Re: [PATCH V11 1/8] block, bfq: split sync bfq_queues on a per-actuator basis

[Damien Le Moal]

On 2022/12/20 18:50, Paolo Valente wrote:
> Single-LUN multi-actuator SCSI drives, as well as all multi-actuator
> SATA drives appear as a single device to the I/O subsystem [1].  Yet
> they address commands to different actuators internally, as a function
> of Logical Block Addressing (LBAs). A given sector is reachable by
> only one of the actuators. For example, Seagate’s Serial Advanced
> Technology Attachment (SATA) version contains two actuators and maps
> the lower half of the SATA LBA space to the lower actuator and the
> upper half to the upper actuator.
> 
> Evidently, to fully utilize actuators, no actuator must be left idle
> or underutilized while there is pending I/O for it. The block layer
> must somehow control the load of each actuator individually. This
> commit lays the ground for allowing BFQ to provide such a per-actuator
> control.
> 
> BFQ associates an I/O-request sync bfq_queue with each process doing
> synchronous I/O, or with a group of processes, in case of queue
> merging. Then BFQ serves one bfq_queue at a time. While in service, a
> bfq_queue is emptied in request-position order. Yet the same process,
> or group of processes, may generate I/O for different actuators. In
> this case, different streams of I/O (each for a different actuator)
> get all inserted into the same sync bfq_queue. So there is basically
> no individual control on when each stream is served, i.e., on when the
> I/O requests of the stream are picked from the bfq_queue and
> dispatched to the drive.
> 
> This commit enables BFQ to control the service of each actuator
> individually for synchronous I/O, by simply splitting each sync
> bfq_queue into N queues, one for each actuator. In other words, a sync
> bfq_queue is now associated to a pair (process, actuator). As a
> consequence of this split, the per-queue proportional-share policy
> implemented by BFQ will guarantee that the sync I/O generated for each
> actuator, by each process, receives its fair share of service.
> 
> This is just a preparatory patch. If the I/O of the same process
> happens to be sent to different queues, then each of these queues may
> undergo queue merging. To handle this event, the bfq_io_cq data
> structure must be properly extended. In addition, stable merging must
> be disabled to avoid loss of control on individual actuators. Finally,
> also async queues must be split. These issues are described in detail
> and addressed in next commits. As for this commit, although multiple
> per-process bfq_queues are provided, the I/O of each process or group
> of processes is still sent to only one queue, regardless of the
> actuator the I/O is for. The forwarding to distinct bfq_queues will be
> enabled after addressing the above issues.
> 
> [1] https://www.linaro.org/blog/budget-fair-queueing-bfq-linux-io-scheduler-optimizations-for-multi-actuator-sata-hard-drives/
> 
> Signed-off-by: Gabriele Felici <felicigb@xxxxxxxxx>
> Signed-off-by: Carmine Zaccagnino <carmine@xxxxxxxxxxxxxxx>
> Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
> ---

[...]

If you want me to review, can you please add me to the CC list of this series ?
I am only getting the patches that I already reviewed, which is not very useful...

> @@ -672,9 +682,9 @@ static void bfq_limit_depth(blk_opf_t opf, struct blk_mq_alloc_data *data)
>  {
>  	struct bfq_data *bfqd = data->q->elevator->elevator_data;
>  	struct bfq_io_cq *bic = bfq_bic_lookup(data->q);
> -	struct bfq_queue *bfqq = bic ? bic_to_bfqq(bic, op_is_sync(opf)) : NULL;
>  	int depth;
>  	unsigned limit = data->q->nr_requests;
> +	unsigned int act_idx;
>  
>  	/* Sync reads have full depth available */
>  	if (op_is_sync(opf) && !op_is_write(opf)) {
> @@ -684,14 +694,21 @@ static void bfq_limit_depth(blk_opf_t opf, struct blk_mq_alloc_data *data)
>  		limit = (limit * depth) >> bfqd->full_depth_shift;
>  	}
>  
> -	/*
> -	 * Does queue (or any parent entity) exceed number of requests that
> -	 * should be available to it? Heavily limit depth so that it cannot
> -	 * consume more available requests and thus starve other entities.
> -	 */
> -	if (bfqq && bfqq_request_over_limit(bfqq, limit))
> -		depth = 1;
> +	for (act_idx = 0; act_idx < bfqd->num_actuators; act_idx++) {
> +		struct bfq_queue *bfqq =
> +			bic ? bic_to_bfqq(bic, op_is_sync(opf), act_idx) : NULL;

Ignored my comment again... Oh well. If you prefer the code this way... I do not
find it pretty nor solid as is though.

