Re: [PATCH BUGFIX] block, bfq: postpone rq preparation to insert or merge

Mike Galbraith
2018-05-07T05:56:37Z

On Sun, 2018-05-06 at 09:42 +0200, Paolo Valente wrote:
>=20
> diff --git a/block/bfq-mq-iosched.c b/block/bfq-mq-iosched.c
> index 118f319af7c0..6662efe29b69 100644
> --- a/block/bfq-mq-iosched.c
> +++ b/block/bfq-mq-iosched.c
> @@ -525,8 +525,13 @@ static void bfq_limit_depth(unsigned int op, struct =
blk_mq_alloc_data *data)
> if (unlikely(bfqd->sb_shift !=3D bt->sb.shift))
> bfq_update_depths(bfqd, bt);
> =20
> +#if 0
> data->shallow_depth =3D
> bfqd->word_depths[!!bfqd->wr_busy_queues][op_is_sync(op)]=
;
^^^^^^^^^^^^^

Q: why doesn't the top of this function look like so?

---
block/bfq-iosched.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -539,7 +539,7 @@ static void bfq_limit_depth(unsigned int
struct bfq_data *bfqd =3D data->q->elevator->elevator_data;
struct sbitmap_queue *bt;
=20
- if (op_is_sync(op) && !op_is_write(op))
+ if (!op_is_write(op))
return;
=20
if (data->flags & BLK_MQ_REQ_RESERVED) {

It looks a bit odd that these elements exist...

+ =A0=A0=A0=A0=A0=A0/*
+ =A0=A0=A0=A0=A0=A0=A0* no more than 75% of tags for sync writes (25% extr=
a tags
+ =A0=A0=A0=A0=A0=A0=A0* w.r.t. async I/O, to prevent async I/O from starvi=
ng sync
+ =A0=A0=A0=A0=A0=A0=A0* writes)
+ =A0=A0=A0=A0=A0=A0=A0*/
+ =A0=A0=A0=A0=A0=A0bfqd->word_depths[0][1] =3D max(((1U<<bfqd->sb_shift) *=
3)>>2, 1U);

+ =A0=A0=A0=A0=A0=A0/* no more than ~37% of tags for sync writes (~20% extr=
a tags) */
+ =A0=A0=A0=A0=A0=A0bfqd->word_depths[1][1] =3D max(((1U<<bfqd->sb_shift) *=
6)>>4, 1U);

...yet we index via and log a guaranteed zero.

-Mike