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

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Date:	Thu, 16 Dec 2010 20:41:41 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Jason Baron <jbaron@...hat.com>
Cc:	hpa@...or.com, rostedt@...dmis.org, mingo@...e.hu,
	mathieu.desnoyers@...ymtl.ca, 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 14:36 -0500, Jason Baron wrote:
> On Thu, Dec 16, 2010 at 08:33:51PM +0100, Peter Zijlstra wrote:
> > On Thu, 2010-12-16 at 14:23 -0500, Jason Baron wrote:
> > > 
> > > For the jump label disabled case, perf is using atomic_inc/dec and atomic_read
> > > to check if enabled. While other consumers (tracepoints) are just using an
> > > 'int'. I didn't want hurt the jump label disabled case for tracepoints.
> > > If we can agree to use atomic ops for tracepoints, or drop atomics from
> > > perf, that would simplify things. 
> > 
> > I had a quick look at the tracepoint stuff but got lost, but surely it
> > has a reference count somewhere as well, it needs to know when the last
> > probe goes away.. or does it check if the list is empty?
> > 
> > Anyway, tracepoint enable/disable isn't a real fast-path, surely it
> > could suffer an atomic op?
> 
> It is the atomic_read() at the tracepoint site that I am concerned
> about.

Look at the implementation :-), its just wrapper foo, its a regular read
for everything except some really weird archs (you really shouldn't care
about).

static inline int atomic_read(const atomic_t *v)
{
        return (*(volatile int *)&(v)->counter);
}

The volatile simply forces a load to be emitted.
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