Re: [PATCH 8/8] block: Always initialize bio IO priority on submit

[Jan Kara <jack@suse.cz>]

On Fri 17-06-22 09:05:12, Damien Le Moal wrote:
> On 6/16/22 20:23, Jan Kara wrote:
> > On Thu 16-06-22 12:15:25, Damien Le Moal wrote:
> >> On 6/16/22 01:16, Jan Kara wrote:
> >>> Currently, IO priority set in task's IO context is not reflected in the
> >>> bio->bi_ioprio for most IO (only io_uring and direct IO set it). This
> >>> results in odd results where process is submitting some bios with one
> >>> priority and other bios with a different (unset) priority and due to
> >>> differing priorities bios cannot be merged. Make sure bio->bi_ioprio is
> >>> always set on bio submission.
> >>>
> >>> Signed-off-by: Jan Kara <jack@suse.cz>
> >>> ---
> >>>   block/blk-mq.c | 6 ++++++
> >>>   1 file changed, 6 insertions(+)
> >>>
> >>> diff --git a/block/blk-mq.c b/block/blk-mq.c
> >>> index e17d822e6051..e976f696babc 100644
> >>> --- a/block/blk-mq.c
> >>> +++ b/block/blk-mq.c
> >>> @@ -2793,7 +2793,13 @@ static inline struct request *blk_mq_get_cached_request(struct request_queue *q,
> >>>   static void bio_set_ioprio(struct bio *bio)
> >>>   {
> >>> +	unsigned short ioprio_class;
> >>> +
> >>>   	blkcg_set_ioprio(bio);
> >>
> >> Shouldn't this function set the default using the below "if" code ?
> >>
> >>> +	ioprio_class = IOPRIO_PRIO_CLASS(bio->bi_ioprio);
> >>> +	/* Nobody set ioprio so far? Initialize it based on task's nice value */
> >>
> >> I do not think that the ioprio_class variable is useful.
> >> This can be:
> >>
> >> 	if (IOPRIO_PRIO_CLASS(bio->bi_ioprio) == IOPRIO_CLASS_NONE)
> >> 		bio->bi_ioprio = get_current_ioprio();
> > 
> > You are right. Fixed.
> 
> What about moving this inside blkcg_set_ioprio() ? bio_set_ioprio() would
> then not be needed at all and blkcg_set_ioprio() called directly ?

What I dislike about this is that it is counterintuitive that
blkcg_set_prio() does anything when blkcgs are disabled in kernel config
(and it would have to to keep things working). So if you dislike
bio_set_ioprio(), I can just opencode this function in its single call
site. Would that be better?

								Honza
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR