On 10/13/21 11:38 PM, Christoph Hellwig wrote:
> On Wed, Oct 13, 2021 at 02:44:14PM -0600, Jens Axboe wrote:
>> The fast path is no splitting needed. Separate the handling into a
>> check part we can inline, and an out-of-line handling path if we do
>> need to split.
>>
>> Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
>
> What about something like this version instead?
A bit of a combo, I think this will do fine.
commit d997c5f4001031863de1c8c437bd2fcc6a4f79a2
Author: Jens Axboe <axboe@xxxxxxxxx>
Date: Wed Oct 13 12:43:41 2021 -0600
block: handle fast path of bio splitting inline
The fast path is no splitting needed. Separate the handling into a
check part we can inline, and an out-of-line handling path if we do
need to split.
Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>
diff --git a/block/blk-merge.c b/block/blk-merge.c
index 4da2bf18fa4d..f390a8753268 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -324,6 +324,7 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
/**
* __blk_queue_split - split a bio and submit the second half
+ * @q: [in] request_queue new bio is being queued at
* @bio: [in, out] bio to be split
* @nr_segs: [out] number of segments in the first bio
*
@@ -334,9 +335,9 @@ static struct bio *blk_bio_segment_split(struct request_queue *q,
* of the caller to ensure that q->bio_split is only released after processing
* of the split bio has finished.
*/
-void __blk_queue_split(struct bio **bio, unsigned int *nr_segs)
+void __blk_queue_split(struct request_queue *q, struct bio **bio,
+ unsigned int *nr_segs)
{
- struct request_queue *q = (*bio)->bi_bdev->bd_disk->queue;
struct bio *split = NULL;
switch (bio_op(*bio)) {
@@ -353,21 +354,6 @@ void __blk_queue_split(struct bio **bio, unsigned int *nr_segs)
nr_segs);
break;
default:
- /*
- * All drivers must accept single-segments bios that are <=
- * PAGE_SIZE. This is a quick and dirty check that relies on
- * the fact that bi_io_vec[0] is always valid if a bio has data.
- * The check might lead to occasional false negatives when bios
- * are cloned, but compared to the performance impact of cloned
- * bios themselves the loop below doesn't matter anyway.
- */
- if (!q->limits.chunk_sectors &&
- (*bio)->bi_vcnt == 1 &&
- ((*bio)->bi_io_vec[0].bv_len +
- (*bio)->bi_io_vec[0].bv_offset) <= PAGE_SIZE) {
- *nr_segs = 1;
- break;
- }
split = blk_bio_segment_split(q, *bio, &q->bio_split, nr_segs);
break;
}
@@ -397,9 +383,11 @@ void __blk_queue_split(struct bio **bio, unsigned int *nr_segs)
*/
void blk_queue_split(struct bio **bio)
{
+ struct request_queue *q = (*bio)->bi_bdev->bd_disk->queue;
unsigned int nr_segs;
- __blk_queue_split(bio, &nr_segs);
+ if (blk_may_split(q, *bio))
+ __blk_queue_split(q, bio, &nr_segs);
}
EXPORT_SYMBOL(blk_queue_split);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index dd4121dcd3ce..0cca4b7a4d16 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -2521,11 +2521,12 @@ void blk_mq_submit_bio(struct bio *bio)
struct request *rq;
struct blk_plug *plug;
struct request *same_queue_rq = NULL;
- unsigned int nr_segs;
+ unsigned int nr_segs = 1;
blk_status_t ret;
blk_queue_bounce(q, &bio);
- __blk_queue_split(&bio, &nr_segs);
+ if (blk_may_split(q, bio))
+ __blk_queue_split(q, &bio, &nr_segs);
if (!bio_integrity_prep(bio))
goto queue_exit;
diff --git a/block/blk.h b/block/blk.h
index fa23338449ed..f6e61cebd6ae 100644
--- a/block/blk.h
+++ b/block/blk.h
@@ -264,7 +264,32 @@ ssize_t part_timeout_show(struct device *, struct device_attribute *, char *);
ssize_t part_timeout_store(struct device *, struct device_attribute *,
const char *, size_t);
-void __blk_queue_split(struct bio **bio, unsigned int *nr_segs);
+static inline bool blk_may_split(struct request_queue *q, struct bio *bio)
+{
+ switch (bio_op(bio)) {
+ case REQ_OP_DISCARD:
+ case REQ_OP_SECURE_ERASE:
+ case REQ_OP_WRITE_ZEROES:
+ case REQ_OP_WRITE_SAME:
+ return true; /* non-trivial splitting decisions */
+ default:
+ break;
+ }
+
+ /*
+ * All drivers must accept single-segments bios that are <= PAGE_SIZE.
+ * This is a quick and dirty check that relies on the fact that
+ * bi_io_vec[0] is always valid if a bio has data. The check might
+ * lead to occasional false negatives when bios are cloned, but compared
+ * to the performance impact of cloned bios themselves the loop below
+ * doesn't matter anyway.
+ */
+ return q->limits.chunk_sectors || bio->bi_vcnt != 1 ||
+ bio->bi_io_vec->bv_len + bio->bi_io_vec->bv_offset > PAGE_SIZE;
+}
+
+void __blk_queue_split(struct request_queue *q, struct bio **bio,
+ unsigned int *nr_segs);
int ll_back_merge_fn(struct request *req, struct bio *bio,
unsigned int nr_segs);
bool blk_attempt_req_merge(struct request_queue *q, struct request *rq,
--
Jens Axboe
