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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>
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@ -143,7 +143,7 @@ void __fsnotify_update_child_dentry_flags(struct inode *inode)
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}
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/* Notify this dentry's parent about a child's events. */
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int fsnotify_parent(struct dentry *dentry, __u32 mask, const void *data,
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int __fsnotify_parent(struct dentry *dentry, __u32 mask, const void *data,
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int data_type)
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{
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struct dentry *parent;
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@ -174,7 +174,7 @@ int fsnotify_parent(struct dentry *dentry, __u32 mask, const void *data,
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return ret;
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}
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EXPORT_SYMBOL_GPL(fsnotify_parent);
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EXPORT_SYMBOL_GPL(__fsnotify_parent);
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static int send_to_group(struct inode *to_tell,
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__u32 mask, const void *data,
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@ -315,17 +315,11 @@ int fsnotify(struct inode *to_tell, __u32 mask, const void *data, int data_is,
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struct fsnotify_iter_info iter_info = {};
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struct super_block *sb = to_tell->i_sb;
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struct mount *mnt = NULL;
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__u32 mnt_or_sb_mask = sb->s_fsnotify_mask;
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int ret = 0;
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__u32 test_mask = (mask & ALL_FSNOTIFY_EVENTS);
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__u32 test_mask, marks_mask;
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if (path) {
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if (path)
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mnt = real_mount(path->mnt);
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mnt_or_sb_mask |= mnt->mnt_fsnotify_mask;
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}
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/* An event "on child" is not intended for a mount/sb mark */
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if (mask & FS_EVENT_ON_CHILD)
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mnt_or_sb_mask = 0;
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/*
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* Optimization: srcu_read_lock() has a memory barrier which can
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@ -337,13 +331,22 @@ int fsnotify(struct inode *to_tell, __u32 mask, const void *data, int data_is,
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if (!to_tell->i_fsnotify_marks && !sb->s_fsnotify_marks &&
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(!mnt || !mnt->mnt_fsnotify_marks))
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return 0;
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/* An event "on child" is not intended for a mount/sb mark */
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marks_mask = to_tell->i_fsnotify_mask;
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if (!(mask & FS_EVENT_ON_CHILD)) {
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marks_mask |= sb->s_fsnotify_mask;
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if (mnt)
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marks_mask |= mnt->mnt_fsnotify_mask;
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}
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/*
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* if this is a modify event we may need to clear the ignored masks
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* otherwise return if neither the inode nor the vfsmount/sb care about
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* this type of event.
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*/
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if (!(mask & FS_MODIFY) &&
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!(test_mask & (to_tell->i_fsnotify_mask | mnt_or_sb_mask)))
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test_mask = (mask & ALL_FSNOTIFY_EVENTS);
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if (!(mask & FS_MODIFY) && !(test_mask & marks_mask))
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return 0;
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iter_info.srcu_idx = srcu_read_lock(&fsnotify_mark_srcu);
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@ -44,6 +44,16 @@ static inline void fsnotify_dirent(struct inode *dir, struct dentry *dentry,
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fsnotify_name(dir, mask, d_inode(dentry), &dentry->d_name, 0);
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}
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/* Notify this dentry's parent about a child's events. */
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static inline int fsnotify_parent(struct dentry *dentry, __u32 mask,
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const void *data, int data_type)
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{
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if (!(dentry->d_flags & DCACHE_FSNOTIFY_PARENT_WATCHED))
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return 0;
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return __fsnotify_parent(dentry, mask, data, data_type);
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}
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/*
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* Simple wrappers to consolidate calls fsnotify_parent()/fsnotify() when
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* an event is on a file/dentry.
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@ -379,7 +379,7 @@ struct fsnotify_mark {
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/* main fsnotify call to send events */
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extern int fsnotify(struct inode *to_tell, __u32 mask, const void *data,
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int data_type, const struct qstr *name, u32 cookie);
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extern int fsnotify_parent(struct dentry *dentry, __u32 mask, const void *data,
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extern int __fsnotify_parent(struct dentry *dentry, __u32 mask, const void *data,
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int data_type);
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extern void __fsnotify_inode_delete(struct inode *inode);
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extern void __fsnotify_vfsmount_delete(struct vfsmount *mnt);
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@ -541,7 +541,7 @@ static inline int fsnotify(struct inode *to_tell, __u32 mask, const void *data,
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return 0;
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}
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static inline int fsnotify_parent(struct dentry *dentry, __u32 mask,
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static inline int __fsnotify_parent(struct dentry *dentry, __u32 mask,
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const void *data, int data_type)
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{
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return 0;
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