
> > 
> > static __always_inline bool static_branch(struct jump_label_key *key);
> > 
> > instead of the old JUMP_LABEL(key, label) macro.
> > 
> > In this way, jump labels become really easy to use:
> > 
> > Define:
> > 
> >         struct jump_label_key jump_key;
> > 
> > Can be used as:
> > 
> >         if (static_branch(&jump_key))
> >                 do unlikely code
> > 
> > enable/disale via:
> > 
> >         jump_label_enable(&jump_key);
> >         jump_label_disable(&jump_key);
> > 
> > that's it!
> > 
> > For perf, which also uses jump labels, I've left the reference counting
> > out of the jump label layer, thus removing the 'jump_label_inc()' and
> > 'jump_label_dec()' interface. Hopefully, this is a more palatable solution.
> 
> Right, lets go with this. Maybe we'll manage to come up with something
> saner than _else_atomic_read(), but for now its an improvement over what
> we have.

I agree that keeping jump_label.h with the minimal clean API is a good
goal, and this patchset is almost there (maybe except for the
_else_atomic_read() part).

Hrm, given that the atomic inc/dec return and test for 1/0 is moved into
the Perf code, I wonder if it would make sense to move the
"_else_atomic_read()" oddness into the perf code too ? Perf could
declare, in its own header, a wrapper over __static_branch, e.g. put in
perf_event.h:

#ifdef HAVE_JUMP_LABEL
static __always_inline
bool perf_sw_event_static_branch_refcount(struct jump_label_key *key,
                                          atomic_t *ref)
{
	return __static_branch(key);
}
#else
static __always_inline
bool perf_sw_event_static_branch_refcount(struct jump_label_key *key,
                                          atomic_t *ref)
{
	if (unlikely(atomic_read(ref)))
		return true;
	return false;
}
#endif

Otherwise, jump_label_ref.h looks like an half-baked interface that only
provides the "test" API but not the ref/unref. If we have only a single
user interested in refcounting, it might make more sense to put the code
in perf_event.h. If we have many users using an atomic refcount like
this, then we should extend jump_label_ref.h to also provide the
ref/unref API too. I don't care much about where it ends up, as long as
it's a consistent choice.

Thoughts ?

Mathieu

-- 
Mathieu Desnoyers
Operating System Efficiency R&D Consultant
EfficiOS Inc.
http://www.efficios.com
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