On Mon, Jun 29, 2020 at 06:32:39PM +0800, Ming Lei wrote:
> On Mon, Jun 29, 2020 at 10:04:52AM +0100, Christoph Hellwig wrote:
> > > +/*
> > > + * We know bfq and deadline apply single scheduler queue instead of multi
> > > + * queue. However, the two are often used on single queue devices, also
> > > + * the current @hctx should affect the real device status most of times
> > > + * because of locality principle.
> > > + *
> > > + * So use current hctx->dispatch_busy directly for figuring out batching
> > > + * dispatch count.
> > > + */
> >
> > I don't really understand this comment. Also I think the code might
> > be cleaner if this function is inlined as an if/else in the only
> > caller.
> >
> > > +static inline bool blk_mq_do_dispatch_rq_lists(struct blk_mq_hw_ctx *hctx,
> > > + struct list_head *lists, bool multi_hctxs, unsigned count)
> > > +{
> > > + bool ret;
> > > +
> > > + if (!count)
> > > + return false;
> > > +
> > > + if (likely(!multi_hctxs))
> > > + return blk_mq_dispatch_rq_list(hctx, lists, count);
> >
> > Keeping these checks in the callers would keep things a little cleaner,
> > especially as the multi hctx case only really needs the lists argument.
> >
> > > + LIST_HEAD(list);
> > > + struct request *new, *rq = list_first_entry(lists,
> > > + struct request, queuelist);
> > > + unsigned cnt = 0;
> > > +
> > > + list_for_each_entry(new, lists, queuelist) {
> > > + if (new->mq_hctx != rq->mq_hctx)
> > > + break;
> > > + cnt++;
> > > + }
> > > +
> > > + if (new->mq_hctx == rq->mq_hctx)
> > > + list_splice_tail_init(lists, &list);
> > > + else
> > > + list_cut_before(&list, lists, &new->queuelist);
> > > +
> > > + ret = blk_mq_dispatch_rq_list(rq->mq_hctx, &list, cnt);
> > > + }
> >
> > I think this has two issues: for one ret should be ORed as any dispatch
> > or error should leaave ret set. Also we need to splice the dispatch
>
> OK.
>
> > list back onto the main list here, otherwise we can lose those requests.
>
> The dispatch list always becomes empty after blk_mq_dispatch_rq_list()
> returns, so no need to splice it back.
>
> >
> > FYI, while reviewing this I ended up editing things into a shape I
> > could better understand. Let me know what you think of this version?
> >
> >
> > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> > index 4c72073830f3cb..466dce99699ae4 100644
> > --- a/block/blk-mq-sched.c
> > +++ b/block/blk-mq-sched.c
> > @@ -7,6 +7,7 @@
> > #include <linux/kernel.h>
> > #include <linux/module.h>
> > #include <linux/blk-mq.h>
> > +#include <linux/list_sort.h>
> >
> > #include <trace/events/block.h>
> >
> > @@ -80,6 +81,38 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
> > blk_mq_run_hw_queue(hctx, true);
> > }
> >
> > +static int sched_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
> > +{
> > + struct request *rqa = container_of(a, struct request, queuelist);
> > + struct request *rqb = container_of(b, struct request, queuelist);
> > +
> > + return rqa->mq_hctx > rqb->mq_hctx;
> > +}
> > +
> > +static bool blk_mq_dispatch_hctx_list(struct list_head *rq_list)
> > +{
> > + struct blk_mq_hw_ctx *hctx =
> > + list_first_entry(rq_list, struct request, queuelist)->mq_hctx;
> > + struct request *rq;
> > + LIST_HEAD(hctx_list);
> > + unsigned int count = 0;
> > + bool ret;
> > +
> > + list_for_each_entry(rq, rq_list, queuelist) {
> > + if (rq->mq_hctx != hctx) {
> > + list_cut_before(&hctx_list, rq_list, &rq->queuelist);
> > + goto dispatch;
> > + }
> > + count++;
> > + }
> > + list_splice_tail_init(rq_list, &hctx_list);
> > +
> > +dispatch:
> > + ret = blk_mq_dispatch_rq_list(hctx, &hctx_list, count);
> > + list_splice(&hctx_list, rq_list);
>
> The above line isn't needed.
>
> > + return ret;
> > +}
> > +
> > #define BLK_MQ_BUDGET_DELAY 3 /* ms units */
> >
> > /*
> > @@ -90,20 +123,29 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
> > * Returns -EAGAIN if hctx->dispatch was found non-empty and run_work has to
> > * be run again. This is necessary to avoid starving flushes.
> > */
> > -static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> > +static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
>
> The return type can be changed to 'bool'.
>
> > {
> > struct request_queue *q = hctx->queue;
> > struct elevator_queue *e = q->elevator;
> > + bool multi_hctxs = false, run_queue = false;
> > + bool dispatched = false, busy = false;
> > + unsigned int max_dispatch;
> > LIST_HEAD(rq_list);
> > - int ret = 0;
> > - struct request *rq;
> > + int count = 0;
> > +
> > + if (hctx->dispatch_busy)
> > + max_dispatch = 1;
> > + else
> > + max_dispatch = hctx->queue->nr_requests;
> >
> > do {
> > + struct request *rq;
> > +
> > if (e->type->ops.has_work && !e->type->ops.has_work(hctx))
> > break;
> >
> > if (!list_empty_careful(&hctx->dispatch)) {
> > - ret = -EAGAIN;
> > + busy = true;
> > break;
> > }
> >
> > @@ -120,7 +162,7 @@ static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> > * no guarantee anyone will kick the queue. Kick it
> > * ourselves.
