xfs: minimize overhead of drain wakeups by using jump labels

To reduce the runtime overhead even further when online fsck isn't
running, use a static branch key to decide if we call wake_up on the
drain. For compilers that support jump labels, the call to wake_up is
replaced by a nop sled when nobody is waiting for intents to drain.

From my initial microbenchmarking, every transition of the static key
between the on and off states takes about 22000ns to complete; this is
paid entirely by the xfs_scrub process. When the static key is off
(which it should be when fsck isn't running), the nop sled adds an
overhead of approximately 0.36ns to runtime code. The post-atomic
lockless waiter check adds about 0.03ns, which is basically free.

For the few compilers that don't support jump labels, runtime code pays
the cost of calling wake_up on an empty waitqueue, which was observed to
be about 30ns. However, most architectures that have sufficient memory
and CPU capacity to run XFS also support jump labels, so this is not
much of a worry.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>

xfs: allow queued AG intents to drain before scrubbing

When a writer thread executes a chain of log intent items, the AG header
buffer locks will cycle during a transaction roll to get from one intent
item to the next in a chain. Although scrub takes all AG header buffer
locks, this isn't sufficient to guard against scrub checking an AG while
that writer thread is in the middle of finishing a chain because there's
no higher level locking primitive guarding allocation groups.

When there's a collision, cross-referencing between data structures
(e.g. rmapbt and refcountbt) yields false corruption events; if repair
is running, this results in incorrect repairs, which is catastrophic.

Fix this by adding to the perag structure the count of active intents
and make scrub wait until it has both AG header buffer locks and the
intent counter reaches zero.

One quirk of the drain code is that deferred bmap updates also bump and
drop the intent counter. A fundamental decision made during the design
phase of the reverse mapping feature is that updates to the rmapbt
records are always made by the same code that updates the primary
metadata. In other words, callers of bmapi functions expect that the
bmapi functions will queue deferred rmap updates.

Some parts of the reflink code queue deferred refcount (CUI) and bmap
(BUI) updates in the same head transaction, but the deferred work
manager completely finishes the CUI before the BUI work is started. As
a result, the CUI drops the intent count long before the deferred rmap
(RUI) update even has a chance to bump the intent count. The only way
to keep the intent count elevated between the CUI and RUI is for the BUI
to bump the counter until the RUI has been created.

A second quirk of the intent drain code is that deferred work items must
increment the intent counter as soon as the work item is added to the
transaction. When a BUI completes and queues an RUI, the RUI must
increment the counter before the BUI decrements it. The only way to
accomplish this is to require that the counter be bumped as soon as the
deferred work item is created in memory.

In the next patches we'll improve on this facility, but this patch
provides the basic functionality.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Reviewed-by: Dave Chinner <dchinner@redhat.com>