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Date: Mon, 10 Jul 2023 18:55:16 +0800
From: chengming.zhou@...ux.dev
To: axboe@...nel.dk, hch@....de, tj@...nel.org, ming.lei@...hat.com
Cc: linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
zhouchengming@...edance.com
Subject: [PATCH v5] blk-mq: fix start_time_ns and alloc_time_ns for pre-allocated rq
From: Chengming Zhou <zhouchengming@...edance.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().
Fix it by setting alloc_time_ns and start_time_ns to now when the allocated
request is actually used.
Signed-off-by: Chengming Zhou <zhouchengming@...edance.com>
---
v5:
- Don't reuse __blk_mq_alloc_requests(), which makes it much complex.
- Always initialize the time stamps after blk_mq_rq_ctx_init(), which
is clearer, as suggested by Christoph Hellwig.
- We still need to pass optional alloc_time_ns to blk_mq_rq_time_init(),
which includes depth and tag waits time by definition.
- [v4] https://lore.kernel.org/all/20230629121302.1124851-1-chengming.zhou@linux.dev/
v4:
- Use blk_mq_alloc_data to pass start_time_ns instead of passing down
yet another parameter. Thanks Christoph Hellwig.
- [v3] https://lore.kernel.org/all/20230628124546.1056698-1-chengming.zhou@linux.dev/
v3:
- Skip setting the alloc_time_ns and start_time_ns during pre-allocation,
which is clearer, as suggested by Tejun.
- [v2] https://lore.kernel.org/all/20230626050405.781253-1-chengming.zhou@linux.dev/
---
block/blk-mq.c | 47 ++++++++++++++++++++++++++++++-----------------
1 file changed, 30 insertions(+), 17 deletions(-)
diff --git a/block/blk-mq.c b/block/blk-mq.c
index decb6ab2d508..2352fed460f8 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -337,8 +337,24 @@ void blk_rq_init(struct request_queue *q, struct request *rq)
}
EXPORT_SYMBOL(blk_rq_init);
+/* Set start and alloc time when the allocated request is actually used */
+static inline void blk_mq_rq_time_init(struct request *rq, u64 alloc_time_ns)
+{
+ if (blk_mq_need_time_stamp(rq))
+ rq->start_time_ns = ktime_get_ns();
+ else
+ rq->start_time_ns = 0;
+
+#ifdef CONFIG_BLK_RQ_ALLOC_TIME
+ if (blk_queue_rq_alloc_time(rq->q))
+ rq->alloc_time_ns = alloc_time_ns ?: rq->start_time_ns;
+ else
+ rq->alloc_time_ns = 0;
+#endif
+}
+
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)
+ struct blk_mq_tags *tags, unsigned int tag)
{
struct blk_mq_ctx *ctx = data->ctx;
struct blk_mq_hw_ctx *hctx = data->hctx;
@@ -365,14 +381,7 @@ static struct request *blk_mq_rq_ctx_init(struct blk_mq_alloc_data *data,
}
rq->timeout = 0;
- if (blk_mq_need_time_stamp(rq))
- rq->start_time_ns = ktime_get_ns();
- else
- rq->start_time_ns = 0;
rq->part = NULL;
-#ifdef CONFIG_BLK_RQ_ALLOC_TIME
- rq->alloc_time_ns = alloc_time_ns;
-#endif
rq->io_start_time_ns = 0;
rq->stats_sectors = 0;
rq->nr_phys_segments = 0;
@@ -402,8 +411,7 @@ 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;
@@ -422,7 +430,7 @@ __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);
+ rq = blk_mq_rq_ctx_init(data, tags, tag);
rq_list_add(data->cached_rq, rq);
nr++;
}
@@ -483,9 +491,11 @@ 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(rq, alloc_time_ns);
return rq;
+ }
data->nr_tags = 1;
}
@@ -508,8 +518,9 @@ static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
goto retry;
}
- return blk_mq_rq_ctx_init(data, blk_mq_tags_from_data(data), tag,
- alloc_time_ns);
+ rq = blk_mq_rq_ctx_init(data, blk_mq_tags_from_data(data), tag);
+ blk_mq_rq_time_init(rq, alloc_time_ns);
+ return rq;
}
static struct request *blk_mq_rq_cache_fill(struct request_queue *q,
@@ -564,6 +575,7 @@ 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(rq, 0);
}
rq->cmd_flags = opf;
@@ -665,8 +677,8 @@ struct request *blk_mq_alloc_request_hctx(struct request_queue *q,
tag = blk_mq_get_tag(&data);
if (tag == BLK_MQ_NO_TAG)
goto out_queue_exit;
- rq = blk_mq_rq_ctx_init(&data, blk_mq_tags_from_data(&data), tag,
- alloc_time_ns);
+ rq = blk_mq_rq_ctx_init(&data, blk_mq_tags_from_data(&data), tag);
+ blk_mq_rq_time_init(rq, alloc_time_ns);
rq->__data_len = 0;
rq->__sector = (sector_t) -1;
rq->bio = rq->biotail = NULL;
@@ -2901,6 +2913,7 @@ 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(rq, 0);
rq->cmd_flags = (*bio)->bi_opf;
INIT_LIST_HEAD(&rq->queuelist);
return rq;
--
2.41.0
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