Re: [RESEND RFC PATCH 2/8] block: Allow sending a batch of requests from the scheduler to hardware

Baolin Wang
2020-03-18T10:26:19Z

Hi Ming,

On Wed, Mar 18, 2020 at 6:01 PM Ming Lei <ming.lei@redhat.com> wrote:
>
> On Mon, Mar 16, 2020 at 06:01:19PM +0800, Baolin Wang wrote:
> > As we know, some SD/MMC host controllers can support packed request,
> > that means we can package several requests to host controller at one
> > time to improve performence. So the hardware driver expects the blk-mq
> > can dispatch a batch of requests at one time, and driver can use bd.last
> > to indicate if it is the last request in the batch to help to combine
> > requests as much as possible.
> >
> > Thus we should add batch requests setting from the block driver to tell
> > the scheduler how many requests can be dispatched in a batch, as well
> > as changing the scheduler to dispatch more than one request if setting
> > the maximum batch requests number.
> >
>
> I feel this batch dispatch style is more complicated, and some other
> drivers(virtio blk/scsi) still may get benefit if we can pass real 'last' flag in
> .queue_rq().
>
> So what about the following way by extending .commit_rqs() to this usage?
> And you can do whatever batch processing in .commit_rqs() which will be
> guaranteed to be called if BLK_MQ_F_FORCE_COMMIT_RQS is set by driver.

I'm very appreciated for your good suggestion, which is much simpler than mine.
It seems to solve my problem, and I will try it on my platform to see
if it can work and give you the feadback. Thanks again.

> diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> index 856356b1619e..cd2bbe56f83f 100644
> --- a/block/blk-mq-sched.c
> +++ b/block/blk-mq-sched.c
> @@ -85,11 +85,12 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
> * its queue by itself in its completion handler, so we don't need to
> * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
> */
> -static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> +static bool blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> {
> struct request_queue *q = hctx->queue;
> struct elevator_queue *e = q->elevator;
> LIST_HEAD(rq_list);
> + bool ret = false;
>
> do {
> struct request *rq;
> @@ -112,7 +113,10 @@ static void blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> * in blk_mq_dispatch_rq_list().
> */
> list_add(&rq->queuelist, &rq_list);
> - } while (blk_mq_dispatch_rq_list(q, &rq_list, true));
> + ret = blk_mq_dispatch_rq_list(q, &rq_list, true);
> + } while (ret);
> +
> + return ret;
> }
>
> static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
> @@ -131,11 +135,12 @@ static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
> * its queue by itself in its completion handler, so we don't need to
> * restart queue if .get_budget() returns BLK_STS_NO_RESOURCE.
> */
> -static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
> +static bool blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
> {
> struct request_queue *q = hctx->queue;
> LIST_HEAD(rq_list);
> struct blk_mq_ctx *ctx = READ_ONCE(hctx->dispatch_from);
> + bool ret = false;
>
> do {
> struct request *rq;
> @@ -161,10 +166,12 @@ static void blk_mq_do_dispatch_ctx(struct blk_mq_hw_ctx *hctx)
>
> /* round robin for fair dispatch */
> ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);
> -
> - } while (blk_mq_dispatch_rq_list(q, &rq_list, true));
> + ret = blk_mq_dispatch_rq_list(q, &rq_list, true);
> + } while (ret);
>
> WRITE_ONCE(hctx->dispatch_from, ctx);
> +
> + return ret;
> }
>
> void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> @@ -173,6 +180,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> struct elevator_queue *e = q->elevator;
> const bool has_sched_dispatch = e && e->type->ops.dispatch_request;
> LIST_HEAD(rq_list);
> + bool dispatch_ret;
>
> /* RCU or SRCU read lock is needed before checking quiesced flag */
> if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
> @@ -206,20 +214,26 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
> */
> if (!list_empty(&rq_list)) {
> blk_mq_sched_mark_restart_hctx(hctx);
> - if (blk_mq_dispatch_rq_list(q, &rq_list, false)) {
> + dispatch_ret = blk_mq_dispatch_rq_list(q, &rq_list, false);
> + if (dispatch_ret) {
> if (has_sched_dispatch)
> - blk_mq_do_dispatch_sched(hctx);
> + dispatch_ret = blk_mq_do_dispatch_sched(hctx);
> else
> - blk_mq_do_dispatch_ctx(hctx);
> + dispatch_ret = blk_mq_do_dispatch_ctx(hctx);
> }
> } else if (has_sched_dispatch) {
> - blk_mq_do_dispatch_sched(hctx);
> + dispatch_ret = blk_mq_do_dispatch_sched(hctx);
> } else if (hctx->dispatch_busy) {
> /* dequeue request one by one from sw queue if queue is busy */
> - blk_mq_do_dispatch_ctx(hctx);
> + dispatch_ret = blk_mq_do_dispatch_ctx(hctx);
> } else {
> blk_mq_flush_busy_ctxs(hctx, &rq_list);
> - blk_mq_dispatch_rq_list(q, &rq_list, false);
> + dispatch_ret = blk_mq_dispatch_rq_list(q, &rq_list, false);
> + }
> +
> + if (dispatch_ret) {
> + if (hctx->flags & BLK_MQ_F_FORCE_COMMIT_RQS)
> + hctx->queue->mq_ops->commit_rqs(hctx);
> }
> }
>
> diff --git a/block/blk-mq.c b/block/blk-mq.c
> index 87c6699f35ae..9b46f5d6c7fd 100644
> --- a/block/blk-mq.c
> +++ b/block/blk-mq.c
> @@ -1238,11 +1238,15 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
> * Flag last if we have no more requests, or if we have more
> * but can't assign a driver tag to it.
> */
> - if (list_empty(list))
> - bd.last = true;
> - else {
> - nxt = list_first_entry(list, struct request, queuelist);
> - bd.last = !blk_mq_get_driver_tag(nxt);
> + if (!(hctx->flags & BLK_MQ_F_FORCE_COMMIT_RQS)) {
> + if (list_empty(list))
> + bd.last = true;
> + else {
> + nxt = list_first_entry(list, struct request, queuelist);
> + bd.last = !blk_mq_get_driver_tag(nxt);
> + }
> + } else {
> + bd.last = false;
> }
>
> ret = q->mq_ops->queue_rq(hctx, &bd);
> diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
> index 07fa767bff86..c0ef6990b698 100644
> --- a/include/linux/blk-mq.h
> +++ b/include/linux/blk-mq.h
> @@ -394,6 +394,7 @@ enum {
> BLK_MQ_F_SHOULD_MERGE = 1 << 0,
> BLK_MQ_F_TAG_SHARED = 1 << 1,
> BLK_MQ_F_NO_MANAGED_IRQ = 1 << 2,
> + BLK_MQ_F_FORCE_COMMIT_RQS = 1 << 3,
> BLK_MQ_F_BLOCKING = 1 << 5,
> BLK_MQ_F_NO_SCHED = 1 << 6,
> BLK_MQ_F_ALLOC_POLICY_START_BIT = 8,
>
> Thanks,
> Ming
>

--
Baolin Wang