Re: [PATCH 1/5] block: enable batched allocation for blk_mq_alloc_request()

On 9/23/22 9:13 AM, Pankaj Raghav wrote:

On 2022-09-23 16:52, Pankaj Raghav wrote:

quoted

On Thu, Sep 22, 2022 at 12:28:01PM -0600, Jens Axboe wrote:

quoted

The filesystem IO path can take advantage of allocating batches of
requests, if the underlying submitter tells the block layer about it
through the blk_plug. For passthrough IO, the exported API is the
blk_mq_alloc_request() helper, and that one does not allow for
request caching.

Wire up request caching for blk_mq_alloc_request(), which is generally
done without having a bio available upfront.

Signed-off-by: Jens Axboe <axboe@kernel.dk>
---
 block/blk-mq.c | 80 ++++++++++++++++++++++++++++++++++++++++++++------
 1 file changed, 71 insertions(+), 9 deletions(-)

I think we need this patch to ensure correct behaviour for passthrough:

diff --git a/block/blk-mq.c b/block/blk-mq.c
index c11949d66163..840541c1ab40 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c

@@ -1213,7 +1213,7 @@ void blk_execute_rq_nowait(struct request *rq, bool at_head)
        WARN_ON(!blk_rq_is_passthrough(rq));
 
        blk_account_io_start(rq);
-       if (current->plug)
+       if (blk_mq_plug(rq->bio))
                blk_add_rq_to_plug(current->plug, rq);
        else
                blk_mq_sched_insert_request(rq, at_head, true, false);

As the passthrough path can now support request caching via blk_mq_alloc_request(),
and it uses blk_execute_rq_nowait(), bad things can happen at least for zoned
devices:

static inline struct blk_plug *blk_mq_plug( struct bio *bio)
{
	/* Zoned block device write operation case: do not plug the BIO */
	if (bdev_is_zoned(bio->bi_bdev) && op_is_write(bio_op(bio)))
		return NULL;
..

Thinking more about it, even this will not fix it because op is
REQ_OP_DRV_OUT if it is a NVMe write for passthrough requests.

@Damien Should the condition in blk_mq_plug() be changed to:

static inline struct blk_plug *blk_mq_plug( struct bio *bio)
{
	/* Zoned block device write operation case: do not plug the BIO */
	if (bdev_is_zoned(bio->bi_bdev) && !op_is_read(bio_op(bio)))
		return NULL;

That looks reasonable to me. It'll prevent plug optimizations even
for passthrough on zoned devices, but that's probably fine.

-- 
Jens Axboe