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>

1 files changed, 15 insertions, 0 deletions

diff --git a/fs/xfs/scrub/common.h b/fs/xfs/scrub/common.h
index 273a4331da05..4714e8a43094 100644
--- a/fs/xfs/scrub/common.h
+++ b/fs/xfs/scrub/common.h
@@ -72,6 +72,7 @@ bool xchk_should_check_xref(struct xfs_scrub *sc, int *error,
 			   struct xfs_btree_cur **curpp);
 
 /* Setup functions */
+int xchk_setup_agheader(struct xfs_scrub *sc);
 int xchk_setup_fs(struct xfs_scrub *sc);
 int xchk_setup_ag_allocbt(struct xfs_scrub *sc);
 int xchk_setup_ag_iallocbt(struct xfs_scrub *sc);
@@ -151,4 +152,18 @@ int xchk_ilock_inverted(struct xfs_inode *ip, uint lock_mode);
 void xchk_stop_reaping(struct xfs_scrub *sc);
 void xchk_start_reaping(struct xfs_scrub *sc);
 
+/*
+ * Setting up a hook to wait for intents to drain is costly -- we have to take
+ * the CPU hotplug lock and force an i-cache flush on all CPUs once to set it
+ * up, and again to tear it down.  These costs add up quickly, so we only want
+ * to enable the drain waiter if the drain actually detected a conflict with
+ * running intent chains.
+ */
+static inline bool xchk_need_intent_drain(struct xfs_scrub *sc)
+{
+	return sc->flags & XCHK_TRY_HARDER;
+}
+
+void xchk_fsgates_enable(struct xfs_scrub *sc, unsigned int scrub_fshooks);
+
 #endif	/* __XFS_SCRUB_COMMON_H__ */