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

[Andi Kleen <andi@firstfloor.org>]

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@ZenIV.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@linux.intel.com -- Speaking for myself only.