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

Dan Williams
2016-10-13T20:19:10Z

On Thu, Oct 13, 2016 at 1:09 PM, Jens Axboe <axboe@kernel.dk> wrote:
> On 10/13/2016 02:06 PM, Dan Williams wrote:
>>
>> On Thu, Oct 13, 2016 at 12:53 PM, Adam Manzanares
>> <adam.manzanares@hgst.com> 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@wdc.com>
>>> ---
>>> 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.

Assuming you always trust the kernel to know the right priority...