From: Al Viro <viro@ZenIV.linux.org.uk>
To: Andi Kleen <andi@firstfloor.org>
Cc: Andrew Morton <akpm@linux-foundation.org>,
"Srivatsa S. Bhat" <srivatsa.bhat@linux.vnet.ibm.com>,
mc@linux.vnet.ibm.com, Stephen Boyd <sboyd@codeaurora.org>,
linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
Nick Piggin <npiggin@kernel.dk>,
david@fromorbit.com, Maciej Rutecki <maciej.rutecki@gmail.com>
Subject: Re: [PATCH] VFS: br_write_lock locks on possible CPUs other than online CPUs
Date: Thu, 22 Dec 2011 08:39:52 +0000 [thread overview]
Message-ID: <20111222083952.GN23916@ZenIV.linux.org.uk> (raw)
In-Reply-To: <20111222081757.GC11715@one.firstfloor.org>
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...
next prev parent reply other threads:[~2011-12-22 8:39 UTC|newest]
Thread overview: 37+ messages / expand[flat|nested] mbox.gz Atom feed top
2011-12-19 3:36 [PATCH] VFS: br_write_lock locks on possible CPUs other than online CPUs mengcong
2011-12-19 4:11 ` Al Viro
2011-12-19 5:00 ` Dave Chinner
2011-12-19 6:07 ` mengcong
2011-12-19 7:31 ` Srivatsa S. Bhat
2011-12-19 9:12 ` Stephen Boyd
2011-12-19 11:03 ` Srivatsa S. Bhat
2011-12-19 12:11 ` Al Viro
2011-12-19 20:23 ` Srivatsa S. Bhat
2011-12-19 20:52 ` Al Viro
2011-12-20 4:56 ` Srivatsa S. Bhat
2011-12-20 6:27 ` Al Viro
2011-12-20 7:28 ` Srivatsa S. Bhat
2011-12-20 9:37 ` mengcong
2011-12-20 10:36 ` Srivatsa S. Bhat
2011-12-20 11:08 ` Srivatsa S. Bhat
2011-12-20 12:50 ` Srivatsa S. Bhat
2011-12-20 14:06 ` Al Viro
2011-12-20 14:35 ` Srivatsa S. Bhat
2011-12-20 17:59 ` Al Viro
2011-12-20 19:12 ` Srivatsa S. Bhat
2011-12-20 19:58 ` Al Viro
2011-12-20 22:27 ` Dave Chinner
2011-12-20 23:31 ` Al Viro
2011-12-21 21:15 ` Srivatsa S. Bhat
2011-12-21 22:02 ` Al Viro
2011-12-21 22:12 ` Andrew Morton
2011-12-22 7:02 ` Al Viro
2011-12-22 7:20 ` Andrew Morton
2011-12-22 8:08 ` Al Viro
2011-12-22 8:17 ` Andi Kleen
2011-12-22 8:39 ` Al Viro [this message]
2011-12-22 8:22 ` Andi Kleen
2011-12-20 7:30 ` mengcong
2011-12-20 7:37 ` Srivatsa S. Bhat
2011-12-19 23:56 ` Dave Chinner
2011-12-20 4:05 ` Al Viro
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