On Wed, May 12, 2021 at 08:38:35AM +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. > > Changes since v3: > - remove unneeded indirection when referencing the super block. > - check if inode attribute update is actually needed. > > Changes since v2: > - actually fix the inode attribute update locking. > - drop the patch that tried to stay in rcu-walk mode. > - drop the use a revision to identify if a directory has changed patch. > > Changes since v1: > - fix locking in .permission() and .getattr() by re-factoring the attribute > handling code. > --- > > Ian Kent (5): > kernfs: move revalidate to be near lookup > kernfs: use VFS negative dentry caching > kernfs: switch kernfs to use an rwsem > kernfs: use i_lock to protect concurrent inode updates > kernfs: add kernfs_need_inode_refresh() > > > fs/kernfs/dir.c | 170 ++++++++++++++++++++---------------- > fs/kernfs/file.c | 4 +- > fs/kernfs/inode.c | 45 ++++++++-- > fs/kernfs/kernfs-internal.h | 5 +- > fs/kernfs/mount.c | 12 +-- > fs/kernfs/symlink.c | 4 +- > include/linux/kernfs.h | 2 +- > 7 files changed, 147 insertions(+), 95 deletions(-) > > -- > Ian > Any benchmark numbers that you ran that are better/worse with this patch series? That woul dbe good to know, otherwise you aren't changing functionality here, so why would we take these changes? :) thanks, greg k-h
