Re: [PATCH V7 1/8] block, bfq: split sync bfq_queues on a per-actuator basis

[Damien Le Moal]

On 12/6/22 17:15, 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>
> ---
>  block/bfq-cgroup.c  |  93 ++++++++++++++------------
>  block/bfq-iosched.c | 158 +++++++++++++++++++++++++++++---------------
>  block/bfq-iosched.h |  51 +++++++++++---
>  3 files changed, 196 insertions(+), 106 deletions(-)
> 
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index 7d624a3a3f0f..8275cdd4573f 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -704,6 +704,48 @@ void bfq_bfqq_move(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>  	bfq_put_queue(bfqq);
>  }
>  
> +static void bfq_sync_bfqq_move(struct bfq_data *bfqd,
> +			       struct bfq_queue *sync_bfqq,
> +			       struct bfq_io_cq *bic,
> +			       struct bfq_group *bfqg,
> +			       unsigned int act_idx)
> +{
> +	struct bfq_queue *bfqq;
> +
> +	if (!sync_bfqq->new_bfqq && !bfq_bfqq_coop(sync_bfqq)) {
> +		/* We are the only user of this bfqq, just move it */
> +		if (sync_bfqq->entity.sched_data != &bfqg->sched_data)
> +			bfq_bfqq_move(bfqd, sync_bfqq, bfqg);
> +		return;
> +	}
> +
> +	/*
> +	 * The queue was merged to a different queue. Check
> +	 * that the merge chain still belongs to the same
> +	 * cgroup.
> +	 */
> +	for (bfqq = sync_bfqq; bfqq; bfqq = bfqq->new_bfqq)
> +		if (bfqq->entity.sched_data !=
> +		    &bfqg->sched_data)
> +			break;
> +	if (bfqq) {
> +		/*
> +		 * Some queue changed cgroup so the merge is
> +		 * not valid anymore. We cannot easily just
> +		 * cancel the merge (by clearing new_bfqq) as
> +		 * there may be other processes using this
> +		 * queue and holding refs to all queues below
> +		 * sync_bfqq->new_bfqq. Similarly if the merge
> +		 * already happened, we need to detach from
> +		 * bfqq now so that we cannot merge bio to a
> +		 * request from the old cgroup.
> +		 */

Why the short lines here ? You can format this like this:

/*

 * Some queue changed cgroup so the merge is not valid anymore.

 * We cannot easily just cancel the merge (by clearing new_bfqq)

 * as there may be other processes using this queue and holding

 * refs to all queues below sync_bfqq->new_bfqq. Similarly if

 * the merge already happened, we need to detach from bfqq now

 * so that we cannot merge bio to a request from the old cgroup.

 */

That is still within 80 chars.

> +		bfq_put_cooperator(sync_bfqq);
> +		bfq_release_process_ref(bfqd, sync_bfqq);
> +		bic_set_bfqq(bic, NULL, 1, act_idx);
> +	}
> +}
> +

[...]
>  	return bfqg;
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 7ea427817f7f..127aeecaf903 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -377,14 +377,21 @@ static const unsigned long bfq_late_stable_merging = 600;
>  #define RQ_BIC(rq)		((struct bfq_io_cq *)((rq)->elv.priv[0]))
>  #define RQ_BFQQ(rq)		((rq)->elv.priv[1])
>  
> -struct bfq_queue *bic_to_bfqq(struct bfq_io_cq *bic, bool is_sync)
> +struct bfq_queue *bic_to_bfqq(struct bfq_io_cq *bic, bool is_sync,
> +			      unsigned int actuator_idx)
>  {
> -	return bic->bfqq[is_sync];

See below. But here, you could add:

	if (!bic)
		return NULL;

