[PATCH v5 1/2] block: skip elevator data initialization for flush requests

Mike Snitzer
2011-02-04T16:59:07Z

Skip elevator initialization for flush requests by passing priv=0 to
blk_alloc_request() in get_request(). As such elv_set_request() is
never called for flush requests.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
---
block/blk-core.c | 29 +++++++++++++++++++++++++----
1 file changed, 25 insertions(+), 4 deletions(-)

v5: introduces blk_rq_should_init_elevator() to abstract the elevator
initialization decision to promote future reuse and clarity.

Index: linux-2.6/block/blk-core.c
===================================================================
--- linux-2.6.orig/block/blk-core.c
+++ linux-2.6/block/blk-core.c
@@ -753,6 +753,25 @@ static void freed_request(struct request
}

/*
+ * Determine if elevator data should be initialized when allocating the
+ * request associated with @bio.
+ */
+static bool blk_rq_should_init_elevator(struct bio *bio)
+{
+ if (!bio)
+ return true;
+
+ /*
+ * Flush requests do not use the elevator so skip initialization.
+ * This allows a request to share the flush and elevator data.
+ */
+ if (bio->bi_rw & (REQ_FLUSH | REQ_FUA))
+ return false;
+
+ return true;
+}
+
+/*
* Get a free request, queue_lock must be held.
* Returns NULL on failure, with queue_lock held.
* Returns !NULL on success, with queue_lock *not held*.
@@ -764,7 +783,7 @@ static struct request *get_request(struc
struct request_list *rl = &q->rq;
struct io_context *ioc = NULL;
const bool is_sync = rw_is_sync(rw_flags) != 0;
- int may_queue, priv;
+ int may_queue, priv = 0;

may_queue = elv_may_queue(q, rw_flags);
if (may_queue == ELV_MQUEUE_NO)
@@ -808,9 +827,11 @@ static struct request *get_request(struc
rl->count[is_sync]++;
rl->starved[is_sync] = 0;

- priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
- if (priv)
- rl->elvpriv++;
+ if (blk_rq_should_init_elevator(bio)) {
+ priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
+ if (priv)
+ rl->elvpriv++;
+ }

if (blk_queue_io_stat(q))
rw_flags |= REQ_IO_STAT;