drbd: drbd_al_being_io: short circuit to reduce latency

A request hitting an already "hot" extent should proceed right away,
even if some other requests need to wait for pending transactions.

Without that short-circuit, several simultaneous make_request contexts
race for committing the transaction, possibly penalizing the innocent.

Signed-off-by: Philipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: Lars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 12 insertions, 2 deletions

diff --git a/drivers/block/drbd/drbd_actlog.c b/drivers/block/drbd/drbd_actlog.c
index 82199d9a9a61..1d7244d2a910 100644
--- a/drivers/block/drbd/drbd_actlog.c
+++ b/drivers/block/drbd/drbd_actlog.c
@@ -256,6 +256,7 @@ void drbd_al_begin_io(struct drbd_conf *mdev, struct drbd_interval *i, bool dele
         unsigned first = i->sector >> (AL_EXTENT_SHIFT-9);
         unsigned last = i->size == 0 ? first : (i->sector + (i->size >> 9) - 1) >> (AL_EXTENT_SHIFT-9);
         unsigned enr;
+        bool need_transaction = false;
         bool locked = false;
 
         /* When called through generic_make_request(), we must delegate
@@ -273,8 +274,17 @@ void drbd_al_begin_io(struct drbd_conf *mdev, struct drbd_interval *i, bool dele
         D_ASSERT(first <= last);
         D_ASSERT(atomic_read(&mdev->local_cnt) > 0);
 
-        for (enr = first; enr <= last; enr++)
-                wait_event(mdev->al_wait, _al_get(mdev, enr) != NULL);
+        for (enr = first; enr <= last; enr++) {
+                struct lc_element *al_ext;
+                wait_event(mdev->al_wait, (al_ext = _al_get(mdev, enr)) != NULL);
+                if (al_ext->lc_number != enr)
+                        need_transaction = true;
+        }
+
+        /* If *this* request was to an already active extent,
+         * we're done, even if there are pending changes. */
+        if (!need_transaction)
+                return;
 
         /* Serialize multiple transactions.
          * This uses test_and_set_bit, memory barrier is implicit.