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From: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
To: Ian Kent <raven@themaw.net>
Cc: Tejun Heo <tj@kernel.org>, Eric Sandeen <sandeen@sandeen.net>,
	Fox Chen <foxhlchen@gmail.com>,
	Brice Goglin <brice.goglin@gmail.com>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Rick Lindsley <ricklind@linux.vnet.ibm.com>,
	David Howells <dhowells@redhat.com>,
	Miklos Szeredi <miklos@szeredi.hu>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	Carlos Maiolino <cmaiolino@redhat.com>,
	linux-fsdevel <linux-fsdevel@vger.kernel.org>,
	Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH v8 0/5] kernfs: proposed locking and concurrency improvement
Date: Tue, 27 Jul 2021 12:02:41 +0200	[thread overview]
Message-ID: <YP/ZwYrtx+h/a/Ez@kroah.com> (raw)
In-Reply-To: <162642752894.63632.5596341704463755308.stgit@web.messagingengine.com>

On Fri, Jul 16, 2021 at 05:28:13PM +0800, Ian Kent wrote:
> There have been a few instances of contention on the kernfs_mutex during
> path walks, a case on very large IBM systems seen by myself, a report by
> Brice Goglin and followed up by Fox Chen, and I've since seen a couple
> of other reports by CoreOS users.
> 
> The common thread is a large number of kernfs path walks leading to
> slowness of path walks due to kernfs_mutex contention.
> 
> The problem being that changes to the VFS over some time have increased
> it's concurrency capabilities to an extent that kernfs's use of a mutex
> is no longer appropriate. There's also an issue of walks for non-existent
> paths causing contention if there are quite a few of them which is a less
> common problem.
> 
> This patch series is relatively straight forward.
> 
> All it does is add the ability to take advantage of VFS negative dentry
> caching to avoid needless dentry alloc/free cycles for lookups of paths
> that don't exit and change the kernfs_mutex to a read/write semaphore.
> 
> The patch that tried to stay in VFS rcu-walk mode during path walks has
> been dropped for two reasons. First, it doesn't actually give very much
> improvement and, second, if there's a place where mistakes could go
> unnoticed it would be in that path. This makes the patch series simpler
> to review and reduces the likelihood of problems going unnoticed and
> popping up later.
> 
> Changes since v7:
> - remove extra tab in helper kernfs_dir_changed.
> - fix thinko adding an unnecessary kernfs_inc_rev() in kernfs_rename_ns().

Thanks for sticking with this, I've applied this to my testing branch
and let's see how 0-day does with it :)

greg k-h

  parent reply	other threads:[~2021-07-27 10:03 UTC|newest]

Thread overview: 10+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-07-16  9:28 [PATCH v8 0/5] kernfs: proposed locking and concurrency improvement Ian Kent
2021-07-16  9:28 ` [PATCH v8 1/5] kernfs: add a revision to identify directory node changes Ian Kent
2021-07-16  9:28 ` [PATCH v8 2/5] kernfs: use VFS negative dentry caching Ian Kent
2021-07-16  9:28 ` [PATCH v8 3/5] kernfs: switch kernfs to use an rwsem Ian Kent
2021-07-16  9:28 ` [PATCH v8 4/5] kernfs: use i_lock to protect concurrent inode updates Ian Kent
2021-07-16  9:28 ` [PATCH v8 5/5] kernfs: dont call d_splice_alias() under kernfs node lock Ian Kent
2021-07-16 10:16 ` [PATCH v8 0/5] kernfs: proposed locking and concurrency improvement Ian Kent
2021-07-16 10:20   ` Ian Kent
2021-07-27 10:02 ` Greg Kroah-Hartman [this message]
2021-07-27 10:44   ` Ian Kent

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