Commit ebfd5d8f authored by Lars Ellenberg's avatar Lars Ellenberg Committed by Jens Axboe

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: default avatarPhilipp Reisner <philipp.reisner@linbit.com>
Signed-off-by: default avatarLars Ellenberg <lars.ellenberg@linbit.com>
Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
parent 56392d2f
...@@ -256,6 +256,7 @@ void drbd_al_begin_io(struct drbd_conf *mdev, struct drbd_interval *i, bool dele ...@@ -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 first = i->sector >> (AL_EXTENT_SHIFT-9);
unsigned last = i->size == 0 ? first : (i->sector + (i->size >> 9) - 1) >> (AL_EXTENT_SHIFT-9); unsigned last = i->size == 0 ? first : (i->sector + (i->size >> 9) - 1) >> (AL_EXTENT_SHIFT-9);
unsigned enr; unsigned enr;
bool need_transaction = false;
bool locked = false; bool locked = false;
/* When called through generic_make_request(), we must delegate /* 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 ...@@ -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(first <= last);
D_ASSERT(atomic_read(&mdev->local_cnt) > 0); D_ASSERT(atomic_read(&mdev->local_cnt) > 0);
for (enr = first; enr <= last; enr++) for (enr = first; enr <= last; enr++) {
wait_event(mdev->al_wait, _al_get(mdev, enr) != NULL); 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. /* Serialize multiple transactions.
* This uses test_and_set_bit, memory barrier is implicit. * This uses test_and_set_bit, memory barrier is implicit.
......
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