> +	if (is_sync)
> +		return bic->bfqq[1][actuator_idx];
> +
> +	return bic->bfqq[0][actuator_idx];
>  }
>  
>  static void bfq_put_stable_ref(struct bfq_queue *bfqq);
>  
> -void bic_set_bfqq(struct bfq_io_cq *bic, struct bfq_queue *bfqq, bool is_sync)
> +void bic_set_bfqq(struct bfq_io_cq *bic,
> +		  struct bfq_queue *bfqq,
> +		  bool is_sync,
> +		  unsigned int actuator_idx)
>  {
>  	/*
>  	 * If bfqq != NULL, then a non-stable queue merge between
> @@ -399,7 +406,10 @@ void bic_set_bfqq(struct bfq_io_cq *bic, struct bfq_queue *bfqq, bool is_sync)
>  	 * we cancel the stable merge if
>  	 * bic->stable_merge_bfqq == bfqq.
>  	 */
> -	bic->bfqq[is_sync] = bfqq;
> +	if (is_sync)
> +		bic->bfqq[1][actuator_idx] = bfqq;
> +	else
> +		bic->bfqq[0][actuator_idx] = bfqq;
>  
>  	if (bfqq && bic->stable_merge_bfqq == bfqq) {
>  		/*
> @@ -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;

You could return NULL in bic_to_bfqq() if bic is NULL. That would avoid
this cludge.

>  
> +		/*
> +		 * 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)
> @@ -1812,6 +1829,18 @@ static bool bfq_bfqq_higher_class_or_weight(struct bfq_queue *bfqq,
>  	return bfqq_weight > in_serv_weight;
>  }
>  
> +/*
> + * Get the index of the actuator that will serve bio.
> + */
> +static unsigned int bfq_actuator_index(struct bfq_data *bfqd, struct bio *bio)
> +{
> +	/*
> +	 * Multi-actuator support not complete yet, so always return 0
> +	 * for the moment (to keep incomplete mechanisms off).
> +	 */
> +	return 0;
> +}
> +
>  static bool bfq_better_to_idle(struct bfq_queue *bfqq);
>  
>  static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd,
> @@ -2142,7 +2171,7 @@ static void bfq_check_waker(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>  	 * We reset waker detection logic also if too much time has passed
>   	 * since the first detection. If wakeups are rare, pointless idling
>  	 * doesn't hurt throughput that much. The condition below makes sure
> -	 * we do not uselessly idle blocking waker in more than 1/64 cases. 
> +	 * we do not uselessly idle blocking waker in more than 1/64 cases.
>  	 */
>  	if (bfqd->last_completed_rq_bfqq !=
>  	    bfqq->tentative_waker_bfqq ||
> @@ -2478,7 +2507,8 @@ static bool bfq_bio_merge(struct request_queue *q, struct bio *bio,
>  		 */
>  		bfq_bic_update_cgroup(bic, bio);
>  
> -		bfqd->bio_bfqq = bic_to_bfqq(bic, op_is_sync(bio->bi_opf));
> +		bfqd->bio_bfqq = bic_to_bfqq(bic, op_is_sync(bio->bi_opf),
> +					     bfq_actuator_index(bfqd, bio));

Given that you repeat this pattern a lot, might be worth having a wrapper
like:

static inline struct bfq_queue *bio_to_bfqq(struct bfq_io_cq *bic,
					    struct bio *bio)
{
	return bic_to_bfqq(bic, op_is_sync(bio->bi_opf),
			   bfq_actuator_index(bfqd, bio));
}

The code would be less verbose while still being clear.

>  	} else {
>  		bfqd->bio_bfqq = NULL;
>  	}
> @@ -3174,7 +3204,7 @@ bfq_merge_bfqqs(struct bfq_data *bfqd, struct bfq_io_cq *bic,
>  	/*
>  	 * Merge queues (that is, let bic redirect its requests to new_bfqq)
>  	 */
> -	bic_set_bfqq(bic, new_bfqq, 1);
> +	bic_set_bfqq(bic, new_bfqq, 1, bfqq->actuator_idx);

s/1/true (the third argument is a bool).

same comment for some other calls below.

