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Subject[PATCH v3 1/2] block: skip elevator data initialization for flush requests
Set REQ_SORTED, in the @rw_flags passed to the request allocator, for
any request that may be put on IO scheduler. Skip elevator data
initialization during request allocation if REQ_SORTED is not set.

REQ_SORTED is not set for flush requests because they are never put on
the IO scheduler.

Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Acked-by: Vivek Goyal <vgoyal@redhat.com>
---
block/blk-core.c | 26 +++++++++++++++++++++-----
1 file changed, 21 insertions(+), 5 deletions(-)

v3: edits to patch header and __make_request() comment

Index: linux-2.6/block/blk-core.c
===================================================================
--- linux-2.6.orig/block/blk-core.c
+++ linux-2.6/block/blk-core.c
@@ -764,7 +764,7 @@ 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 +808,14 @@ 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++;
+ /*
+ * Only initialize elevator data if REQ_SORTED is set.
+ */
+ if (rw_flags & REQ_SORTED) {
+ priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
+ if (priv)
+ rl->elvpriv++;
+ }

if (blk_queue_io_stat(q))
rw_flags |= REQ_IO_STAT;
@@ -1197,6 +1202,7 @@ static int __make_request(struct request
const unsigned short prio = bio_prio(bio);
const bool sync = !!(bio->bi_rw & REQ_SYNC);
const bool unplug = !!(bio->bi_rw & REQ_UNPLUG);
+ const bool flush = !!(bio->bi_rw & (REQ_FLUSH | REQ_FUA));
const unsigned long ff = bio->bi_rw & REQ_FAILFAST_MASK;
int where = ELEVATOR_INSERT_SORT;
int rw_flags;
@@ -1210,7 +1216,7 @@ static int __make_request(struct request

spin_lock_irq(q->queue_lock);

- if (bio->bi_rw & (REQ_FLUSH | REQ_FUA)) {
+ if (flush) {
where = ELEVATOR_INSERT_FLUSH;
goto get_rq;
}
@@ -1293,6 +1299,16 @@ get_rq:
rw_flags |= REQ_SYNC;

/*
+ * Set REQ_SORTED for all requests that may be put on IO scheduler.
+ * The request allocator's IO scheduler initialization will be skipped
+ * if REQ_SORTED is not set -- elv_set_request() is avoided so that
+ * that the allocated request's elevator_private pointers are not
+ * initialized and that space can be used by flush request data.
+ */
+ if (!flush)
+ rw_flags |= REQ_SORTED;
+
+ /*
* Grab a free request. This is might sleep but can not fail.
* Returns with the queue unlocked.
*/

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