Re: [PATCH] nvmet: introduce use_vfs ns-attr

[Keith Busch <kbusch@kernel.org>]

On Sun, Oct 27, 2019 at 04:03:30PM +0100, hch@lst.de wrote:
> ---
>  drivers/nvme/target/io-cmd-bdev.c | 3 +++
>  1 file changed, 3 insertions(+)
> 
> diff --git a/drivers/nvme/target/io-cmd-bdev.c b/drivers/nvme/target/io-cmd-bdev.c
> index 04a9cd2a2604..ed1a8d0ab30e 100644
> --- a/drivers/nvme/target/io-cmd-bdev.c
> +++ b/drivers/nvme/target/io-cmd-bdev.c
> @@ -147,6 +147,7 @@ static void nvmet_bdev_execute_rw(struct nvmet_req *req)
>  	int sg_cnt = req->sg_cnt;
>  	struct bio *bio;
>  	struct scatterlist *sg;
> +	struct blk_plug plug;
>  	sector_t sector;
>  	int op, op_flags = 0, i;
>  
> @@ -185,6 +186,7 @@ static void nvmet_bdev_execute_rw(struct nvmet_req *req)
>  	bio->bi_end_io = nvmet_bio_done;
>  	bio_set_op_attrs(bio, op, op_flags);
>  
> +	blk_start_plug(&plug);
>  	for_each_sg(req->sg, sg, req->sg_cnt, i) {
>  		while (bio_add_page(bio, sg_page(sg), sg->length, sg->offset)
>  				!= sg->length) {
> @@ -202,6 +204,7 @@ static void nvmet_bdev_execute_rw(struct nvmet_req *req)
>  		sector += sg->length >> 9;
>  		sg_cnt--;
>  	}
> +	blk_finish_plug(&plug);
>  
>  	submit_bio(bio);
>  }

The blk_finish_plug() should be after the last submit_bio().

I looked at plugging too since that is a difference between the
submit_bio and write_iter paths, but I thought we needed to plug the
entire IO queue drain. Otherwise this random 4k write workload should
plug a single request, which doesn't sound like it would change anything.

Using the block plug for the entire IO queue drain requires quite a bit
larger change, though. Also, I saw a similar performance difference with
a ramdisk back-end, which doesn't use plugs.

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