From: Mikulas Patocka <mpatocka@redhat.com>
To: Bart Van Assche <bvanassche@acm.org>
Cc: Mike Snitzer <snitzer@redhat.com>,
dm-devel@lists.linux.dev, Eric Biggers <ebiggers@kernel.org>,
Jaegeuk Kim <jaegeuk@kernel.org>,
Daniel Lee <chullee@google.com>,
stable@vger.kernel.org
Subject: Re: [PATCH] dm: Change the default value of rq_affinity from 0 into 1
Date: Tue, 16 Apr 2024 15:30:40 +0200 (CEST) [thread overview]
Message-ID: <1494d512-9eb0-2091-8e9f-fbb28d2441d1@redhat.com> (raw)
In-Reply-To: <c107bba4-03a1-4121-af9e-7c93f40c24a0@acm.org>
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
prev parent reply other threads:[~2024-04-16 13:30 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-04-15 19:49 [PATCH] dm: Change the default value of rq_affinity from 0 into 1 Bart Van Assche
2024-04-15 19:56 ` Mikulas Patocka
2024-04-15 20:54 ` Bart Van Assche
2024-04-16 13:30 ` Mikulas Patocka [this message]
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