path: root/fs/file_table.c
diff options
authorMel Gorman <mgorman@techsingularity.net>2020-06-15 13:13:58 +0100
committerJan Kara <jack@suse.cz>2020-06-16 09:40:45 +0200
commite9c15badbb7b20ccdbadf5da14e0a68fbad51015 (patch)
tree9e53ac24e72b8c17d673894097f00c883b5d1263 /fs/file_table.c
parentscripts/decode_stacktrace: warn when modpath is needed but is unset (diff)
fs: Do not check if there is a fsnotify watcher on pseudo inodes
The kernel uses internal mounts created by kern_mount() and populated with files with no lookup path by alloc_file_pseudo() for a variety of reasons. An example of such a mount is for anonymous pipes. For pipes, every vfs_write() regardless of filesystem, calls fsnotify_modify() to notify of any changes which incurs a small amount of overhead in fsnotify even when there are no watchers. It can also trigger for reads and readv and writev, it was simply vfs_write() that was noticed first. A patch is pending that reduces, but does not eliminate, the overhead of fsnotify but for files that cannot be looked up via a path, even that small overhead is unnecessary. The user API for all notification subsystems (inotify, fanotify, ...) is based on the pathname and a dirfd and proc entries appear to be the only visible representation of the files. Proc does not have the same pathname as the internal entry and the proc inode is not the same as the internal inode so even if fanotify is used on a file under /proc/XX/fd, no useful events are notified. This patch changes alloc_file_pseudo() to always opt out of fsnotify by setting FMODE_NONOTIFY flag so that no check is made for fsnotify watchers on pseudo files. This should be safe as the underlying helper for the dentry is d_alloc_pseudo() which explicitly states that no lookups are ever performed meaning that fanotify should have nothing useful to attach to. The test motivating this was "perf bench sched messaging --pipe". On a single-socket machine using threads the difference of the patch was as follows. 5.7.0 5.7.0 vanilla nofsnotify-v1r1 Amean 1 1.3837 ( 0.00%) 1.3547 ( 2.10%) Amean 3 3.7360 ( 0.00%) 3.6543 ( 2.19%) Amean 5 5.8130 ( 0.00%) 5.7233 * 1.54%* Amean 7 8.1490 ( 0.00%) 7.9730 * 2.16%* Amean 12 14.6843 ( 0.00%) 14.1820 ( 3.42%) Amean 18 21.8840 ( 0.00%) 21.7460 ( 0.63%) Amean 24 28.8697 ( 0.00%) 29.1680 ( -1.03%) Amean 30 36.0787 ( 0.00%) 35.2640 * 2.26%* Amean 32 38.0527 ( 0.00%) 38.1223 ( -0.18%) The difference is small but in some cases it's outside the noise so while marginal, there is still some small benefit to ignoring fsnotify for files allocated via alloc_file_pseudo() in some cases. Link: https://lore.kernel.org/r/20200615121358.GF3183@techsingularity.net Signed-off-by: Mel Gorman <mgorman@techsingularity.net> Reviewed-by: Amir Goldstein <amir73il@gmail.com> Signed-off-by: Jan Kara <jack@suse.cz>
Diffstat (limited to 'fs/file_table.c')
1 files changed, 1 insertions, 1 deletions
diff --git a/fs/file_table.c b/fs/file_table.c
index 656647f9575a..65603502fed6 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -230,7 +230,7 @@ struct file *alloc_file_pseudo(struct inode *inode, struct vfsmount *mnt,
d_set_d_op(path.dentry, &anon_ops);
path.mnt = mntget(mnt);
d_instantiate(path.dentry, inode);
- file = alloc_file(&path, flags, fops);
+ file = alloc_file(&path, flags | FMODE_NONOTIFY, fops);
if (IS_ERR(file)) {