>  	bfq_mark_bfqq_coop(new_bfqq);
>  	/*
>  	 * new_bfqq now belongs to at least two bics (it is a shared queue):
> @@ -4808,11 +4838,8 @@ static struct bfq_queue *bfq_select_queue(struct bfq_data *bfqd)
>  	 */
>  	if (bfq_bfqq_wait_request(bfqq) ||
>  	    (bfqq->dispatched != 0 && bfq_better_to_idle(bfqq))) {
> -		struct bfq_queue *async_bfqq =
> -			bfqq->bic && bfqq->bic->bfqq[0] &&
> -			bfq_bfqq_busy(bfqq->bic->bfqq[0]) &&
> -			bfqq->bic->bfqq[0]->next_rq ?
> -			bfqq->bic->bfqq[0] : NULL;
> +		unsigned int act_idx = bfqq->actuator_idx;
> +		struct bfq_queue *async_bfqq = NULL;
>  		struct bfq_queue *blocked_bfqq =
>  			!hlist_empty(&bfqq->woken_list) ?
>  			container_of(bfqq->woken_list.first,
> @@ -4820,6 +4847,10 @@ static struct bfq_queue *bfq_select_queue(struct bfq_data *bfqd)
>  				     woken_list_node)
>  			: NULL;
>  
> +		if (bfqq->bic && bfqq->bic->bfqq[0][act_idx] &&
> +		    bfq_bfqq_busy(bfqq->bic->bfqq[0][act_idx]) &&
> +		    bfqq->bic->bfqq[0][act_idx]->next_rq)
> +			async_bfqq = bfqq->bic->bfqq[0][act_idx];
>  		/*
>  		 * The next four mutually-exclusive ifs decide
>  		 * whether to try injection, and choose the queue to
> @@ -4904,7 +4935,7 @@ static struct bfq_queue *bfq_select_queue(struct bfq_data *bfqd)
>  		    icq_to_bic(async_bfqq->next_rq->elv.icq) == bfqq->bic &&
>  		    bfq_serv_to_charge(async_bfqq->next_rq, async_bfqq) <=
>  		    bfq_bfqq_budget_left(async_bfqq))
> -			bfqq = bfqq->bic->bfqq[0];
> +			bfqq = bfqq->bic->bfqq[0][act_idx];
>  		else if (bfqq->waker_bfqq &&
>  			   bfq_bfqq_busy(bfqq->waker_bfqq) &&
>  			   bfqq->waker_bfqq->next_rq &&
> @@ -5365,49 +5396,55 @@ static void bfq_exit_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq)
>  	bfq_release_process_ref(bfqd, bfqq);
>  }
>  
> -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)
>  		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) {
> +		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);
>  }
>  
>  /*
> @@ -5484,23 +5521,25 @@ static void bfq_check_ioprio_change(struct bfq_io_cq *bic, struct bio *bio)
>  
>  	bic->ioprio = ioprio;
>  
> -	bfqq = bic_to_bfqq(bic, false);
> +	bfqq = bic_to_bfqq(bic, false, bfq_actuator_index(bfqd, bio));
>  	if (bfqq) {
>  		bfq_release_process_ref(bfqd, bfqq);
>  		bfqq = bfq_get_queue(bfqd, bio, false, bic, true);
> -		bic_set_bfqq(bic, bfqq, false);
> +		bic_set_bfqq(bic, bfqq, false, bfq_actuator_index(bfqd, bio));
>  	}
>  
> -	bfqq = bic_to_bfqq(bic, true);
> +	bfqq = bic_to_bfqq(bic, true, bfq_actuator_index(bfqd, bio));
>  	if (bfqq)
>  		bfq_set_next_ioprio_data(bfqq, bic);
>  }
>  
>  static void bfq_init_bfqq(struct bfq_data *bfqd, struct bfq_queue *bfqq,
> -			  struct bfq_io_cq *bic, pid_t pid, int is_sync)
> +			  struct bfq_io_cq *bic, pid_t pid, int is_sync,
> +			  unsigned int act_idx)
>  {
>  	u64 now_ns = ktime_get_ns();
>  
> +	bfqq->actuator_idx = act_idx;
>  	RB_CLEAR_NODE(&bfqq->entity.rb_node);
>  	INIT_LIST_HEAD(&bfqq->fifo);
>  	INIT_HLIST_NODE(&bfqq->burst_list_node);
> @@ -5753,7 +5792,7 @@ static struct bfq_queue *bfq_get_queue(struct bfq_data *bfqd,
>  
>  	if (bfqq) {
>  		bfq_init_bfqq(bfqd, bfqq, bic, current->pid,
> -			      is_sync);
> +			      is_sync, bfq_actuator_index(bfqd, bio));
>  		bfq_init_entity(&bfqq->entity, bfqg);
>  		bfq_log_bfqq(bfqd, bfqq, "allocated");
>  	} else {
> @@ -6068,7 +6107,8 @@ static bool __bfq_insert_request(struct bfq_data *bfqd, struct request *rq)
>  		 * then complete the merge and redirect it to
>  		 * new_bfqq.
>  		 */
> -		if (bic_to_bfqq(RQ_BIC(rq), 1) == bfqq)
> +		if (bic_to_bfqq(RQ_BIC(rq), 1,
> +				bfq_actuator_index(bfqd, rq->bio)) == bfqq)

