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SubjectRe: [PATCH V5 6/7] blk-mq-sched: improve dispatching from sw queue
On Sat, Sep 30, 2017 at 06:27:19PM +0800, Ming Lei wrote:
> SCSI devices use host-wide tagset, and the shared
> driver tag space is often quite big. Meantime
> there is also queue depth for each lun(.cmd_per_lun),
> which is often small.
>
> So lots of requests may stay in sw queue, and we
> always flush all belonging to same hw queue and
> dispatch them all to driver, unfortunately it is
> easy to cause queue busy because of the small
> per-lun queue depth. Once these requests are flushed
> out, they have to stay in hctx->dispatch, and no bio
> merge can participate into these requests, and
> sequential IO performance is hurted.
>
> This patch improves dispatching from sw queue when
> there is per-request-queue queue depth by taking
> request one by one from sw queue, just like the way
> of IO scheduler.

Once again, use your line length real estate for your change logs..

> +static 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 = READ_ONCE(hctx->dispatch_from);
> + bool dispatched;
> +
> + do {
> + struct request *rq;
> +
> + rq = blk_mq_dequeue_from_ctx(hctx, ctx);

This probably should be merged wit hthe patch that introduceѕ
blk_mq_dequeue_from_ctx.

> + if (!rq)
> + break;
> + list_add(&rq->queuelist, &rq_list);

Btw, do we really need to return a request from blk_mq_dequeue_from_ctx,
or would it be easier to just add it directly to the list passed as
an argument. I did wonder that about the existing code already.

> +
> + /* round robin for fair dispatch */
> + ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);
> +
> + dispatched = blk_mq_dispatch_rq_list(q, &rq_list);
> + } while (dispatched);
> +
> + if (!dispatched)
> + WRITE_ONCE(hctx->dispatch_from, ctx);

No need for the dispatched argument, just write this as:

} while (blk_mq_dispatch_rq_list(q, &rq_list));

WRITE_ONCE(hctx->dispatch_from, ctx);

and do an early return instead of a break from inside the loop.

> + } else if (!has_sched_dispatch && !q->queue_depth) {
> + /*
> + * If there is no per-request_queue depth, we
> + * flush all requests in this hw queue, otherwise
> + * pick up request one by one from sw queue for
> + * avoiding to mess up I/O merge when dispatch
> + * run out of resource, which can be triggered
> + * easily by per-request_queue queue depth
> + */

Use your 80 line real estate for comments..

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