netdev - Re: [PATCH] vfs: avoid taking locks if inode not in lists

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Date:	Wed, 27 Jul 2011 16:44:15 -0400
From:	Christoph Hellwig <hch@...radead.org>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Christoph Hellwig <hch@...radead.org>,
	Tim Chen <tim.c.chen@...ux.intel.com>,
	Al Viro <viro@...IV.linux.org.uk>,
	David Miller <davem@...emloft.net>,
	Andi Kleen <andi@...stfloor.org>,
	Matthew Wilcox <matthew@....cx>,
	Anton Blanchard <anton@...ba.org>, npiggin@...nel.dk,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	netdev <netdev@...r.kernel.org>
Subject: Re: [PATCH] vfs: avoid taking locks if inode not in lists

On Wed, Jul 27, 2011 at 05:21:05PM +0200, Eric Dumazet wrote:
> If I am not mistaken, we can add unlocked checks on the three hot spots.
> 
> After following patch, a close(socket(PF_INET, SOCK_DGRAM, 0)) pair on
> my dev machine takes ~3us instead of ~9us.
> 
> Maybe its better to split it in three patches, just let me know.

I think three patches would be a lot cleaner.

As for safety of the unlocked checks:

 - inode are either hashed when created or never, so that one looks
   fine.
 - same for the sb list.
 - the writeback list is a bit more dynamic as we move things around
   quite a bit.  But in additon to the inode_wb_list_del call from
   evict() it only ever gets remove in writeback_single_inode, which
   for a freeing inode can only be called from the callers of evict().

Btw, I wonder if you should micro-optimize things a bit further by
moving the unhashed checks from the deletion functions into the callers
and thus save a function call for each of them.

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