Re: [LKP] [fsnotify] 60f7ed8c7c: will-it-scale.per_thread_ops -5.9% regression

[Amir Goldstein]

On Mon, Oct 15, 2018 at 10:50 AM Rong Chen <rong.a.chen@xxxxxxxxx> wrote:
[...]
> the patch seems not work.
>
> tests: 1
> testcase/path_params/tbox_group/run: will-it-scale/16-thread-unlink2-performance/lkp-bdw-ep3d
>
>  commit:
>    1e6cb72399 ("fsnotify: add super block object type")
>    298cd0b2f4 (the below patch)
>
> 1e6cb72399fd58b3  298cd0b2f481d9cc2e2cd5bfd3
> ----------------  --------------------------
>          %stddev      change         %stddev
>              \          |                \
>     103.21              -5%      98.54        will-it-scale.time.user_time
>      46266              -6%      43516        will-it-scale.time.involuntary_context_switches
>      54483              -7%      50610        will-it-scale.per_thread_ops
>     871749              -7%     809765        will-it-scale.workload

Thanks for testing my patch. As Jan commented, it is not surprising
that the patch
makes no difference.

I would like to clarify a few things about how you ran the test before
I continue to
investigate:

1. When I ran the workload I saw that it writes files to whatever filesystem is
    mounted on /tmp. Can I assume you have tmpfs mounted at /tmp?

2. Can you confirm that there is no fanotify mount mark on the /tmp mount?
    for example:
# ls -l /proc/*/fd/*|grep fanotify
lrwx------ 1 root root 64 Oct 15 08:36 /proc/3927/fd/3 -> anon_inode:[fanotify]
# grep fanotify.mnt_id /proc/3927/fdinfo/3
fanotify mnt_id:33 mflags:0 mask:3b ignored_mask:0
# grep ^$(( 0x33 )) /proc/3927/mountinfo
51 16 0:27 / /tmp rw,relatime shared:18 - tmpfs tmpfs rw

3. I saw that LKP caches the results for a specific commit
    (i.e. 1e6cb72399 ("fsnotify: add super block object type")).
Did you use cached results when comparing to patch or did you re-run the
test with the "good" commit? The reason I am asking is because
sometimes performance result may differ between boots even with no
kernel code change.
Where all the "good" bisect samples taken from the same boot/machine?
or different boots/machines?

4. If this regression is reliably reproduced, then our best bet is on the
cost of access to s_fsnotify_{marks,mask} fields.
The patch below moves those frequently accessed fields near the
frequently accessed fields s_time_gran,s_writers and moves
the seldom accessed fields s_id,s_uuid further away.
Could you please try this patch?

Thanks,
Amir.

---
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 25a449f37bb1..37c19c601979 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1393,9 +1393,6 @@ struct super_block {

        struct sb_writers       s_writers;

-       char                    s_id[32];       /* Informational name */
-       uuid_t                  s_uuid;         /* UUID */
-
        void                    *s_fs_info;     /* Filesystem private info */
        unsigned int            s_max_links;
        fmode_t                 s_mode;
@@ -1403,6 +1400,14 @@ struct super_block {
        /* Granularity of c/m/atime in ns.
           Cannot be worse than a second */
        u32                     s_time_gran;
+#ifdef CONFIG_FSNOTIFY
+       __u32                   s_fsnotify_mask;
+       struct fsnotify_mark_connector __rcu    *s_fsnotify_marks;
+#endif
+
+       /* Seldom accessed fields: */
+       char                    s_id[32];       /* Informational name */
+       uuid_t                  s_uuid;         /* UUID */

        /*
         * The next field is for VFS *only*. No filesystems have any business
@@ -1464,11 +1469,6 @@ struct super_block {

        spinlock_t              s_inode_wblist_lock;
        struct list_head        s_inodes_wb;    /* writeback inodes */
-
-#ifdef CONFIG_FSNOTIFY
-       __u32                   s_fsnotify_mask;
-       struct fsnotify_mark_connector __rcu    *s_fsnotify_marks;
-#endif
 } __randomize_layout;