On Mon, 15 Apr 2024, Bart Van Assche wrote:
> On 4/15/24 12:56, Mikulas Patocka wrote:
> > I am wondering how should QUEUE_FLAG_SAME_COMP work for bio-based
> > devices.
> >
> > I grepped the kernel for QUEUE_FLAG_SAME_COMP and it is tested in
> > block/blk-mq.c in blk_mq_complete_need_ipi (this code path is taken only
> > for request-based devices) and in block/blk-sysfs.c in
> > queue_rq_affinity_show (this just displays the value in sysfs). There are
> > no other places where QUEUE_FLAG_SAME_COMP is tested, so I don't see what
> > effect is it supposed to have.
>
> I think the answer depends on whether or not the underlying device
> defines the .submit_bio() callback. From block/blk-core.c:
>
> static void __submit_bio(struct bio *bio)
> {
> if (unlikely(!blk_crypto_bio_prep(&bio)))
> return;
>
> if (!bio->bi_bdev->bd_has_submit_bio) {
> blk_mq_submit_bio(bio);
> } else if (likely(bio_queue_enter(bio) == 0)) {
> struct gendisk *disk = bio->bi_bdev->bd_disk;
>
> disk->fops->submit_bio(bio);
> blk_queue_exit(disk->queue);
> }
> }
>
> In other words, if the .submit_bio() callback is defined, that function
> is called. If it is not defined, blk_mq_submit_bio() converts the bio
> into a request. QUEUE_FLAG_SAME_COMP affects the request completion
> path. On my test setup there are multiple dm instances defined on top of
> SCSI devices. The SCSI core does not implement the .submit_bio()
> callback.
>
> Thanks,
>
> Bart.
Yes, setting the flag QUEUE_FLAG_SAME_COMP for bio-based drivers (those
that define .submit_bio) has no effect. So, I think that you patch doesn't
have any effect too.
Mikulas
