linux-kernel - Re: [PATCH] VFS: br_write_lock locks on possible CPUs other than online CPUs

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Message-ID: <20111222081757.GC11715@one.firstfloor.org>
Date:	Thu, 22 Dec 2011 09:17:57 +0100
From:	Andi Kleen <andi@...stfloor.org>
To:	Al Viro <viro@...IV.linux.org.uk>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	mc@...ux.vnet.ibm.com, Stephen Boyd <sboyd@...eaurora.org>,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	Nick Piggin <npiggin@...nel.dk>, david@...morbit.com,
	Maciej Rutecki <maciej.rutecki@...il.com>,
	Andi Kleen <andi@...stfloor.org>
Subject: Re: [PATCH] VFS: br_write_lock locks on possible CPUs other than online CPUs

On Thu, Dec 22, 2011 at 08:08:56AM +0000, Al Viro wrote:
> On Wed, Dec 21, 2011 at 11:20:47PM -0800, Andrew Morton wrote:
> > On Thu, 22 Dec 2011 07:02:15 +0000 Al Viro <viro@...IV.linux.org.uk> wrote:
> > 
> > > On Wed, Dec 21, 2011 at 02:12:29PM -0800, Andrew Morton wrote:
> > > > off-topic, but the lockdep stuff in include/linux/lglock.h
> > > > (LOCKDEP_INIT_MAP and DEFINE_LGLOCK_LOCKDEP) appears to be dead code.
> > > 
> > > Um?  See ..._lock_init(); it's used there.
> > 
> > oops, I had Andi's patch applied.
> > 
> > Wanna take a look at it while things are fresh in your mind?
> 
> a) tons of trivial conflicts with fs/namespace.c changes in my tree
> b) more seriously, the question of overhead - see the mail you replied
> to.
> 

The costly operations here are the atomics and nothing really changes
for them. So I don't expect any measurable difference.

I actually have an idea to remove them for the common case, but not in 
that patchkit or cycle :)

I can run a ftrace if you want, but I expect any difference to be below
the measurement inaccuracy.

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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