On 1/12/24 6:58 AM, Jens Axboe wrote:
> On 1/12/24 6:00 AM, Amir Goldstein wrote:
>> On Fri, Jan 12, 2024 at 1:09?PM Jan Kara <jack@xxxxxxx> wrote:
>>>
>>> On Thu 11-01-24 17:22:33, Amir Goldstein wrote:
>>>> Commit e43de7f0862b ("fsnotify: optimize the case of no marks of any type")
>>>> optimized the case where there are no fsnotify watchers on any of the
>>>> filesystem's objects.
>>>>
>>>> It is quite common for a system to have a single local filesystem and
>>>> it is quite common for the system to have some inotify watches on some
>>>> config files or directories, so the optimization of no marks at all is
>>>> often not in effect.
>>>>
>>>> Content event (i.e. access,modify) watchers on sb/mount more rare, so
>>>> optimizing the case of no sb/mount marks with content events can improve
>>>> performance for more systems, especially for performance sensitive io
>>>> workloads.
>>>>
>>>> Set a per-sb flag SB_I_CONTENT_WATCHED if sb/mount marks with content
>>>> events in their mask exist and use that flag to optimize out the call to
>>>> __fsnotify_parent() and fsnotify() in fsnotify access/modify hooks.
>>>>
>>>> Signed-off-by: Amir Goldstein <amir73il@xxxxxxxxx>
>>>
>>> ...
>>>
>>>> -static inline int fsnotify_file(struct file *file, __u32 mask)
>>>> +static inline int fsnotify_path(const struct path *path, __u32 mask)
>>>> {
>>>> - const struct path *path;
>>>> + struct dentry *dentry = path->dentry;
>>>>
>>>> - if (file->f_mode & FMODE_NONOTIFY)
>>>> + if (!fsnotify_sb_has_watchers(dentry->d_sb))
>>>> return 0;
>>>>
>>>> - path = &file->f_path;
>>>> + /* Optimize the likely case of sb/mount/parent not watching content */
>>>> + if (mask & FSNOTIFY_CONTENT_EVENTS &&
>>>> + likely(!(dentry->d_flags & DCACHE_FSNOTIFY_PARENT_WATCHED)) &&
>>>> + likely(!(dentry->d_sb->s_iflags & SB_I_CONTENT_WATCHED))) {
>>>> + /*
>>>> + * XXX: if SB_I_CONTENT_WATCHED is not set, checking for content
>>>> + * events in s_fsnotify_mask is redundant, but it will be needed
>>>> + * if we use the flag FS_MNT_CONTENT_WATCHED to indicate the
>>>> + * existence of only mount content event watchers.
>>>> + */
>>>> + __u32 marks_mask = d_inode(dentry)->i_fsnotify_mask |
>>>> + dentry->d_sb->s_fsnotify_mask;
>>>> +
>>>> + if (!(mask & marks_mask))
>>>> + return 0;
>>>> + }
>>>
>>> So I'm probably missing something but how is all this patch different from:
>>>
>>> if (likely(!(dentry->d_flags & DCACHE_FSNOTIFY_PARENT_WATCHED))) {
>>> __u32 marks_mask = d_inode(dentry)->i_fsnotify_mask |
>>> path->mnt->mnt_fsnotify_mask |
>>
>> It's actually:
>>
>> real_mount(path->mnt)->mnt_fsnotify_mask
>>
>> and this requires including "internal/mount.h" in all the call sites.
>>
>>> dentry->d_sb->s_fsnotify_mask;
>>> if (!(mask & marks_mask))
>>> return 0;
>>> }
>>>
>>> I mean (mask & FSNOTIFY_CONTENT_EVENTS) is true for the frequent events
>>> (read & write) we care about. In Jens' case
>>>
>>> !(dentry->d_flags & DCACHE_FSNOTIFY_PARENT_WATCHED) &&
>>> !(dentry->d_sb->s_iflags & SB_I_CONTENT_WATCHED)
>>>
>>> is true as otherwise we'd go right to fsnotify_parent() and so Jens
>>> wouldn't see the performance benefit. But then with your patch you fetch
>>> i_fsnotify_mask and s_fsnotify_mask anyway for the test so the only
>>> difference to what I suggest above is the path->mnt->mnt_fsnotify_mask
>>> fetch but that is equivalent to sb->s_iflags (or wherever we store that
>>> bit) fetch?
>>>
>>> So that would confirm that the parent handling costs in fsnotify_parent()
>>> is what's really making the difference and just avoiding that by checking
>>> masks early should be enough?
>>
>> Can't the benefit be also related to saving a function call?
>>
>> Only one way to find out...
>>
>> Jens,
>>
>> Can you please test attached v3 with a non-inlined fsnotify_path() helper?
>
> I can run it since it doesn't take much to do that, but there's no way
> parallel universe where saving a function call would yield those kinds
> of wins (or have that much cost).
Ran this patch, and it's better than mainline for sure, but it does have
additional overhead that the previous version did not:
+1.46% [kernel.vmlinux] [k] fsnotify_path
--
Jens Axboe
