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SubjectRe: [RESEND RFC PATCH 2/8] block: Allow sending a batch of requests from the scheduler to hardware
On Mon, Mar 23, 2020 at 11:44 AM Ming Lei <ming.lei@redhat.com> wrote:
>
> On Fri, Mar 20, 2020 at 06:27:41PM +0800, Baolin Wang wrote:
> > Hi Ming,
> >
> > On Wed, Mar 18, 2020 at 6:26 PM Baolin Wang <baolin.wang7@gmail.com> wrote:
> > >
> > > 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.
> >
> > I tried your approach on my platform, but met some problems, see below.
> >
> > >
> > > > 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);
> >
> > If we dispatched a request successfully by blk_mq_dispatch_rq_list(),
> > and got dispatch_ret = true now. Then we will try to dispatch more
> > reuqests from scheduler by blk_mq_do_dispatch_sched(), but if now no
> > more requests in scheduler, then we will got dispatch_ret = false. In
>
> 'dispatch_ret' always holds result of the last blk_mq_do_dispatch_sched().
> When any one request has been dispatched successfully, 'dispatch_ret'
> is true. New request is always added to list before calling
> blk_mq_do_dispatch_sched(), so once blk_mq_do_dispatch_sched() returns
> false, it means that .commit_rqs() has been called.

Not really, if no requests int the IO cheduler, we will break the loop
in blk_mq_do_dispatch_sched() and return false without calling
.commit_rqs().

So in this case, blk_mq_do_dispatch_sched() will return 'false', which
overlapped the return value of 'true' from blk_mq_dispatch_rq_list(),
and did not call .commit_rqs(). Then the IO processing will be stuck.

static void 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;

if (e->type->ops.has_work && !e->type->ops.has_work(hctx))
break;

.......
} while (ret);

return ret;
}

>
> > this case, we will not issue commit_rqs() to tell the hardware to
> > handle previous request dispatched from &rq_list.
> >
> > So I think we should not overlap the 'dispatch_ret'? Or do you have
> > any other thoughts to fix?
> >
> > > > 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;
> >
> > If we enabled BLK_MQ_F_FORCE_COMMIT_RQS flag, we will always get
> > bd.last = false even for the real last request in the IO scheduler. I
> > know you already use commit_irqs() to help to kick driver. But I
> > worried if it is reasonable that drivers always get bd.last = false.
> >
>
> BLK_MQ_F_FORCE_COMMIT_RQS means the .last flag is ignored, and we can
> document this usage.

OK. Thanks for your comments.

--
Baolin Wang

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