On Tue, Sep 19, 2017 at 02:37:50PM -0600, Jens Axboe wrote:
> On 09/02/2017 09:17 AM, Ming Lei wrote:
> > @@ -142,18 +178,31 @@ 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) {
> > + /*
> > + * 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
> > + * is busy, which can be triggered easily by
> > + * per-request_queue queue depth
> > + */
> > blk_mq_flush_busy_ctxs(hctx, &rq_list);
> > blk_mq_dispatch_rq_list(q, &rq_list);
> > }
>
> I don't like this part at all. It's a heuristic, and on top of that,
> it's a heuristic based on a weak assumption that if q->queue_depth == 0,
> then we never run into BUSY constraints. I think that would be stronger
> if it was based on "is this using shared tags", but even that is not
> great at all. It's perfectly possible to have a shared tags setup and
> never run into resource constraints. The opposite condition is also true
> - you can run without shared tags, and yet hit resource constraints
> constantly. Hence this is NOT a good test for deciding whether to flush
> everything at once or not. In short, I think a much better test case
> would be "has this device ever returned BLK_STS_RESOURCE. As it so
> happens, that's easy enough to keep track of and base this test on.
Hi Jens,
The attached patch follows your suggestion, and uses EWMA to
compute the average length of hctx->dispatch, then only flush
all requests from ctxs if the average length is zero, what do
you think about this approach? Or other suggestions?
diff --git a/block/blk-mq-debugfs.c b/block/blk-mq-debugfs.c
index 7a27f262c96a..c420c775b9c0 100644
--- a/block/blk-mq-debugfs.c
+++ b/block/blk-mq-debugfs.c
@@ -232,6 +232,14 @@ static int hctx_flags_show(void *data, struct seq_file *m)
return 0;
}
+static int hctx_dispatch_len_show(void *data, struct seq_file *m)
+{
+ struct blk_mq_hw_ctx *hctx = data;
+
+ seq_printf(m, "%u\n", hctx->dispatch_len);
+ return 0;
+}
+
#define REQ_OP_NAME(name) [REQ_OP_##name] = #name
static const char *const op_name[] = {
REQ_OP_NAME(READ),
@@ -763,6 +771,7 @@ static const struct blk_mq_debugfs_attr
blk_mq_debugfs_hctx_attrs[] = {
{"state", 0400, hctx_state_show},
{"flags", 0400, hctx_flags_show},
{"dispatch", 0400, .seq_ops = &hctx_dispatch_seq_ops},
+ {"dispatch_length", 0400, hctx_dispatch_len_show},
{"busy", 0400, hctx_busy_show},
{"ctx_map", 0400, hctx_ctx_map_show},
{"tags", 0400, hctx_tags_show},
diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
index a5712dd67783..91a6eeaadaf1 100644
--- a/block/blk-mq-sched.c
+++ b/block/blk-mq-sched.c
@@ -102,7 +102,7 @@ static void blk_mq_do_dispatch_sched(struct request_queue
*q,
if (!rq)
break;
list_add(&rq->queuelist, &rq_list);
- } while (blk_mq_dispatch_rq_list(q, &rq_list));
+ } while (blk_mq_dispatch_rq_list(q, &rq_list, 1));
}
static struct blk_mq_ctx *blk_mq_next_ctx(struct blk_mq_hw_ctx *hctx,
@@ -134,19 +134,51 @@ static void blk_mq_do_dispatch_ctx(struct request_queue
*q,
/* round robin for fair dispatch */
ctx = blk_mq_next_ctx(hctx, rq->mq_ctx);
- dispatched = blk_mq_dispatch_rq_list(q, &rq_list);
+ dispatched = blk_mq_dispatch_rq_list(q, &rq_list, 1);
} while (dispatched);
if (!dispatched)
WRITE_ONCE(hctx->dispatch_from, ctx);
}
+/* borrowed from bcache */
+void ewma_add(unsigned *ewma, unsigned val, unsigned weight)
+{
+ unsigned result = READ_ONCE(*ewma);
+
+ if (val == 1) {
+ result += 1;
+ } else {
+ result *= weight - 1;
+ result += val;
+ result /= weight;
+ }
+ WRITE_ONCE(*ewma, result);
+}
+
+/*
+ * We use EWMA to compute the average length of dispatch list.
+ * It is totally lockless, but we can survive that since it
+ * is just a hint.
