On Mon, Jan 15, 2018 at 02:41:12PM -0500, Mike Snitzer wrote:
> On Mon, Jan 15 2018 at 12:29pm -0500,
> Jens Axboe <ax...@kernel.dk> wrote:
>
> > On 1/15/18 9:58 AM, Ming Lei wrote:
> > > No functional change, just to clean up code a bit, so that the following
> > > change of using direct issue for blk_mq_request_bypass_insert() which is
> > > needed by DM can be easier to do.
> > >
> > > Signed-off-by: Ming Lei <ming....@redhat.com>
> > > ---
> > > block/blk-mq.c | 39 +++++++++++++++++++++++++++------------
> > > 1 file changed, 27 insertions(+), 12 deletions(-)
> > >
> > > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > > index edb1291a42c5..bf8d6651f40e 100644
> > > --- a/block/blk-mq.c
> > > +++ b/block/blk-mq.c
> > > @@ -1696,15 +1696,37 @@ static blk_qc_t request_to_qc_t(struct
> > > blk_mq_hw_ctx *hctx, struct request *rq)
> > > return blk_tag_to_qc_t(rq->internal_tag, hctx->queue_num, true);
> > > }
> > >
> > > -static void __blk_mq_try_issue_directly(struct blk_mq_hw_ctx *hctx,
> > > - struct request *rq,
> > > - blk_qc_t *cookie)
> > > +static blk_status_t __blk_mq_issue_req(struct blk_mq_hw_ctx *hctx,
> > > + struct request *rq,
> > > + blk_qc_t *new_cookie)
> > > {
> > > + blk_status_t ret;
> > > struct request_queue *q = rq->q;
> > > struct blk_mq_queue_data bd = {
> > > .rq = rq,
> > > .last = true,
> > > };
> > > +
> > > + if (!blk_mq_get_driver_tag(rq, NULL, false))
> > > + return BLK_STS_AGAIN;
> > > +
> > > + if (!blk_mq_get_dispatch_budget(hctx)) {
> > > + blk_mq_put_driver_tag(rq);
> > > + return BLK_STS_AGAIN;
> > > + }
> > > +
> > > + *new_cookie = request_to_qc_t(hctx, rq);
> > > +
> > > + ret = q->mq_ops->queue_rq(hctx, &bd);
> > > +
> > > + return ret;
> >
> > return q->mq_ops->queue_rq(hctx, &bd);
> >
> > and kill 'ret', it's not used.
>
> Yeap, good point.
>
> > But more importantly, who puts the
> > driver tag and the budget if we get != OK for ->queue_rq()?
>
> __blk_mq_try_issue_directly() processes the returned value same as before
> this patch. Means this patch isn't making any functional change:
> If BLK_STS_RESOURCE: __blk_mq_requeue_request() is called.
> __blk_mq_requeue_request() will blk_mq_put_driver_tag().
> Otherwise, all other errors result in blk_mq_end_request(rq, ret);
>
> So ignoring this patch, are you concerned that:
> 1) Does blk_mq_end_request() put both? Looks like blk_mq_free_request()
> handles rq->tag != -1 but why not have it use __blk_mq_put_driver_tag()?
> I'm not seeing where the budget is put from blk_mq_end_request()...
blk_mq_free_request() releases driver tag, and budget is owned by driver
once .queue_rq is called.
>
> 2) Nothing seems to be putting the budget in
> __blk_mq_try_issue_directly()'s BLK_STS_RESOURCE error path? I share
> your concern now (for drivers who set {get,put}_budget in mq_ops).
> Should __blk_mq_requeue_request() be updated to also
> blk_mq_put_dispatch_budget()?
No, at least it is current protocol of using budget, please see
scsi_mq_queue_rq() and comment of blk_mq_do_dispatch_sched().
--
Ming
