netdev - [PATCH v2 4/5] fs: Introduce SINGLE dentries for pipes, socket, anon fd

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Date:	Sat, 29 Nov 2008 09:44:23 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Ingo Molnar <mingo@...e.hu>, Christoph Hellwig <hch@...radead.org>
CC:	David Miller <davem@...emloft.net>,
	"Rafael J. Wysocki" <rjw@...k.pl>, linux-kernel@...r.kernel.org,
	"kernel-testers@...r.kernel.org >> Kernel Testers List" 
	<kernel-testers@...r.kernel.org>, Mike Galbraith <efault@....de>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Linux Netdev List <netdev@...r.kernel.org>,
	Christoph Lameter <cl@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, Al Viro <viro@...IV.linux.org.uk>
Subject: [PATCH v2 4/5] fs: Introduce SINGLE dentries for pipes, socket, anon
 fd

Sockets, pipes and anonymous fds have interesting properties.

Like other files, they use a dentry and an inode.

But dentries for these kind of files are not hashed into dcache,
since there is no way someone can lookup such a file in the vfs tree.
(/proc/{pid}/fd/{number} uses a different mechanism)

Still, allocating and freeing such dentries are expensive processes,
because we currently take dcache_lock inside d_alloc(), d_instantiate(),
and dput(). This lock is very contended on SMP machines.

This patch defines a new DCACHE_SINGLE flag, to mark a dentry as
a single one (for sockets, pipes, anonymous fd), and a new
d_alloc_single(const struct qstr *name, struct inode *inode)
method, called by the three subsystems.

Internally, dput() can take a fast path to dput_single() for
SINGLE dentries. No more atomic_dec_and_lock()
for such dentries.

Differences betwen an SINGLE dentry and a normal one are :

1) SINGLE dentry has the DCACHE_SINGLE flag
2) SINGLE dentry's parent is itself (DCACHE_DISCONNECTED)
This to avoid taking a reference on sb 'root' dentry, shared
by too many dentries.
3) They are not hashed into global hash table (DCACHE_UNHASHED)
4) Their d_alias list is empty

(socket8 bench result : from 25s to 19.9s)

Signed-off-by: Eric Dumazet <dada1@...mosbay.com>
---
 fs/anon_inodes.c       |   16 ------------
 fs/dcache.c            |   51 +++++++++++++++++++++++++++++++++++++++
 fs/pipe.c              |   23 +----------------
 include/linux/dcache.h |    9 ++++++
 net/socket.c           |   24 +-----------------
 5 files changed, 65 insertions(+), 58 deletions(-)

View attachment "dcache_single.patch" of type "text/plain" (7887 bytes)