linux-kernel - [PATCH 2/6] fs: Introduce special dentries for pipes, socket, anon fd

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Date:	Thu, 27 Nov 2008 00:31:40 +0100
From:	Eric Dumazet <dada1@...mosbay.com>
To:	Ingo Molnar <mingo@...e.hu>
CC:	"David S. Miller" <davem@...emloft.net>,
	"Rafael J. Wysocki" <rjw@...k.pl>,
	linux kernel <linux-kernel@...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>,
	Christoph Hellwig <hch@...radead.org>
Subject: [PATCH 2/6] fs: Introduce special 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_SPECIAL flag, to mark a dentry as
a special one (for sockets, pipes, anonymous fd), and a new
d_alloc_special(const struct qstr *name, struct inode *inode)
method, called by the three subsystems.

Internally, dput() can take a fast path to dput_special() for
special dentries, avoiding an expensive atomic_dec_and_lock()

Differences betwen a special dentry and a normal one are :

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

(socket8 bench result : from 27.5s to 24s)

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 |    2 +
 net/socket.c           |   24 +-----------------
 5 files changed, 58 insertions(+), 58 deletions(-)

View attachment "special_dentries.patch" of type "text/plain" (7688 bytes)