linux-kernel - Re: [rfc][patch] store-free path walking

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Message-ID: <20091009104808.GZ9228@kernel.dk>
Date:	Fri, 9 Oct 2009 12:48:09 +0200
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Nick Piggin <npiggin@...e.de>
Cc:	David Miller <davem@...emloft.net>, torvalds@...ux-foundation.org,
	linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
	kiran@...lex86.org, peterz@...radead.org
Subject: Re: [rfc][patch] store-free path walking

On Fri, Oct 09 2009, Nick Piggin wrote:
> On Thu, Oct 08, 2009 at 11:15:27PM -0700, David Miller wrote:
> > From: Nick Piggin <npiggin@...e.de>
> > Date: Fri, 9 Oct 2009 05:50:50 +0200
> > 
> > > OK, I got rid of this guy from the RCU walk. Basically now hold
> > > vfsmount_lock over the entire RCU path walk (which also pins the mnt)
> > > and use a seqlock in the fs struct to get a consistent mnt,dentry
> > > pair. This also simplifies the walk because we don't need the
> > > complexity to avoid mntget/mntput (just do one final mntget on the
> > > resulting mnt before dropping vfsmount_lock).
> > > 
> > > vfsmount_lock adds one per-cpu atomic for the spinlock, and we
> > > remove two thread-shared atomics for fs->lock so a net win for
> > > both single threaded performance and thread-shared scalability.
> > > Latency is no problem because we hold rcu_read_lock for the same
> > > length of time anyway.
> > > 
> > > The parallel git diff workload is improved by serveral percent.
> > 
> > Sounds sweet Nick, can't wait to play with your next set of
> > patches here.
> 
> Oh and the RCU patch there just in case you want to try some file
> open/close benchmarks. Turns out I broke Paul's new scalable RCU ;)

In that case, you may want to fix the permissions on that file :-)

I'll try the new rolled up patch.

-- 
Jens Axboe

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