+ *
+ * We only flush requests from all ctxs if the average length
+ * of dispatch list is zero, otherwise the hw queue can be
+ * thought as busy and we dequeue request from ctxs one by
+ * one
+ */
+static void blk_mq_update_dispatch_length(struct blk_mq_hw_ctx *hctx,
+ unsigned cnt)
+{
+ ewma_add(&hctx->dispatch_len, cnt, 8);
+}
+
void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx *hctx)
{
struct request_queue *q = hctx->queue;
struct elevator_queue *e = q->elevator;
const bool has_sched_dispatch = e && e->type->ops.mq.dispatch_request;
LIST_HEAD(rq_list);
+ unsigned rq_cnt = 0;
/* RCU or SRCU read lock is needed before checking quiesced flag */
if (unlikely(blk_mq_hctx_stopped(hctx) || blk_queue_quiesced(q)))
@@ -162,9 +194,14 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx
*hctx)
spin_lock(&hctx->lock);
if (!list_empty(&hctx->dispatch))
list_splice_init(&hctx->dispatch, &rq_list);
+ rq_cnt = hctx->cnt;
+ hctx->cnt = 0;
spin_unlock(&hctx->lock);
}
+ if (!has_sched_dispatch)
+ blk_mq_update_dispatch_length(hctx, rq_cnt);
+
/*
* Only ask the scheduler for requests, if we didn't have residual
* requests from the dispatch list. This is to avoid the case where
@@ -176,7 +213,7 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx
*hctx)
*/
if (!list_empty(&rq_list)) {
blk_mq_sched_mark_restart_hctx(hctx);
- blk_mq_dispatch_rq_list(q, &rq_list);
+ blk_mq_dispatch_rq_list(q, &rq_list, rq_cnt);
/*
* We may clear DISPATCH_BUSY just after it
@@ -204,16 +241,16 @@ void blk_mq_sched_dispatch_requests(struct blk_mq_hw_ctx
*hctx)
if (!has_sched_dispatch) {
/*
- * If there is no per-request_queue depth, we
+ * If dispatch doesn't run out of resource, 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
*/
- if (!q->queue_depth) {
- blk_mq_flush_busy_ctxs(hctx, &rq_list);
- blk_mq_dispatch_rq_list(q, &rq_list);
+ if (!hctx->dispatch_len) {
+ rq_cnt = blk_mq_flush_busy_ctxs(hctx, &rq_list);
+ blk_mq_dispatch_rq_list(q, &rq_list, rq_cnt);
} else {
blk_mq_do_dispatch_ctx(q, hctx);
}
@@ -346,6 +383,7 @@ static bool blk_mq_sched_bypass_insert(struct blk_mq_hw_ctx
*hctx,
* the dispatch list.
*/
spin_lock(&hctx->lock);
+ hctx->cnt++;
list_add(&rq->queuelist, &hctx->dispatch);
set_bit(BLK_MQ_S_DISPATCH_BUSY, &hctx->state);
spin_unlock(&hctx->lock);
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 1bb45e995da3..b2db5d6d568e 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -850,6 +850,7 @@ static void blk_mq_timeout_work(struct work_struct *work)
struct flush_busy_ctx_data {
struct blk_mq_hw_ctx *hctx;
struct list_head *list;
+ unsigned cnt;
};
static bool flush_busy_ctx(struct sbitmap *sb, unsigned int bitnr, void *data)
@@ -857,11 +858,16 @@ static bool flush_busy_ctx(struct sbitmap *sb, unsigned
int bitnr, void *data)
struct flush_busy_ctx_data *flush_data = data;
struct blk_mq_hw_ctx *hctx = flush_data->hctx;
struct blk_mq_ctx *ctx = hctx->ctxs[bitnr];
+ unsigned cnt = 0;
sbitmap_clear_bit(sb, bitnr);
spin_lock(&ctx->lock);
list_splice_tail_init(&ctx->rq_list, flush_data->list);
+ cnt = ctx->cnt;
+ ctx->cnt = 0;;
spin_unlock(&ctx->lock);
+ flush_data->cnt += cnt;
+
return true;
}
@@ -869,14 +875,17 @@ static bool flush_busy_ctx(struct sbitmap *sb, unsigned
int bitnr, void *data)
* Process software queues that have been marked busy, splicing them
* to the for-dispatch
*/
-void blk_mq_flush_busy_ctxs(struct blk_mq_hw_ctx *hctx, struct list_head *list)
+unsigned blk_mq_flush_busy_ctxs(struct blk_mq_hw_ctx *hctx, struct list_head
*list)
{
struct flush_busy_ctx_data data = {
.hctx = hctx,
.list = list,
+ .cnt = 0,
};
sbitmap_for_each_set(&hctx->ctx_map, flush_busy_ctx, &data);
