From: Fox Chen <foxhlchen@gmail.com>
To: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Ian Kent <raven@themaw.net>, Tejun Heo <tj@kernel.org>,
Eric Sandeen <sandeen@sandeen.net>,
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>,
linux-fsdevel <linux-fsdevel@vger.kernel.org>,
Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [REPOST PATCH v4 0/5] kernfs: proposed locking and concurrency improvement
Date: Fri, 28 May 2021 19:56:10 +0800 [thread overview]
Message-ID: <CAC2o3D+GH31Q2oxDgBVu8BfEUXuDovUM7uKJq=uHb3Ay+WjhSQ@mail.gmail.com> (raw)
In-Reply-To: <YLCwIfYxM7jYKQxe@kroah.com>
On Fri, May 28, 2021 at 4:56 PM Greg Kroah-Hartman
<gregkh@linuxfoundation.org> wrote:
>
> On Fri, May 28, 2021 at 02:33:42PM +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.
> >
> > The patch to use a revision to identify if a directory has changed has
> > also been dropped. If the directory has changed the dentry revision
> > needs to be updated to avoid subsequent rb tree searches and after
> > changing to use a read/write semaphore the update also requires a lock.
> > But the d_lock is the only lock available at this point which might
> > itself be contended.
>
> Fox, can you take some time and test these to verify it all still works
> properly with your benchmarks?
Sure, I will take a look.
Actually, I've tested it before, but I will test it again to confirm it.
> thanks,
>
> greg k-h
thanks,
fox
next prev parent reply other threads:[~2021-05-28 11:56 UTC|newest]
Thread overview: 30+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-05-28 6:33 [REPOST PATCH v4 0/5] kernfs: proposed locking and concurrency improvement Ian Kent
2021-05-28 6:33 ` [REPOST PATCH v4 1/5] kernfs: move revalidate to be near lookup Ian Kent
2021-06-03 14:50 ` Eric W. Biederman
2021-06-04 2:29 ` Ian Kent
2021-05-28 6:34 ` [REPOST PATCH v4 2/5] kernfs: use VFS negative dentry caching Ian Kent
2021-06-01 12:41 ` Miklos Szeredi
2021-06-02 3:44 ` Ian Kent
2021-06-02 8:58 ` Miklos Szeredi
2021-06-02 10:57 ` Ian Kent
2021-06-03 2:15 ` Ian Kent
2021-06-03 23:57 ` Ian Kent
2021-06-04 1:07 ` Ian Kent
2021-06-03 17:26 ` Eric W. Biederman
2021-06-03 18:06 ` Miklos Szeredi
2021-06-03 22:02 ` Eric W. Biederman
2021-06-04 3:14 ` Ian Kent
2021-06-04 14:28 ` Eric W. Biederman
2021-06-05 3:19 ` Ian Kent
2021-06-05 20:52 ` Eric W. Biederman
2021-05-28 6:34 ` [REPOST PATCH v4 3/5] kernfs: switch kernfs to use an rwsem Ian Kent
2021-06-01 13:11 ` Miklos Szeredi
2021-06-03 16:59 ` Eric W. Biederman
2021-05-28 6:34 ` [REPOST PATCH v4 4/5] kernfs: use i_lock to protect concurrent inode updates Ian Kent
2021-05-31 14:53 ` [kernfs] 9a658329cd: stress-ng.get.ops_per_sec 191.4% improvement kernel test robot
2021-06-01 13:18 ` [REPOST PATCH v4 4/5] kernfs: use i_lock to protect concurrent inode updates Miklos Szeredi
2021-06-02 5:41 ` Ian Kent
2021-05-28 6:34 ` [REPOST PATCH v4 5/5] kernfs: add kernfs_need_inode_refresh() Ian Kent
2021-05-28 8:56 ` [REPOST PATCH v4 0/5] kernfs: proposed locking and concurrency improvement Greg Kroah-Hartman
2021-05-28 11:56 ` Fox Chen [this message]
2021-05-30 4:44 ` Fox Chen
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