linux-kernel - Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless

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Message-ID: <20081113182937.GZ3810@one.firstfloor.org>
Date:	Thu, 13 Nov 2008 19:29:37 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Steven Rostedt <rostedt@...dmis.org>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 1/2] tracing/function-return-tracer: Make the function return tracer lockless

On Thu, Nov 13, 2008 at 12:32:23PM -0500, Steven Rostedt wrote:
> 
> On Thu, 13 Nov 2008, Andi Kleen wrote:
> 
> > > So the answer to this is:
> > > 
> > >   i = index++;
> > >   barrier();
> > >   write to index i (not index);
> > 
> > That was my first thought when I wrote the original email,
> > but the disadvantage is that barrier() is a big hammer
> > that flushes everything and can make the code much worse.
> > That is why I suggested local_add_return() instead.
> 
> barrier() is a compiler barrier, does nothing with the caches, and is 
> quite cheap. We only need a compiler barrier because we are only 

I did not refer to CPU caches, but the compiler's register allocation
[ok if you want the registers are the "level 0 cache"]. A memory barrier
all messes it up. That is why it is better to only clobber specific
memory regions, which is what local_* does.

-Andi

-- 
ak@...ux.intel.com
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