diff mbox series

[-next] blk-mq: fix tag_get wait task can't be awakened

Message ID 20210913081248.3201596-1-qiulaibin@huawei.com (mailing list archive)
State New, archived
Headers show
Series [-next] blk-mq: fix tag_get wait task can't be awakened | expand

Commit Message

QiuLaibin Sept. 13, 2021, 8:12 a.m. UTC 
When multiple hctx share one tagset. The wake_batch is calculated
during initialization by queue_depth. But when multiple hctx share one
tagset. The queue depth assigned to each user may be smaller than
wakup_batch. This may cause the waiting queue to fail to wakup and leads
to Hang.

Fix this by recalculating wake_batch when inc or dec active_queues.

Fixes: 0d2602ca30e41 ("blk-mq: improve support for shared tags maps")
Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>
---
 block/blk-mq-tag.c      | 44 +++++++++++++++++++++++++++++++++++++++--
 include/linux/sbitmap.h |  8 ++++++++
 lib/sbitmap.c           |  3 ++-
 3 files changed, 52 insertions(+), 3 deletions(-)

Comments

Ming Lei Sept. 26, 2021, 12:47 p.m. UTC | #1 
Hi Laibin,

On Mon, Sep 13, 2021 at 04:12:48PM +0800, Laibin Qiu wrote:
> When multiple hctx share one tagset. The wake_batch is calculated
> during initialization by queue_depth. But when multiple hctx share one
> tagset. The queue depth assigned to each user may be smaller than
> wakup_batch. This may cause the waiting queue to fail to wakup and leads
> to Hang.

In case of shared tags, there might be more than one hctx which
allocates tag from single tags, and each hctx is limited to allocate
at most:

 	hctx_max_depth = max((bt->sb.depth + users - 1) / users, 4U);

	and

	users = atomic_read(&hctx->tags->active_queues)

See hctx_may_queue().

tag idle detection is lazy, and may be delayed for 30sec, so
there could be just one real active hctx(queue) but all others are
actually idle and still accounted as active because of the lazy
idle detection. Then if wake_batch is > hctx_max_depth, driver
tag allocation may wait forever on this real active hctx.

Correct me if my understanding is wrong.

> 
> Fix this by recalculating wake_batch when inc or dec active_queues.
> 
> Fixes: 0d2602ca30e41 ("blk-mq: improve support for shared tags maps")
> Signed-off-by: Laibin Qiu <qiulaibin@huawei.com>
> ---
>  block/blk-mq-tag.c      | 44 +++++++++++++++++++++++++++++++++++++++--
>  include/linux/sbitmap.h |  8 ++++++++
>  lib/sbitmap.c           |  3 ++-
>  3 files changed, 52 insertions(+), 3 deletions(-)
> 
> diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
> index 86f87346232a..d02f5ac0004c 100644
> --- a/block/blk-mq-tag.c
> +++ b/block/blk-mq-tag.c
> @@ -16,6 +16,27 @@
>  #include "blk-mq-sched.h"
>  #include "blk-mq-tag.h"
>  
> +static void bt_update_wake_batch(struct sbitmap_queue *bt, unsigned int users)
> +{
> +	unsigned int depth;
> +
> +	depth = max((bt->sb.depth + users - 1) / users, 4U);
> +	sbitmap_queue_update_wake_batch(bt, depth);
> +}

Use the hctx's max queue depth could reduce wake_batch a lot, then
performance may be degraded.

Just wondering why not set sbq->wake_batch as hctx_max_depth if
sbq->wake_batch is < hctx_max_depth?



Thanks,
Ming
diff mbox series

Patch

diff --git a/block/blk-mq-tag.c b/block/blk-mq-tag.c
index 86f87346232a..d02f5ac0004c 100644
--- a/block/blk-mq-tag.c
+++ b/block/blk-mq-tag.c
@@ -16,6 +16,27 @@ 
 #include "blk-mq-sched.h"
 #include "blk-mq-tag.h"
 