+
+ return data.cnt;
}
EXPORT_SYMBOL_GPL(blk_mq_flush_busy_ctxs);
@@ -897,6 +906,7 @@ static bool dispatch_rq_from_ctx(struct sbitmap *sb,
unsigned int bitnr, void *d
list_del_init(&dispatch_data->rq->queuelist);
if (list_empty(&ctx->rq_list))
sbitmap_clear_bit(sb, bitnr);
+ ctx->cnt--;
}
spin_unlock(&ctx->lock);
@@ -1052,7 +1062,9 @@ static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx
*hctx)
return true;
}
-bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
+/* @cnt represents the number of requests in @list */
+bool blk_mq_dispatch_rq_list(struct request_queue *q,
+ struct list_head *list, unsigned cnt)
{
struct blk_mq_hw_ctx *hctx;
struct request *rq;
@@ -1139,6 +1151,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q,
struct list_head *list)
blk_mq_put_driver_tag(rq);
spin_lock(&hctx->lock);
+ hctx->cnt += cnt - queued - errors;
list_splice_init(list, &hctx->dispatch);
/*
* DISPATCH_BUSY won't be cleared until all requests
@@ -1431,6 +1444,7 @@ static inline void __blk_mq_insert_req_list(struct
blk_mq_hw_ctx *hctx,
list_add(&rq->queuelist, &ctx->rq_list);
else
list_add_tail(&rq->queuelist, &ctx->rq_list);
+ ctx->cnt++;
}
void __blk_mq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
@@ -1454,6 +1468,7 @@ void blk_mq_request_bypass_insert(struct request *rq)
struct blk_mq_hw_ctx *hctx = blk_mq_map_queue(rq->q, ctx->cpu);
spin_lock(&hctx->lock);
+ hctx->cnt++;
list_add_tail(&rq->queuelist, &hctx->dispatch);
set_bit(BLK_MQ_S_DISPATCH_BUSY, &hctx->state);
spin_unlock(&hctx->lock);
@@ -1941,6 +1956,7 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu,
struct hlist_node *node)
if (!list_empty(&ctx->rq_list)) {
list_splice_init(&ctx->rq_list, &tmp);
blk_mq_hctx_clear_pending(hctx, ctx);
+ ctx->cnt = 0;
}
spin_unlock(&ctx->lock);
@@ -1950,6 +1966,7 @@ static int blk_mq_hctx_notify_dead(unsigned int cpu,
struct hlist_node *node)
spin_lock(&hctx->lock);
list_splice_tail_init(&tmp, &hctx->dispatch);
set_bit(BLK_MQ_S_DISPATCH_BUSY, &hctx->state);
+ hctx->cnt++;
spin_unlock(&hctx->lock);
blk_mq_run_hw_queue(hctx, true);
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 231cfb0d973b..0da5a81cc41f 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -9,6 +9,7 @@ struct blk_mq_ctx {
struct {
spinlock_t lock;
struct list_head rq_list;
+ unsigned cnt;
} ____cacheline_aligned_in_smp;
unsigned int cpu;
@@ -30,8 +31,9 @@ void blk_mq_freeze_queue(struct request_queue *q);
void blk_mq_free_queue(struct request_queue *q);
int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr);
void blk_mq_wake_waiters(struct request_queue *q);
-bool blk_mq_dispatch_rq_list(struct request_queue *, struct list_head *);
-void blk_mq_flush_busy_ctxs(struct blk_mq_hw_ctx *hctx, struct list_head
*list);
+bool blk_mq_dispatch_rq_list(struct request_queue *, struct list_head *,
+ unsigned);
+unsigned blk_mq_flush_busy_ctxs(struct blk_mq_hw_ctx *hctx, struct list_head
*list);
bool blk_mq_hctx_has_pending(struct blk_mq_hw_ctx *hctx);
bool blk_mq_get_driver_tag(struct request *rq, struct blk_mq_hw_ctx **hctx,
bool wait);
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 13f6c25fa461..778dbdd596f3 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -11,6 +11,7 @@ struct blk_flush_queue;
struct blk_mq_hw_ctx {
struct {
spinlock_t lock;
+ unsigned cnt;
struct list_head dispatch;
unsigned long state; /* BLK_MQ_S_* flags */
} ____cacheline_aligned_in_smp;
@@ -31,6 +32,7 @@ struct blk_mq_hw_ctx {
struct sbitmap ctx_map;
struct blk_mq_ctx *dispatch_from;
+ unsigned dispatch_len;
struct blk_mq_ctx **ctxs;
unsigned int nr_ctx;
Thanks,
Ming
