linux-kernel - Re: [PATCH 25/33] aio: use xchg() instead of completion

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Message-ID: <20130402163514.GI31577@thunk.org>
Date:	Tue, 2 Apr 2013 12:35:14 -0400
From:	Theodore Ts'o <tytso@....edu>
To:	Kent Overstreet <koverstreet@...gle.com>
Cc:	linux-kernel@...r.kernel.org, linux-aio@...ck.org,
	akpm@...ux-foundation.org, Zach Brown <zab@...hat.com>,
	Felipe Balbi <balbi@...com>,
	Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
	Mark Fasheh <mfasheh@...e.com>,
	Joel Becker <jlbec@...lplan.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Jens Axboe <axboe@...nel.dk>,
	Asai Thambi S P <asamymuthupa@...ron.com>,
	Selvan Mani <smani@...ron.com>,
	Sam Bradshaw <sbradshaw@...ron.com>,
	Jeff Moyer <jmoyer@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>,
	Benjamin LaHaise <bcrl@...ck.org>
Subject: Re: [PATCH 25/33] aio: use xchg() instead of completion_lock

On Thu, Mar 21, 2013 at 09:35:46AM -0700, Kent Overstreet wrote:
> So, for sticking kiocb completions on the kioctx ringbuffer, we need a
> lock - it unfortunately can't be lockless.
> 
> When the kioctx is shared between threads on different cpus and the rate
> of completions is high, this lock sees quite a bit of contention - in
> terms of cacheline contention it's the hottest thing in the aio subsystem.
> 
> That means, with a regular spinlock, we're going to take a cache miss to
> grab the lock, then another cache miss when we touch the data the lock
> protects - if it's on the same cacheline as the lock, other cpus spinning
> on the lock are going to be pulling it out from under us as we're using
> it.
> 
> So, we use an old trick to get rid of this second forced cache miss - make
> the data the lock protects be the lock itself, so we grab them both at
> once.

Reviewed-by: "Theodore Ts'o" <tytso@....edu>
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