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authorMel Gorman <mgorman@techsingularity.net>2020-07-08 14:11:36 +0300
committerJan Kara <jack@suse.cz>2020-07-15 16:29:10 +0300
commit71d734103edfa2b4c6657578a3082ee0e51d767e (patch)
tree77a67eb2805d3bbf8e6c48d3b4bb68f2ede48ce2 /include/linux/fsnotify.h
parent47aaabdedf366ac5894c7fddec388832f0d8193e (diff)
downloadlinux-71d734103edfa2b4c6657578a3082ee0e51d767e.tar.xz
fsnotify: Rearrange fast path to minimise overhead when there is no watcher
The fsnotify paths are trivial to hit even when there are no watchers and they are surprisingly expensive. For example, every successful vfs_write() hits fsnotify_modify which calls both fsnotify_parent and fsnotify unless FMODE_NONOTIFY is set which is an internal flag invisible to userspace. As it stands, fsnotify_parent is a guaranteed functional call even if there are no watchers and fsnotify() does a substantial amount of unnecessary work before it checks if there are any watchers. A perf profile showed that applying mnt->mnt_fsnotify_mask in fnotify() was almost half of the total samples taken in that function during a test. This patch rearranges the fast paths to reduce the amount of work done when there are no watchers. The test motivating this was "perf bench sched messaging --pipe". Despite the fact the pipes are anonymous, fsnotify is still called a lot and the overhead is noticeable even though it's completely pointless. It's likely the overhead is negligible for real IO so this is an extreme example. This is a comparison of hackbench using processes and pipes on a 1-socket machine with 8 CPU threads without fanotify watchers. 5.7.0 5.7.0 vanilla fastfsnotify-v1r1 Amean 1 0.4837 ( 0.00%) 0.4630 * 4.27%* Amean 3 1.5447 ( 0.00%) 1.4557 ( 5.76%) Amean 5 2.6037 ( 0.00%) 2.4363 ( 6.43%) Amean 7 3.5987 ( 0.00%) 3.4757 ( 3.42%) Amean 12 5.8267 ( 0.00%) 5.6983 ( 2.20%) Amean 18 8.4400 ( 0.00%) 8.1327 ( 3.64%) Amean 24 11.0187 ( 0.00%) 10.0290 * 8.98%* Amean 30 13.1013 ( 0.00%) 12.8510 ( 1.91%) Amean 32 13.9190 ( 0.00%) 13.2410 ( 4.87%) 5.7.0 5.7.0 vanilla fastfsnotify-v1r1 Duration User 157.05 152.79 Duration System 1279.98 1219.32 Duration Elapsed 182.81 174.52 This is showing that the latencies are improved by roughly 2-9%. The variability is not shown but some of these results are within the noise as this workload heavily overloads the machine. That said, the system CPU usage is reduced by quite a bit so it makes sense to avoid the overhead even if it is a bit tricky to detect at times. A perf profile of just 1 group of tasks showed that 5.14% of samples taken were in either fsnotify() or fsnotify_parent(). With the patch, 2.8% of samples were in fsnotify, mostly function entry and the initial check for watchers. The check for watchers is complicated enough that inlining it may be controversial. [Amir] Slightly simplify with mnt_or_sb_mask => marks_mask Link: https://lore.kernel.org/r/20200708111156.24659-1-amir73il@gmail.com Signed-off-by: Mel Gorman <mgorman@techsingularity.net> Signed-off-by: Amir Goldstein <amir73il@gmail.com> Signed-off-by: Jan Kara <jack@suse.cz>
Diffstat (limited to 'include/linux/fsnotify.h')
-rw-r--r--include/linux/fsnotify.h10
1 files changed, 10 insertions, 0 deletions
diff --git a/include/linux/fsnotify.h b/include/linux/fsnotify.h
index 5ab28f6c7d26..508f6bb0b06b 100644
--- a/include/linux/fsnotify.h
+++ b/include/linux/fsnotify.h
@@ -44,6 +44,16 @@ static inline void fsnotify_dirent(struct inode *dir, struct dentry *dentry,
fsnotify_name(dir, mask, d_inode(dentry), &dentry->d_name, 0);
}
+/* Notify this dentry's parent about a child's events. */
+static inline int fsnotify_parent(struct dentry *dentry, __u32 mask,
+ const void *data, int data_type)
+{
+ if (!(dentry->d_flags & DCACHE_FSNOTIFY_PARENT_WATCHED))
+ return 0;
+
+ return __fsnotify_parent(dentry, mask, data, data_type);
+}
+
/*
* Simple wrappers to consolidate calls fsnotify_parent()/fsnotify() when
* an event is on a file/dentry.