>  
> +		/*
> +		 * Does queue (or any parent entity) exceed number of
> +		 * requests that should be available to it? Heavily
> +		 * limit depth so that it cannot consume more
> +		 * available requests and thus starve other entities.
> +		 */
> +		if (bfqq && bfqq_request_over_limit(bfqq, limit)) {
> +			depth = 1;
> +			break;
> +		}
> +	}
>  	bfq_log(bfqd, "[%s] wr_busy %d sync %d depth %u",
>  		__func__, bfqd->wr_busy_queues, op_is_sync(opf), depth);
>  	if (depth)

[...]

> -static void bfq_exit_icq_bfqq(struct bfq_io_cq *bic, bool is_sync)
> +static void bfq_exit_icq_bfqq(struct bfq_io_cq *bic, bool is_sync,
> +			      unsigned int actuator_idx)
>  {
> -	struct bfq_queue *bfqq = bic_to_bfqq(bic, is_sync);
> +	struct bfq_queue *bfqq = bic_to_bfqq(bic, is_sync, actuator_idx);
>  	struct bfq_data *bfqd;
>  
>  	if (bfqq)

With your current bic_to_bfqq() implementation, you will *never* get NULL as a
return value. So why is this if necessary ?

>  		bfqd = bfqq->bfqd; /* NULL if scheduler already exited */
>  
>  	if (bfqq && bfqd) {
> -		unsigned long flags;
> -
> -		spin_lock_irqsave(&bfqd->lock, flags);
>  		bfqq->bic = NULL;
>  		bfq_exit_bfqq(bfqd, bfqq);
> -		bic_set_bfqq(bic, NULL, is_sync);
> -		spin_unlock_irqrestore(&bfqd->lock, flags);
> +		bic_set_bfqq(bic, NULL, is_sync, actuator_idx);
>  	}
>  }
>  
>  static void bfq_exit_icq(struct io_cq *icq)
>  {
>  	struct bfq_io_cq *bic = icq_to_bic(icq);
> +	struct bfq_data *bfqd = bic_to_bfqd(bic);
> +	unsigned long flags;
> +	unsigned int act_idx;
> +	/*
> +	 * If bfqd and thus bfqd->num_actuators is not available any
> +	 * longer, then cycle over all possible per-actuator bfqqs in
> +	 * next loop. We rely on bic being zeroed on creation, and
> +	 * therefore on its unused per-actuator fields being NULL.
> +	 */
> +	unsigned int num_actuators = BFQ_MAX_ACTUATORS;
>  
> -	if (bic->stable_merge_bfqq) {
> -		struct bfq_data *bfqd = bic->stable_merge_bfqq->bfqd;
> +	/*
> +	 * bfqd is NULL if scheduler already exited, and in that case
> +	 * this is the last time these queues are accessed.
> +	 */
> +	if (bfqd) {

Same here. bfqd can never be NULL. Or I am really missing something... Lots of
other places like this where checking bic_to_bfqd() seems unnecessary.

> +		spin_lock_irqsave(&bfqd->lock, flags);
> +		num_actuators = bfqd->num_actuators;
> +	}
>  
> -		/*
> -		 * bfqd is NULL if scheduler already exited, and in
> -		 * that case this is the last time bfqq is accessed.
> -		 */
> -		if (bfqd) {
> -			unsigned long flags;
> +	if (bic->stable_merge_bfqq)
> +		bfq_put_stable_ref(bic->stable_merge_bfqq);
>  
> -			spin_lock_irqsave(&bfqd->lock, flags);
> -			bfq_put_stable_ref(bic->stable_merge_bfqq);
> -			spin_unlock_irqrestore(&bfqd->lock, flags);
> -		} else {
> -			bfq_put_stable_ref(bic->stable_merge_bfqq);
> -		}
> +	for (act_idx = 0; act_idx < num_actuators; act_idx++) {
> +		bfq_exit_icq_bfqq(bic, true, act_idx);
> +		bfq_exit_icq_bfqq(bic, false, act_idx);
>  	}
>  
> -	bfq_exit_icq_bfqq(bic, true);
> -	bfq_exit_icq_bfqq(bic, false);
> +	if (bfqd)
> +		spin_unlock_irqrestore(&bfqd->lock, flags);
>  }


-- 
Damien Le Moal
Western Digital Research