Re: [PATCH V2 05/20] blk-mq-sched: improve dispatching from sw queue

Ming Lei
2017-08-24T05:53:01Z

On Tue, Aug 22, 2017 at 07:55:55PM +0000, Bart Van Assche wrote:
> On Sat, 2017-08-05 at 14:56 +0800, Ming Lei wrote:
> > easy to cause queue busy becasue of the small
> ^^^^^^^
> because?
>
> > -static void blk_mq_do_dispatch(struct request_queue *q,
> > - struct elevator_queue *e,
> > - struct blk_mq_hw_ctx *hctx)
> > +static inline void blk_mq_do_dispatch_sched(struct request_queue *q,
> > + struct elevator_queue *e,
> > + struct blk_mq_hw_ctx *hctx)
> > {
> > LIST_HEAD(rq_list);
>
> Why to declare this function "inline"? Are you sure that the compiler
> is not smart enough to decide on its own whether or not to inline this
> function?

OK.

>
> > +static inline struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
> > + struct blk_mq_ctx *ctx)
> > +{
> > + unsigned idx = ctx->index_hw;
> > +
> > + if (++idx == hctx->nr_ctx)
> > + idx = 0;
> > +
> > + return hctx->ctxs[idx];
> > +}
> > +
> > +static inline void blk_mq_do_dispatch_ctx(struct request_queue *q,
> > + struct blk_mq_hw_ctx *hctx)
> > +{
> > + LIST_HEAD(rq_list);
> > + struct blk_mq_ctx *ctx = NULL;
> > +
> > + do {
> > + struct request *rq;
> > +
> > + rq = blk_mq_dispatch_rq_from_ctx(hctx, ctx);
> > + if (!rq)
> > + break;
> > + list_add(&rq->queuelist, &rq_list);
> > +
> > + /* round robin for fair dispatch */
> > + ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);
> > + } while (blk_mq_dispatch_rq_list(q, &rq_list));
> > +}
>
> Please consider to move the blk_mq_next_ctx() functionality into
> blk_mq_dispatch_rq_from_ctx() as requested in a comment on a previous patch.

I believe this way is more clean and readable, otherwise
blk_mq_dispatch_rq_from_next_ctx() can be a bit ugly.

>
> > void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> > {
> > struct request_queue *q = hctx->queue;
> > @@ -142,18 +172,31 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> > if (!list_empty(&rq_list)) {
> > blk_mq_sched_mark_restart_hctx(hctx);
> > do_sched_dispatch = blk_mq_dispatch_rq_list(q, &rq_list);
> > - } else if (!has_sched_dispatch) {
> > + } else if (!has_sched_dispatch & !q->queue_depth) {
>
> Please use "&&" instead of "&" to represent logical and.

Hamm, I remember that another one is fixed against V1, but
this one is missed.

>
> Otherwise this patch looks fine to me.

Thanks.

--
Ming