linux-kernel - Re: [PATCH v8 0/5] kernfs: proposed locking and concurrency improvement

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Date:   Tue, 27 Jul 2021 18:44:18 +0800
From:   Ian Kent <raven@...maw.net>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Tejun Heo <tj@...nel.org>, Eric Sandeen <sandeen@...deen.net>,
        Fox Chen <foxhlchen@...il.com>,
        Brice Goglin <brice.goglin@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        Rick Lindsley <ricklind@...ux.vnet.ibm.com>,
        David Howells <dhowells@...hat.com>,
        Miklos Szeredi <miklos@...redi.hu>,
        Marcelo Tosatti <mtosatti@...hat.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Carlos Maiolino <cmaiolino@...hat.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v8 0/5] kernfs: proposed locking and concurrency
 improvement

On Tue, 2021-07-27 at 12:02 +0200, Greg Kroah-Hartman wrote:
> 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 :)

That's great news Greg, and thanks for putting up with me too, ;)

Ian