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

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Message-ID: <20111222083952.GN23916@ZenIV.linux.org.uk>
Date:	Thu, 22 Dec 2011 08:39:52 +0000
From:	Al Viro <viro@...IV.linux.org.uk>
To:	Andi Kleen <andi@...stfloor.org>
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>
Subject: Re: [PATCH] VFS: br_write_lock locks on possible CPUs other than
 online CPUs

On Thu, Dec 22, 2011 at 09:17:57AM +0100, Andi Kleen wrote:

> 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.

What I'm concerned with is not the cost of extra dereference per se; it's
more about cacheline bouncing - note that with this fix in place you'll
end up with spinlock in the same cacheline as the pointer to per-cpu stuff.
Hell knows; it might not matter at all, since we take it only for
br_write_lock() (and definitely rare CPU up/down), but still, I'd like to
see the data.  In any case, that's not -stable material.  The race fix,
OTOH, is...
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