Re: [PATCH v4 1/4] block: Add iocontext priority to request

From: Jens Axboe
Date: Thu Oct 13 2016 - 16:10:36 EST

On 10/13/2016 02:06 PM, Dan Williams wrote:
On Thu, Oct 13, 2016 at 12:53 PM, Adam Manzanares
<adam.manzanares@xxxxxxxx> wrote:
Patch adds an association between iocontext ioprio and the ioprio of a
request. This value is set in blk_rq_set_prio which takes the request and
the ioc as arguments. If the ioc is valid in blk_rq_set_prio then the
iopriority of the request is set as the iopriority of the ioc. In
init_request_from_bio a check is made to see if the ioprio of the bio is
valid and if so then the request prio comes from the bio.

Signed-off-by: Adam Manzananares <adam.manzanares@xxxxxxx>
block/blk-core.c | 4 +++-
include/linux/blkdev.h | 14 ++++++++++++++
2 files changed, 17 insertions(+), 1 deletion(-)

diff --git a/block/blk-core.c b/block/blk-core.c
index 14d7c07..361b1b9 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -1153,6 +1153,7 @@ static struct request *__get_request(struct request_list *rl, int op,

blk_rq_init(q, rq);
blk_rq_set_rl(rq, rl);
+ blk_rq_set_prio(rq, ioc);
req_set_op_attrs(rq, op, op_flags | REQ_ALLOCED);

/* init elvpriv */
@@ -1656,7 +1657,8 @@ void init_request_from_bio(struct request *req, struct bio *bio)

req->errors = 0;
req->__sector = bio->bi_iter.bi_sector;
- req->ioprio = bio_prio(bio);
+ if (ioprio_valid(bio_prio(bio)))
+ req->ioprio = bio_prio(bio);

Should we use ioprio_best() here? If req->ioprio and bio_prio()
disagree one side has explicitly asked for a higher priority.

It's a good question - but if priority has been set in the bio, it makes
sense that that would take priority over the general setting for the
task/io context. So I think the patch is correct as-is.

Adam, you'll want to rewrite the commit message though. A good commit
message should explain WHY the change is made, not detail the code
implementation of it.

Jens Axboe