[PATCH 4/6] blk-mq: use EWMA to estimate congestion threshold

Ming Lei
2017-07-11T18:22:14Z

When .queue_rq() returns BLK_STS_RESOURCE(BUSY), we can
consider that there is congestion in either low level
driver or hardware.

This patch uses EWMA to estimate this congestion threshold,
then this threshold can be used to detect/avoid congestion.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
block/blk-mq.c | 14 ++++++++++++++
block/blk-mq.h | 9 +++++++++
include/linux/blk-mq.h | 2 ++
3 files changed, 25 insertions(+)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 6e0fc80aa151..da50c187c508 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -976,6 +976,18 @@ static bool blk_mq_dispatch_wait_add(struct blk_mq_hw_ctx *hctx)
return true;
}

+static void blk_mq_update_req_dispatch_busy(struct blk_mq_hw_ctx *hctx)
+{
+ struct sbitmap_queue *sbq;
+ unsigned depth;
+
+ sbq = &hctx->tags->bitmap_tags;
+ depth = sbitmap_weight(&sbq->sb);
+
+ /* use EWMA to estimate a threshold for detecting congestion */
+ ewma_add(hctx->avg_busy_threshold, depth, 8, 0);
+}
+
bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)
{
struct blk_mq_hw_ctx *hctx;
@@ -1064,6 +1076,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list)

spin_lock(&hctx->lock);
list_splice_init(list, &hctx->dispatch);
+ blk_mq_update_req_dispatch_busy(hctx);
spin_unlock(&hctx->lock);

/*
@@ -1468,6 +1481,7 @@ static void blk_mq_direct_dispatch(struct blk_mq_hw_ctx *hctx,
{
spin_lock(&hctx->lock);
list_add(&rq->queuelist, &hctx->dispatch);
+ blk_mq_update_req_dispatch_busy(hctx);
spin_unlock(&hctx->lock);

blk_mq_run_hw_queue(hctx, false);
diff --git a/block/blk-mq.h b/block/blk-mq.h
index 60b01c0309bc..c4516d2a2d2c 100644
--- a/block/blk-mq.h
+++ b/block/blk-mq.h
@@ -133,4 +133,13 @@ static inline bool blk_mq_hw_queue_mapped(struct blk_mq_hw_ctx *hctx)
return hctx->nr_ctx && hctx->tags;
}

+/* borrowed from bcache */
+#define ewma_add(ewma, val, weight, factor) \
+({ \
+ (ewma) *= (weight) - 1; \
+ (ewma) += (val) << factor; \
+ (ewma) /= (weight); \
+ (ewma) >> factor; \
+})
+
#endif
diff --git a/include/linux/blk-mq.h b/include/linux/blk-mq.h
index 14542308d25b..8694fb39cd80 100644
--- a/include/linux/blk-mq.h
+++ b/include/linux/blk-mq.h
@@ -22,6 +22,8 @@ struct blk_mq_hw_ctx {

unsigned long flags; /* BLK_MQ_F_* flags */

+ unsigned long avg_busy_threshold;
+
void *sched_data;
struct request_queue *queue;
struct blk_flush_queue *fq;
--
2.9.4