Date:   Sun, 30 Sep 2018 12:16:45 +0300
From:   Amir Goldstein <amir73il@...il.com>
To:     rong.a.chen@...el.com
Cc:     Jan Kara <jack@...e.cz>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Stephen Rothwell <sfr@...b.auug.org.au>, LKP <lkp@...org>
Subject: Re: [LKP] [fsnotify] 60f7ed8c7c: will-it-scale.per_thread_ops -5.9% regression

On Sun, Sep 30, 2018 at 12:00 PM Amir Goldstein <amir73il@...il.com> wrote:
>
> On Sun, Sep 30, 2018 at 9:50 AM kernel test robot <rong.a.chen@...el.com> wrote:
> >
> > Greeting,
> >
> > FYI, we noticed a -5.9% regression of will-it-scale.per_thread_ops due to commit:
> >
> >
> > commit: 60f7ed8c7c4d06aeda448c6da74621552ee739aa ("fsnotify: send path type events to group with super block marks")
> > https://git.kernel.org/cgit/linux/kernel/git/next/linux-next.git master
> >
> > in testcase: will-it-scale
> > on test machine: 88 threads Intel(R) Xeon(R) CPU E5-2699 v4 @ 2.20GHz with 64G memory
> > with following parameters:
> >
> >         nr_task: 16
> >         mode: thread
> >         test: unlink2
> >         cpufreq_governor: performance
> >
> > test-description: Will It Scale takes a testcase and runs it from 1 through to n parallel copies to see if the testcase will scale. It builds both a process and threads based test in order to see any differences between the two.
> > test-url: https://github.com/antonblanchard/will-it-scale
> >
> >
> >
> > Details are as below:
> > -------------------------------------------------------------------------------------------------->
> >
> >
> > To reproduce:
> >
> >         git clone https://github.com/intel/lkp-tests.git
> >         cd lkp-tests
> >         bin/lkp install job.yaml  # job file is attached in this email
> >         bin/lkp run     job.yaml
> >
> > =========================================================================================
> > compiler/cpufreq_governor/kconfig/mode/nr_task/rootfs/tbox_group/test/testcase:
> >   gcc-7/performance/x86_64-rhel-7.2/thread/16/debian-x86_64-2018-04-03.cgz/lkp-bdw-ep3d/unlink2/will-it-scale
> >
> > commit:
> >   1e6cb72399 ("fsnotify: add super block object type")
> >   60f7ed8c7c ("fsnotify: send path type events to group with super block marks")
> >
>
> I have to admit this looks strange.
> All this commit does is dereference mnt->mnt.mnt_sb and then
> sb->s_fsnotify_mask/sb->s_fsnotify_marks to find that they are zero.
> AFAICT there should be no extra contention added by this commit and it's
> hard to believe that parallel unlink workload would suffer from this change.
>
> I will try to install lkp-tests to verify this on my own system, but
> until proven
> otherwise I will regard this as false positive.
>

Actually, is it possible to ask for a private test with the following patch
to optimize out an unneeded srcu_derefence().
This optimization (assuming it is correct) could in fact improve scaling
compared to upstream, because there are already 2 calls to
fsnotify_first_mark in the code. The blamed commit just adds a 3rd one.
I am assuming that() in the test there is an fsnotify mount mark (maybe setup
by systemd) otherwise, the optimization in line 351 would have not reached
the extra fsnotify_first_mark() call.
Can you confirm or disprove the assumption that an fanotify mount mark
is present during the test?

Thanks,
Amir.

----
diff --git a/fs/notify/fsnotify.c b/fs/notify/fsnotify.c
index 422fbc6dffde..8d45d82e09ff 100644
--- a/fs/notify/fsnotify.c
+++ b/fs/notify/fsnotify.c
@@ -246,6 +246,9 @@ static struct fsnotify_mark
*fsnotify_first_mark(struct fsnotify_mark_connector
        struct fsnotify_mark_connector *conn;
        struct hlist_node *node = NULL;

+       if (!*connp)
+               return NULL;
+
        conn = srcu_dereference(*connp, &fsnotify_mark_srcu);
        if (conn)
                node = srcu_dereference(conn->list.first, &fsnotify_mark_srcu);

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