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

[Jens Axboe]

On 10/13/2016 02:59 PM, Dan Williams wrote:
> On Thu, Oct 13, 2016 at 1:24 PM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> > On 10/13/2016 02:19 PM, Dan Williams wrote:
> > > On Thu, Oct 13, 2016 at 1:09 PM, Jens Axboe <axboe@xxxxxxxxx> wrote:
> > > > 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.
> > >
> > >
> > > Assuming you always trust the kernel to know the right priority...
> >
> >
> > If it set it in the bio, it better know what it's doing. Besides,
> > there's nothing stopping the caller from checking the task priority when
> > it sets it. If we do ioprio_best(), then we are effectively preventing
> > anyone from submitting a bio with a lower priority than the task has
> > generally set.
>
> Ah, that makes sense.  Move the ioprio_best() decision out to whatever
> code is setting bio_prio() to allow for cases where the kernel knows
> best.

Yes, precisely.

--
Jens Axboe