[PATCH RFC 4/8] fsnotify: stop walking child dentries if remaining tail is negative

Konstantin Khlebnikov
2020-05-08T12:23:28Z

When notification starts/stops listening events from inode's children it
have to update dentry->d_flags of all positive child dentries. Scanning
may took a long time if directory has a lot of negative child dentries.

This is main beneficiary of sweeping cached negative dentries to the end.

Before patch:

nr_dentry = 24172597 24.2M
nr_buckets = 8388608 2.9 avg
nr_unused = 24158110 99.9%
nr_negative = 24142810 99.9%

inotify time: 0.507182 seconds

After patch:

nr_dentry = 24562747 24.6M
nr_buckets = 8388608 2.9 avg
nr_unused = 24548714 99.9%
nr_negative = 24543867 99.9%

inotify time: 0.000010 seconds

Negative dentries no longer slow down inotify op at parent directory.

Signed-off-by: Konstantin Khlebnikov <khlebnikov@yandex-team.ru>
---
fs/notify/fsnotify.c | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)

diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index 72d332ce8e12..072974302950 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -127,8 +127,12 @@ void __fsnotify_update_child_dentry_flags(struct inode *inode)
* original inode) */
spin_lock(&alias->d_lock);
list_for_each_entry(child, &alias->d_subdirs, d_child) {
- if (!child->d_inode)
+ if (!child->d_inode) {
+ /* all remaining children are negative */
+ if (d_is_tail_negative(child))
+ break;
continue;
+ }

spin_lock_nested(&child->d_lock, DENTRY_D_LOCK_NESTED);
if (watched)