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

[Chengming Zhou <chengming.zhou@linux.dev>]

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;