[PATCH v3 3/3] blk-mq: fix start_time_ns and alloc_time_ns for pre-allocated rq

chengming.zhou
2023-06-28T12:48:01Z

From: Chengming Zhou <zhouchengming@bytedance.com>

The iocost rely on rq start_time_ns and alloc_time_ns to tell saturation
state of the block device. Most of the time request is allocated after
rq_qos_throttle() and its alloc_time_ns or start_time_ns won't be affected.

But for plug batched allocation introduced by the commit 47c122e35d7e
("block: pre-allocate requests if plug is started and is a batch"), we can
rq_qos_throttle() after the allocation of the request. This is what the
blk_mq_get_cached_request() does.

In this case, the cached request alloc_time_ns or start_time_ns is much
ahead if blocked in any qos ->throttle().

This patch fix it by setting alloc_time_ns and start_time_ns to now
when the pre-allocated rq is actually used. And we skip setting the
alloc_time_ns and start_time_ns during pre-allocation, so just pass 0
in __blk_mq_alloc_requests_batch().

Signed-off-by: Chengming Zhou <zhouchengming@bytedance.com>
---
block/blk-mq.c | 35 ++++++++++++++++++++++++++---------
1 file changed, 26 insertions(+), 9 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 8b981d0a868e..55a2e600f943 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -337,6 +337,24 @@ void blk_rq_init(struct request_queue *q, struct request *rq)
}
EXPORT_SYMBOL(blk_rq_init);

+/* Set rq alloc and start time when pre-allocated rq is actually used */
+static inline void blk_mq_rq_time_init(struct request_queue *q, struct request *rq)
+{
+ if (blk_mq_need_time_stamp(rq->rq_flags)) {
+ u64 now = ktime_get_ns();
+
+#ifdef CONFIG_BLK_RQ_ALLOC_TIME
+ /*
+ * alloc time is only used by iocost for now,
+ * only possible when blk_mq_need_time_stamp().
+ */
+ if (blk_queue_rq_alloc_time(q))
+ rq->alloc_time_ns = now;
+#endif
+ rq->start_time_ns = now;
+ }
+}
+
static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
struct blk_mq_tags *tags, unsigned int tag,
u64 alloc_time_ns, u64 start_time_ns)
@@ -395,23 +413,18 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
}

static inline struct request *
-__blk_mq_alloc_requests_batch(struct blk_mq_alloc_data *data,
- u64 alloc_time_ns)
+__blk_mq_alloc_requests_batch(struct blk_mq_alloc_data *data)
{
unsigned int tag, tag_offset;
struct blk_mq_tags *tags;
struct request *rq;
unsigned long tag_mask;
int i, nr = 0;
- u64 start_time_ns = 0;

tag_mask = blk_mq_get_tags(data, data->nr_tags, &tag_offset);
if (unlikely(!tag_mask))
return NULL;

- if (blk_mq_need_time_stamp(data->rq_flags))
- start_time_ns = ktime_get_ns();
-
tags = blk_mq_tags_from_data(data);
for (i = 0; tag_mask; i++) {
if (!(tag_mask & (1UL << i)))
@@ -419,7 +432,7 @@ __blk_mq_alloc_requests_batch(struct blk_mq_alloc_data *data,
tag = tag_offset + i;
prefetch(tags->static_rqs[tag]);
tag_mask &= ~(1UL << i);
- rq = blk_mq_rq_ctx_init(data, tags, tag, alloc_time_ns, start_time_ns);
+ rq = blk_mq_rq_ctx_init(data, tags, tag, 0, 0);
rq_list_add(data->cached_rq, rq);
nr++;
}
@@ -490,9 +503,11 @@ static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
* Try batched alloc if we want more than 1 tag.
*/
if (data->nr_tags > 1) {
- rq = __blk_mq_alloc_requests_batch(data, alloc_time_ns);
- if (rq)
+ rq = __blk_mq_alloc_requests_batch(data);
+ if (rq) {
+ blk_mq_rq_time_init(q, rq);
return rq;
+ }
data->nr_tags = 1;
}

@@ -575,6 +590,7 @@ static struct request *blk_mq_alloc_cached_request(struct request_queue *q,
return NULL;

plug->cached_rq = rq_list_next(rq);
+ blk_mq_rq_time_init(q, rq);
}

rq->cmd_flags = opf;
@@ -2896,6 +2912,7 @@ static inline struct request *blk_mq_get_cached_request(struct request_queue *q,
plug->cached_rq = rq_list_next(rq);
rq_qos_throttle(q, *bio);

+ blk_mq_rq_time_init(q, rq);
rq->cmd_flags = (*bio)->bi_opf;
INIT_LIST_HEAD(&rq->queuelist);
return rq;
--
2.39.2