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

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

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