s/1/true

>  			bfq_merge_bfqqs(bfqd, RQ_BIC(rq),
>  					bfqq, new_bfqq);
>  
> @@ -6622,7 +6662,7 @@ bfq_split_bfqq(struct bfq_io_cq *bic, struct bfq_queue *bfqq)
>  		return bfqq;
>  	}
>  
> -	bic_set_bfqq(bic, NULL, 1);
> +	bic_set_bfqq(bic, NULL, 1, bfqq->actuator_idx);

Same.

>  
>  	bfq_put_cooperator(bfqq);
>  
> @@ -6636,7 +6676,8 @@ static struct bfq_queue *bfq_get_bfqq_handle_split(struct bfq_data *bfqd,
>  						   bool split, bool is_sync,
>  						   bool *new_queue)
>  {
> -	struct bfq_queue *bfqq = bic_to_bfqq(bic, is_sync);
> +	unsigned int act_idx = bfq_actuator_index(bfqd, bio);
> +	struct bfq_queue *bfqq = bic_to_bfqq(bic, is_sync, act_idx);
>  
>  	if (likely(bfqq && bfqq != &bfqd->oom_bfqq))
>  		return bfqq;
> @@ -6648,7 +6689,7 @@ static struct bfq_queue *bfq_get_bfqq_handle_split(struct bfq_data *bfqd,
>  		bfq_put_queue(bfqq);
>  	bfqq = bfq_get_queue(bfqd, bio, is_sync, bic, split);
>  
> -	bic_set_bfqq(bic, bfqq, is_sync);
> +	bic_set_bfqq(bic, bfqq, is_sync, act_idx);
>  	if (split && is_sync) {
>  		if ((bic->was_in_burst_list && bfqd->large_burst) ||
>  		    bic->saved_in_large_burst)
> @@ -7090,8 +7131,10 @@ static int bfq_init_queue(struct request_queue *q, struct elevator_type *e)
>  	 * Our fallback bfqq if bfq_find_alloc_queue() runs into OOM issues.
>  	 * Grab a permanent reference to it, so that the normal code flow
>  	 * will not attempt to free it.
> +	 * Set zero as actuator index: we will pretend that
> +	 * all I/O requests are for the same actuator.
>  	 */
> -	bfq_init_bfqq(bfqd, &bfqd->oom_bfqq, NULL, 1, 0);
> +	bfq_init_bfqq(bfqd, &bfqd->oom_bfqq, NULL, 1, 0, 0);
>  	bfqd->oom_bfqq.ref++;
>  	bfqd->oom_bfqq.new_ioprio = BFQ_DEFAULT_QUEUE_IOPRIO;
>  	bfqd->oom_bfqq.new_ioprio_class = IOPRIO_CLASS_BE;
> @@ -7110,6 +7153,13 @@ static int bfq_init_queue(struct request_queue *q, struct elevator_type *e)
>  
>  	bfqd->queue = q;
>  
> +	/*
> +	 * Multi-actuator support not complete yet, unconditionally
> +	 * set to only one actuator for the moment (to keep incomplete
> +	 * mechanisms off).
> +	 */
> +	bfqd->num_actuators = 1;
> +
>  	INIT_LIST_HEAD(&bfqd->dispatch);
>  
>  	hrtimer_init(&bfqd->idle_slice_timer, CLOCK_MONOTONIC,
> diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
> index 71f721670ab6..2b413ddffbb9 100644
> --- a/block/bfq-iosched.h
> +++ b/block/bfq-iosched.h
> @@ -33,6 +33,14 @@
>   */
>  #define BFQ_SOFTRT_WEIGHT_FACTOR	100
>  
> +/*
> + * Maximum number of actuators supported. This constant is used simply
> + * to define the size of the static array that will contain
> + * per-actuator data. The current value is hopefully a good upper
> + * bound to the possible number of actuators of any actual drive.
> + */
> +#define BFQ_MAX_ACTUATORS 8
> +
>  struct bfq_entity;
>  
>  /**
> @@ -225,12 +233,14 @@ struct bfq_ttime {
>   * struct bfq_queue - leaf schedulable entity.
>   *
>   * A bfq_queue is a leaf request queue; it can be associated with an
> - * io_context or more, if it  is  async or shared  between  cooperating