> > */
> > - blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY);
> > + run_queue = true;
> > break;
> > }
> >
> > @@ -130,7 +172,43 @@ static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
> > * in blk_mq_dispatch_rq_list().
> > */
> > list_add(&rq->queuelist, &rq_list);
>
> The above should change to list_add_tail(&rq->queuelist, &rq_list).
Hi Christoph,
Follows the revised patch, and will post it as V7 if you are fine:
diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index 4c72073830f3..1c52e56a19b1 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -7,6 +7,7 @@
#include <linux/kernel.h>
#include <linux/module.h>
#include <linux/blk-mq.h>
+#include <linux/list_sort.h>
#include <trace/events/block.h>
@@ -80,6 +81,37 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
blk_mq_run_hw_queue(hctx, true);
}
+static int sched_rq_cmp(void *priv, struct list_head *a, struct list_head *b)
+{
+ struct request *rqa = container_of(a, struct request, queuelist);
+ struct request *rqb = container_of(b, struct request, queuelist);
+
+ return rqa->mq_hctx > rqb->mq_hctx;
+}
+
+static bool blk_mq_dispatch_hctx_list(struct list_head *rq_list)
+{
+ struct blk_mq_hw_ctx *hctx =
+ list_first_entry(rq_list, struct request, queuelist)->mq_hctx;
+ struct request *rq;
+ LIST_HEAD(hctx_list);
+ unsigned int count = 0;
+ bool ret;
+
+ list_for_each_entry(rq, rq_list, queuelist) {
+ if (rq->mq_hctx != hctx) {
+ list_cut_before(&hctx_list, rq_list, &rq->queuelist);
+ goto dispatch;
+ }
+ count++;
+ }
+ list_splice_tail_init(rq_list, &hctx_list);
+
+dispatch:
+ ret = blk_mq_dispatch_rq_list(hctx, &hctx_list, count);
+ return ret;
+}
+
#define BLK_MQ_BUDGET_DELAY 3 /* ms units */
/*
@@ -90,20 +122,29 @@ void blk_mq_sched_restart(struct blk_mq_hw_ctx *hctx)
* Returns -EAGAIN if hctx->dispatch was found non-empty and run_work has to
* be run again. This is necessary to avoid starving flushes.
*/
-static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
+static int __blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
{
struct request_queue *q = hctx->queue;
struct elevator_queue *e = q->elevator;
+ bool multi_hctxs = false, run_queue = false;
+ bool dispatched = false, busy = false;
+ unsigned int max_dispatch;
LIST_HEAD(rq_list);
- int ret = 0;
- struct request *rq;
+ int count = 0;
+
+ if (hctx->dispatch_busy)
+ max_dispatch = 1;
+ else
+ max_dispatch = hctx->queue->nr_requests;
do {
+ struct request *rq;
+
if (e->type->ops.has_work && !e->type->ops.has_work(hctx))
break;
if (!list_empty_careful(&hctx->dispatch)) {
- ret = -EAGAIN;
+ busy = true;
break;
}
@@ -120,7 +161,7 @@ static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
* no guarantee anyone will kick the queue. Kick it
* ourselves.
*/
- blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY);
+ run_queue = true;
break;
}
@@ -129,8 +170,42 @@ static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
* if this rq won't be queued to driver via .queue_rq()
* in blk_mq_dispatch_rq_list().
*/
- list_add(&rq->queuelist, &rq_list);
- } while (blk_mq_dispatch_rq_list(rq->mq_hctx, &rq_list, 1));
+ list_add_tail(&rq->queuelist, &rq_list);
+ if (rq->mq_hctx != hctx)
+ multi_hctxs = true;
+ } while (++count < max_dispatch);
+
+ if (!count) {
+ if (run_queue)
+ blk_mq_delay_run_hw_queues(q, BLK_MQ_BUDGET_DELAY);
+ } else if (multi_hctxs) {
+ /*
+ * Requests from different hctx may be dequeued from some
+ * schedulers, such as bfq and deadline.
+ *
+ * Sort the requests in the list according to their hctx,
+ * dispatch batching requests from same hctx at a time.
+ */
+ list_sort(NULL, &rq_list, sched_rq_cmp);
+ do {
+ dispatched |= blk_mq_dispatch_hctx_list(&rq_list);
+ } while (!list_empty(&rq_list));
+ } else {
+ dispatched = blk_mq_dispatch_rq_list(hctx, &rq_list, count);
+ }
+
+ if (busy)
+ return -EAGAIN;
+ return !!dispatched;
+}
+
+static int blk_mq_do_dispatch_sched(struct blk_mq_hw_ctx *hctx)
+{
+ int ret;
+
+ do {
+ ret = __blk_mq_do_dispatch_sched(hctx);
+ } while (ret == 1);
return ret;
}
diff --git a/block/blk-mq.c b/block/blk-mq.c
index d273a56f11c0..57ae018d5cc8 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -1323,8 +1323,6 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx *hctx, struct list_head *list,
if (list_empty(list))
return false;
- WARN_ON(!list_is_singular(list) && nr_budgets);
-
/*
* Now process all the entries, sending them to the driver.
*/
Thanks,
Ming
