Re: [PATCH] block: improve error checking in blkdev_bio_end_io_async()

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On 10/19/21 19:59, Jens Axboe wrote:
Track the current error status of the dio with DIO_ERROR in the flags,
which can then avoid diving into dio->bio for the fast path of not
having any errors. This reduces the overhead of the function nicely,
which was previously dominated by this seemingly cheap check:

      4.55%     -1.13%  [kernel.vmlinux]  [k] blkdev_bio_end_io_async

Jens, something gone wrong here. blkdev_bio_end_io_async() is a
function from my not yet published branch, the perf here is for it,
but the patch tackles blkdev_bio_end_io().

P.s. once blkdev_bio_end_io_async() doesn't need it, and once in
normal blkdev_bio_end_io() will be a slow-ish path, so probably
we don't care much.



Signed-off-by: Jens Axboe <axboe@xxxxxxxxx>

---

diff --git a/block/fops.c b/block/fops.c
index d4f4fffb7d32..21d265caecff 100644
--- a/block/fops.c
+++ b/block/fops.c
@@ -127,6 +127,7 @@ enum {
  	DIO_MULTI_BIO		= 1,
  	DIO_SHOULD_DIRTY	= 2,
  	DIO_IS_SYNC		= 4,
+	DIO_ERROR		= 8,
  };
struct blkdev_dio {
@@ -147,8 +148,10 @@ static void blkdev_bio_end_io(struct bio *bio)
  	struct blkdev_dio *dio = bio->bi_private;
  	unsigned int flags = dio->flags;
- if (bio->bi_status && !dio->bio.bi_status)
+	if (!(flags & DIO_ERROR) && !dio->bio.bi_status) {
  		dio->bio.bi_status = bio->bi_status;
+		flags |= DIO_ERROR;
+	}
if (!(flags & DIO_MULTI_BIO) || atomic_dec_and_test(&dio->ref)) {
  		if (!(flags & DIO_IS_SYNC)) {
@@ -157,7 +160,7 @@ static void blkdev_bio_end_io(struct bio *bio)
WRITE_ONCE(iocb->private, NULL); - if (likely(!dio->bio.bi_status)) {
+			if (likely(!(flags & DIO_ERROR))) {
  				ret = dio->size;
  				iocb->ki_pos += ret;
  			} else {


--
Pavel Begunkov



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