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

Chengming Zhou
2023-07-10T11:08:21Z

On 2023/7/10 15:36, Christoph Hellwig wrote:
> 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.

You are right.

I will not mess with reusing __blk_mq_alloc_requests() in the next version.

>
>> 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:

I get what you mean: always initialize the two time stamps after blk_mq_rq_ctx_init(),
so we know whether the time stamps are needed in blk_mq_rq_time_init().

It seems better and clearer indeed, I will try to change as you suggest.

>
> 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)

We need to pass "u64 alloc_time_ns" here, which includes depth and tag waits time by definition.

So:
1. for non-batched request that need alloc_time_ns: passed alloc_time_ns != 0
2. for batched request that need alloc_time_ns: passed alloc_time_ns == 0, will be set to start_time_ns

I have just updated the patch:
https://lore.kernel.org/all/20230710105516.2053478-1-chengming.zhou@linux.dev/

Thanks!

> +{
> + 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;