+static void bt_update_wake_batch(struct sbitmap_queue *bt, unsigned int users)
+{
+	unsigned int depth;
+
+	depth = max((bt->sb.depth + users - 1) / users, 4U);
+	sbitmap_queue_update_wake_batch(bt, depth);
+}
+
+/*
+ * Recalculate wakeup batch when tag is shared by hctx.
+ */
+static void blk_mq_update_wake_batch(struct sbitmap_queue *bitmap_tags,
+		struct sbitmap_queue *breserved_tags, unsigned int users)
+{
+	if (!users)
+		return;
+
+	bt_update_wake_batch(bitmap_tags, users);
+	bt_update_wake_batch(breserved_tags, users);
+}
+
 /*
  * If a previously inactive queue goes active, bump the active user count.
  * We need to do this before try to allocate driver tag, then even if fail
@@ -24,17 +45,29 @@ 
  */
 bool __blk_mq_tag_busy(struct blk_mq_hw_ctx *hctx)
 {
+	unsigned int users;
+
 	if (blk_mq_is_sbitmap_shared(hctx->flags)) {
 		struct request_queue *q = hctx->queue;
 		struct blk_mq_tag_set *set = q->tag_set;
 
 		if (!test_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags) &&
-		    !test_and_set_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags))
+		    !test_and_set_bit(QUEUE_FLAG_HCTX_ACTIVE, &q->queue_flags)) {
 			atomic_inc(&set->active_queues_shared_sbitmap);
+
+			users = atomic_read(&set->active_queues_shared_sbitmap);
+			blk_mq_update_wake_batch(&set->__bitmap_tags,
+					&set->__breserved_tags, users);
+		}
 	} else {
 		if (!test_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state) &&
-		    !test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
+		    !test_and_set_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state)) {
 			atomic_inc(&hctx->tags->active_queues);
+
+			users = atomic_read(&hctx->tags->active_queues);
+			blk_mq_update_wake_batch(&hctx->tags->__bitmap_tags,
+					&hctx->tags->__breserved_tags, users);
+		}
 	}
 
 	return true;
@@ -59,16 +92,23 @@  void __blk_mq_tag_idle(struct blk_mq_hw_ctx *hctx)
 	struct blk_mq_tags *tags = hctx->tags;
 	struct request_queue *q = hctx->queue;
 	struct blk_mq_tag_set *set = q->tag_set;
+	unsigned int users;
 
 	if (blk_mq_is_sbitmap_shared(hctx->flags)) {
 		if (!test_and_clear_bit(QUEUE_FLAG_HCTX_ACTIVE,
 					&q->queue_flags))
 			return;
 		atomic_dec(&set->active_queues_shared_sbitmap);
+		users = atomic_read(&set->active_queues_shared_sbitmap);
+		blk_mq_update_wake_batch(&set->__bitmap_tags,
+				&set->__breserved_tags, users);
 	} else {
 		if (!test_and_clear_bit(BLK_MQ_S_TAG_ACTIVE, &hctx->state))
 			return;
 		atomic_dec(&tags->active_queues);
+		users = atomic_read(&tags->active_queues);
+		blk_mq_update_wake_batch(&tags->__bitmap_tags,
+				&tags->__breserved_tags, users);
 	}
 
 	blk_mq_tag_wakeup_all(tags, false);
diff --git a/include/linux/sbitmap.h b/include/linux/sbitmap.h
index 2713e689ad66..d49e4f054bfe 100644
--- a/include/linux/sbitmap.h
+++ b/include/linux/sbitmap.h
@@ -406,6 +406,14 @@  static inline void sbitmap_queue_free(struct sbitmap_queue *sbq)
 	sbitmap_free(&sbq->sb);
 }
 
+/**
+ * sbitmap_queue_update_wake_batch() - Recalucate wake batch.
+ * @sbq: Bitmap queue.
+ * @depth: New number of queue depth.
+ */
+void sbitmap_queue_update_wake_batch(struct sbitmap_queue *sbq,
+				     unsigned int depth);
+
 /**
  * sbitmap_queue_resize() - Resize a &struct sbitmap_queue.
  * @sbq: Bitmap queue to resize.
diff --git a/lib/sbitmap.c b/lib/sbitmap.c
index b25db9be938a..bbe1d663763f 100644
--- a/lib/sbitmap.c
+++ b/lib/sbitmap.c
@@ -457,7 +457,7 @@  int sbitmap_queue_init_node(struct sbitmap_queue *sbq, unsigned int depth,
 }
 EXPORT_SYMBOL_GPL(sbitmap_queue_init_node);
 
-static void sbitmap_queue_update_wake_batch(struct sbitmap_queue *sbq,
+void sbitmap_queue_update_wake_batch(struct sbitmap_queue *sbq,
 					    unsigned int depth)
 {
 	unsigned int wake_batch = sbq_calc_wake_batch(sbq, depth);
@@ -475,6 +475,7 @@  static void sbitmap_queue_update_wake_batch(struct sbitmap_queue *sbq,
 			atomic_set(&sbq->ws[i].wait_cnt, 1);
 	}
 }
+EXPORT_SYMBOL_GPL(sbitmap_queue_update_wake_batch);
 
 void sbitmap_queue_resize(struct sbitmap_queue *sbq, unsigned int depth)
 {