> - * processes. @cgroup holds a reference to the cgroup, to be sure that it
> - * does not disappear while a bfqq still references it (mostly to avoid
> - * races between request issuing and task migration followed by cgroup
> - * destruction).
> - * All the fields are protected by the queue lock of the containing bfqd.
> + * io_context or more, if it is async or shared between cooperating
> + * processes. Besides, it contains I/O requests for only one actuator
> + * (an io_context is associated with a different bfq_queue for each
> + * actuator it generates I/O for). @cgroup holds a reference to the
> + * cgroup, to be sure that it does not disappear while a bfqq still
> + * references it (mostly to avoid races between request issuing and
> + * task migration followed by cgroup destruction).  All the fields are
> + * protected by the queue lock of the containing bfqd.
>   */
>  struct bfq_queue {
>  	/* reference counter */
> @@ -395,6 +405,9 @@ struct bfq_queue {
>  	 * the woken queues when this queue exits.
>  	 */
>  	struct hlist_head woken_list;
> +
> +	/* index of the actuator this queue is associated with */
> +	unsigned int actuator_idx;
>  };
>  
>  /**
> @@ -403,8 +416,17 @@ struct bfq_queue {
>  struct bfq_io_cq {
>  	/* associated io_cq structure */
>  	struct io_cq icq; /* must be the first member */
> -	/* array of two process queues, the sync and the async */
> -	struct bfq_queue *bfqq[2];
> +	/*
> +	 * Matrix of associated process queues: first row for async
> +	 * queues, second row sync queues. Each row contains one
> +	 * column for each actuator. An I/O request generated by the
> +	 * process is inserted into the queue pointed by bfqq[i][j] if
> +	 * the request is to be served by the j-th actuator of the
> +	 * drive, where i==0 or i==1, depending on whether the request
> +	 * is async or sync. So there is a distinct queue for each
> +	 * actuator.
> +	 */
> +	struct bfq_queue *bfqq[2][BFQ_MAX_ACTUATORS];
>  	/* per (request_queue, blkcg) ioprio */
>  	int ioprio;
>  #ifdef CONFIG_BFQ_GROUP_IOSCHED
> @@ -768,6 +790,13 @@ struct bfq_data {
>  	 */
>  	unsigned int word_depths[2][2];
>  	unsigned int full_depth_shift;
> +
> +	/*
> +	 * Number of independent actuators. This is equal to 1 in
> +	 * case of single-actuator drives.
> +	 */
> +	unsigned int num_actuators;
> +
>  };
>  
>  enum bfqq_state_flags {
> @@ -964,8 +993,10 @@ struct bfq_group {
>  
>  extern const int bfq_timeout;
>  
> -struct bfq_queue *bic_to_bfqq(struct bfq_io_cq *bic, bool is_sync);
> -void bic_set_bfqq(struct bfq_io_cq *bic, struct bfq_queue *bfqq, bool is_sync);
> +struct bfq_queue *bic_to_bfqq(struct bfq_io_cq *bic, bool is_sync,
> +				unsigned int actuator_idx);
> +void bic_set_bfqq(struct bfq_io_cq *bic, struct bfq_queue *bfqq, bool is_sync,
> +				unsigned int actuator_idx);
>  struct bfq_data *bic_to_bfqd(struct bfq_io_cq *bic);
>  void bfq_pos_tree_add_move(struct bfq_data *bfqd, struct bfq_queue *bfqq);
>  void bfq_weights_tree_add(struct bfq_data *bfqd, struct bfq_queue *bfqq,

-- 
Damien Le Moal
Western Digital Research