Re: [locks] 6d390e4b5d: will-it-scale.per_process_ops -96.6% regression

[Linus Torvalds <torvalds@linux-foundation.org>]

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On Mon, Mar 9, 2020 at 7:36 AM Jeff Layton <jlayton@kernel.org> wrote:
>
> On Sun, 2020-03-08 at 22:03 +0800, kernel test robot wrote:
> >
> > FYI, we noticed a -96.6% regression of will-it-scale.per_process_ops due to commit:
>
> This is not completely unexpected as we're banging on the global
> blocked_lock_lock now for every unlock. This test just thrashes file
> locks and unlocks without doing anything in between, so the workload
> looks pretty artificial [1].
>
> It would be nice to avoid the global lock in this codepath, but it
> doesn't look simple to do. I'll keep thinking about it, but for now I'm
> inclined to ignore this result unless we see a problem in more realistic
> workloads.

That is a _huge_ regression, though.

What about something like the attached? Wouldn't that work? And make
the code actually match the old comment about wow "fl_blocker" being
NULL being special.

The old code seemed to not know about things like memory ordering either.

Patch is entirely untested, but aims to have that "smp_store_release()
means I'm done and not going to touch it any more", making that
smp_load_acquire() test hopefully be valid as per the comment..

Hmm?

                    Linus

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 fs/locks.c | 29 ++++++++++++++++++++++++++++-
 1 file changed, 28 insertions(+), 1 deletion(-)

diff --git a/fs/locks.c b/fs/locks.c
index 426b55d333d5..bc5ca54a0749 100644
--- a/fs/locks.c
+++ b/fs/locks.c
@@ -725,7 +725,6 @@ static void __locks_delete_block(struct file_lock *waiter)
 {
 	locks_delete_global_blocked(waiter);
 	list_del_init(&waiter->fl_blocked_member);
-	waiter->fl_blocker = NULL;
 }
 
 static void __locks_wake_up_blocks(struct file_lock *blocker)
@@ -740,6 +739,12 @@ static void __locks_wake_up_blocks(struct file_lock *blocker)
 			waiter->fl_lmops->lm_notify(waiter);
 		else
 			wake_up(&waiter->fl_wait);
+
+		/*
+		 * Tell the world we're done with it - see comment at
+		 * top of locks_delete_block().
+		 */
+		smp_store_release(&waiter->fl_blocker, NULL);
 	}
 }
 
@@ -753,11 +758,33 @@ int locks_delete_block(struct file_lock *waiter)
 {
 	int status = -ENOENT;
 
+	/*
+	 * If fl_blocker is NULL, it won't be set again as this thread
+	 * "owns" the lock and is the only one that might try to claim
+	 * the lock.  So it is safe to test fl_blocker locklessly.
+	 * Also if fl_blocker is NULL, this waiter is not listed on
+	 * fl_blocked_requests for some lock, so no other request can
+	 * be added to the list of fl_blocked_requests for this
+	 * request.  So if fl_blocker is NULL, it is safe to
+	 * locklessly check if fl_blocked_requests is empty.  If both
+	 * of these checks succeed, there is no need to take the lock.
+	 */
+	if (!smp_load_acquire(&waiter->fl_blocker)) {
+		if (list_empty(&waiter->fl_blocked_requests))
+		        return status;
+	}
+
 	spin_lock(&blocked_lock_lock);
 	if (waiter->fl_blocker)
 		status = 0;
 	__locks_wake_up_blocks(waiter);
 	__locks_delete_block(waiter);
+
+	/*
+	 * Tell the world we're done with it - see commit at top
+	 * of this function
+	 */
+	smp_store_release(&waiter->fl_blocker, NULL);
 	spin_unlock(&blocked_lock_lock);
 	return status;
 }