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

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Message-ID: <20101216194852.GD2856@redhat.com>
Date:	Thu, 16 Dec 2010 14:48:53 -0500
From:	Jason Baron <jbaron@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>, athieu.desnoyers@...ymtl.ca
Cc:	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 08:41:41PM +0100, Peter Zijlstra wrote:
> 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

i did.

> for everything except some really weird archs (you really shouldn't care
> about).

right, I wasn't sure how much those mattered.

> 
> static inline int atomic_read(const atomic_t *v)
> {
>         return (*(volatile int *)&(v)->counter);
> }
> 
> The volatile simply forces a load to be emitted.

Mathieu, what do you think? Are you ok with an atomic_read() for
checking if a tracepoint is enabled, when jump labels are disabled?

thanks,

-Jason
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