Re: [PATCH v3 1/3] blk-mq: always use __blk_mq_alloc_requests() to alloc and init rq

Christoph Hellwig
2023-07-10T07:36:41Z

On Thu, Jun 29, 2023 at 03:40:03PM +0800, Chengming Zhou wrote:
> Thanks for your review!
>
> Since hctx-specific allocation path always has BLK_MQ_REQ_NOWAIT flag,
> it won't retry.
>
> But I agree, this makes the general __blk_mq_alloc_requests() more complex.

And also very confusing as it pretends to share some code, while almost
nothing of __blk_mq_alloc_requests is actually used.

> The reason is blk_mq_rq_ctx_init() has some data->rq_flags initialization:
>
> ```
> if (data->flags & BLK_MQ_REQ_PM)
> data->rq_flags |= RQF_PM;
> if (blk_queue_io_stat(q))
> data->rq_flags |= RQF_IO_STAT;
> rq->rq_flags = data->rq_flags;
> ```
>
> Because we need this data->rq_flags to tell if we need start_time_ns,
> we need to put these initialization in the callers of blk_mq_rq_ctx_init().

Why can't we just always initialize the time stampts after
blk_mq_rq_ctx_init? Something like this (untested) variant of your
patch 2 from the latest iteration:

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 5504719b970d59..55bf1009f3e32a 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -328,8 +328,26 @@ void blk_rq_init(struct request_queue *q, struct request *rq)
}
EXPORT_SYMBOL(blk_rq_init);

+/* Set alloc and start time when pre-allocated rq is actually used */
+static inline void blk_mq_rq_time_init(struct request *rq, bool set_alloc_time)
+{
+ if (blk_mq_need_time_stamp(rq)) {
+ u64 now = ktime_get_ns();
+
+#ifdef CONFIG_BLK_RQ_ALLOC_TIME
+ /*
+ * The alloc time is only used by iocost for now,
+ * only possible when blk_mq_need_time_stamp().
+ */
+ if (set_alloc_time)
+ 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)
+ struct blk_mq_tags *tags, unsigned int tag)
{
struct blk_mq_ctx *ctx = data->ctx;
struct blk_mq_hw_ctx *hctx = data->hctx;
@@ -356,14 +374,7 @@ 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;
@@ -393,8 +404,7 @@ 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;
@@ -413,7 +423,7 @@ __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++;
}
@@ -427,12 +437,13 @@ __blk_mq_alloc_requests_batch(struct blk_mq_alloc_data *data,
static struct request *__blk_mq_alloc_requests(struct blk_mq_alloc_data *data)
{
struct request_queue *q = data->q;
+ bool set_alloc_time = blk_queue_rq_alloc_time(q);
u64 alloc_time_ns = 0;
struct request *rq;
unsigned int tag;

/* alloc_time includes depth and tag waits */
- if (blk_queue_rq_alloc_time(q))
+ if (set_alloc_time)
alloc_time_ns = ktime_get_ns();

if (data->cmd_flags & REQ_NOWAIT)
@@ -474,9 +485,11 @@ 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, true);
return rq;
+ }
data->nr_tags = 1;
}

@@ -499,8 +512,10 @@ 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);
+ if (rq)
+ blk_mq_rq_time_init(rq, set_alloc_time);
+ return rq;
}

static struct request *blk_mq_rq_cache_fill(struct request_queue *q,
@@ -555,6 +570,7 @@ 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, blk_queue_rq_alloc_time(rq->q));
}

rq->cmd_flags = opf;
@@ -656,8 +672,8 @@ 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, blk_queue_rq_alloc_time(rq->q));
rq->__data_len = 0;
rq->__sector = (sector_t) -1;
rq->bio = rq->biotail = NULL;
@@ -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(rq, blk_queue_rq_alloc_time(rq->q));
rq->cmd_flags = (*bio)->bi_opf;
INIT_LIST_HEAD(&rq->queuelist);
return rq;