fs/locks: create a tree of dependent requests.

When we find an existing lock which conflicts with a request,
and the request wants to wait, we currently add the request
to a list. When the lock is removed, the whole list is woken.
This can cause the thundering-herd problem.
To reduce the problem, we make use of the (new) fact that
a pending request can itself have a list of blocked requests.
When we find a conflict, we look through the existing blocked requests.
If any one of them blocks the new request, the new request is attached
below that request, otherwise it is added to the list of blocked
requests, which are now known to be mutually non-conflicting.

This way, when the lock is released, only a set of non-conflicting
locks will be woken, the rest can stay asleep.
If the lock request cannot be granted and the request needs to be
requeued, all the other requests it blocks will then be woken

To make this more concrete:

If you have a many-core machine, and have many threads all wanting to
briefly lock a give file (udev is known to do this), you can get quite
poor performance.

When one thread releases a lock, it wakes up all other threads that
are waiting (classic thundering-herd) - one will get the lock and the
others go to sleep.
When you have few cores, this is not very noticeable: by the time the
4th or 5th thread gets enough CPU time to try to claim the lock, the
earlier threads have claimed it, done what was needed, and released.
So with few cores, many of the threads don't end up contending.
With 50+ cores, lost of threads can get the CPU at the same time,
and the contention can easily be measured.

This patchset creates a tree of pending lock requests in which siblings
don't conflict and each lock request does conflict with its parent.
When a lock is released, only requests which don't conflict with each
other a woken.

Testing shows that lock-acquisitions-per-second is now fairly stable
even as the number of contending process goes to 1000. Without this
patch, locks-per-second drops off steeply after a few 10s of
processes.

There is a small cost to this extra complexity.
At 20 processes running a particular test on 72 cores, the lock
acquisitions per second drops from 1.8 million to 1.4 million with
this patch. For 100 processes, this patch still provides 1.4 million
while without this patch there are about 700,000.

Reported-and-tested-by: Martin Wilck <[e-mail]>
Signed-off-by: NeilBrown <[e-mail]>
Reviewed-by: J. Bruce Fields <[e-mail]>
Signed-off-by: Jeff Layton <[e-mail]>

fs/locks.c

···
  *  Leases and LOCK_MAND
  *  Matthew Wilcox <[e-mail]>, June, 2000.
  *  Stephen Rothwell <[e-mail]>, June, 2000.
+ *
+ * Locking conflicts and dependencies:
+ * If multiple threads attempt to lock the same byte (or flock the same file)
+ * only one can be granted the lock, and other must wait their turn.
+ * The first lock has been "applied" or "granted", the others are "waiting"
+ * and are "blocked" by the "applied" lock..
+ *
+ * Waiting and applied locks are all kept in trees whose properties are:
+ *
+ *	- the root of a tree may be an applied or waiting lock.
+ *	- every other node in the tree is a waiting lock that
+ *	  conflicts with every ancestor of that node.
+ *
+ * Every such tree begins life as a waiting singleton which obviously
+ * satisfies the above properties.
+ *
+ * The only ways we modify trees preserve these properties:
+ *
+ *	1. We may add a new leaf node, but only after first verifying that it
+ *	   conflicts with all of its ancestors.
+ *	2. We may remove the root of a tree, creating a new singleton
+ *	   tree from the root and N new trees rooted in the immediate
+ *	   children.
+ *	3. If the root of a tree is not currently an applied lock, we may
+ *	   apply it (if possible).
+ *	4. We may upgrade the root of the tree (either extend its range,
+ *	   or upgrade its entire range from read to write).
+ *
+ * When an applied lock is modified in a way that reduces or downgrades any
+ * part of its range, we remove all its children (2 above).  This particularly
+ * happens when a lock is unlocked.
+ *
+ * For each of those child trees we "wake up" the thread which is
+ * waiting for the lock so it can continue handling as follows: if the
+ * root of the tree applies, we do so (3).  If it doesn't, it must
+ * conflict with some applied lock.  We remove (wake up) all of its children
+ * (2), and add it is a new leaf to the tree rooted in the applied
+ * lock (1).  We then repeat the process recursively with those
+ * children.
+ *
  */
 
 #include <linux/capability.h>
···
  * but by ensuring that the flc_lock is also held on insertions we can avoid
  * taking the blocked_lock_lock in some cases when we see that the
  * fl_blocked_requests list is empty.
+ *
+ * Rather than just adding to the list, we check for conflicts with any existing
+ * waiters, and add beneath any waiter that blocks the new waiter.
+ * Thus wakeups don't happen until needed.
  */
 static void __locks_insert_block(struct file_lock *blocker,
-					struct file_lock *waiter)
+				 struct file_lock *waiter,
+				 bool conflict(struct file_lock *,
+					       struct file_lock *))
 {
+	struct file_lock *fl;
 	BUG_ON(!list_empty(&waiter->fl_blocked_member));
+
+new_blocker:
+	list_for_each_entry(fl, &blocker->fl_blocked_requests, fl_blocked_member)
+		if (conflict(fl, waiter)) {
+			blocker =  fl;
+			goto new_blocker;
+		}
 	waiter->fl_blocker = blocker;
 	list_add_tail(&waiter->fl_blocked_member, &blocker->fl_blocked_requests);
 	if (IS_POSIX(blocker) && !IS_OFDLCK(blocker))
···
 
 /* Must be called with flc_lock held. */
 static void locks_insert_block(struct file_lock *blocker,
-					struct file_lock *waiter)
+			       struct file_lock *waiter,
+			       bool conflict(struct file_lock *,
+					     struct file_lock *))
 {
 	spin_lock(&blocked_lock_lock);
-	__locks_insert_block(blocker, waiter);
+	__locks_insert_block(blocker, waiter, conflict);
 	spin_unlock(&blocked_lock_lock);
 }
 
···
 		if (!(request->fl_flags & FL_SLEEP))
 			goto out;
 		error = FILE_LOCK_DEFERRED;
-		locks_insert_block(fl, request);
+		locks_insert_block(fl, request, flock_locks_conflict);
 		goto out;
 	}
 	if (request->fl_flags & FL_ACCESS)
···
 			spin_lock(&blocked_lock_lock);
 			if (likely(!posix_locks_deadlock(request, fl))) {
 				error = FILE_LOCK_DEFERRED;
-				__locks_insert_block(fl, request);
+				__locks_insert_block(fl, request,
+						     posix_locks_conflict);
 			}
 			spin_unlock(&blocked_lock_lock);
 			goto out;
···
 		break_time -= jiffies;
 	if (break_time == 0)
 		break_time++;
-	locks_insert_block(fl, new_fl);
+	locks_insert_block(fl, new_fl, leases_conflict);
 	trace_break_lease_block(inode, new_fl);
 	spin_unlock(&ctx->flc_lock);
 	percpu_up_read_preempt_enable(&file_rwsem);