Date:   Wed, 5 Oct 2022 08:04:03 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     Paolo Valente <paolo.valente@...aro.org>,
        Jens Axboe <axboe@...nel.dk>
Cc:     linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        jack@...e.cz, andrea.righi@...onical.com, glen.valante@...aro.org,
        arie.vanderhoeven@...gate.com, rory.c.chen@...gate.com,
        Gabriele Felici <felicigb@...il.com>
Subject: Re: [PATCH V3 1/8] block, bfq: split sync bfq_queues on a
 per-actuator basis

On 10/4/22 18:40, Paolo Valente wrote:
> Multi-actuator drives appear as a single device to the I/O subsystem [1].

Not necessarilly. Multi-lun scsi model will show up as multiple drives
with one actuator each.

> 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@...il.com>
> Signed-off-by: Paolo Valente <paolo.valente@...aro.org>
> ---
>  block/bfq-cgroup.c  |  95 +++++++++++++++++--------------
>  block/bfq-iosched.c | 135 +++++++++++++++++++++++++++-----------------
>  block/bfq-iosched.h |  38 +++++++++----
>  3 files changed, 164 insertions(+), 104 deletions(-)
> 
> diff --git a/block/bfq-cgroup.c b/block/bfq-cgroup.c
> index 30b15a9a47c4..a745dd9d658e 100644
> --- a/block/bfq-cgroup.c
> +++ b/block/bfq-cgroup.c
> @@ -705,6 +705,48 @@ 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)
> +{
> +	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);
> +	} else {
> +		struct bfq_queue *bfqq;
> +
> +		/*
> +		 * 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.
> +			 */
> +			bfq_put_cooperator(sync_bfqq);
> +			bfq_release_process_ref(bfqd, sync_bfqq);
> +			bic_set_bfqq(bic, NULL, 1, act_idx);
> +		}
> +	}
> +}
> +
> +
>  /**
>   * __bfq_bic_change_cgroup - move @bic to @bfqg.
>   * @bfqd: the queue descriptor.
> @@ -719,53 +761,24 @@ static void *__bfq_bic_change_cgroup(struct bfq_data *bfqd,
>  				     struct bfq_io_cq *bic,
>  				     struct bfq_group *bfqg)
>  {
> -	struct bfq_queue *async_bfqq = bic_to_bfqq(bic, 0);
> -	struct bfq_queue *sync_bfqq = bic_to_bfqq(bic, 1);
>  	struct bfq_entity *entity;
> +	unsigned int act_idx;
>  
> -	if (async_bfqq) {
> -		entity = &async_bfqq->entity;
> -
> -		if (entity->sched_data != &bfqg->sched_data) {
> -			bic_set_bfqq(bic, NULL, 0);
> -			bfq_release_process_ref(bfqd, async_bfqq);
> -		}
> -	}
> +	for (act_idx = 0; act_idx < BFQ_NUM_ACTUATORS; act_idx++) {

Why loop over all BFQ_NUM_ACTUATORS actuators even though this patch
itself is not enough to support multiple actuators ?
You then have patch 5 changing this macro to BFQ_MAX_ACTUATORS and then
patch 6 finally introducing a nr_ia_range bfq field to indicate the
effective number of actuators.

Why not:
1) introduce BFQ_MAX_ACTUATORS in this patch and define the bfqq field
using it
2) introduce a nr_actuators field defaultint to 1 for now and use that as
the upper bound for actuator earch loop

That would be 100% consistent with the current code (no change in
practice) and avoid all the code churn you have in the following patches.

> +		struct bfq_queue *async_bfqq = bic_to_bfqq(bic, 0, act_idx);
> +		struct bfq_queue *sync_bfqq = bic_to_bfqq(bic, 1, act_idx);
>  
> -	if (sync_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);
> -		} else {
> -			struct bfq_queue *bfqq;
> +		if (async_bfqq) {
> +			entity = &async_bfqq->entity;
>  
> -			/*
> -			 * 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.
> -				 */
> -				bfq_put_cooperator(sync_bfqq);
> -				bfq_release_process_ref(bfqd, sync_bfqq);
> -				bic_set_bfqq(bic, NULL, 1);
> +			if (entity->sched_data != &bfqg->sched_data) {
> +				bic_set_bfqq(bic, NULL, 0, act_idx);
> +				bfq_release_process_ref(bfqd, async_bfqq);
>  			}
>  		}
> +
> +		if (sync_bfqq)
> +			bfq_sync_bfqq_move(bfqd, sync_bfqq, bic, bfqg, act_idx);
>  	}
>  
>  	return bfqg;
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index c740b41fe0a4..c2485b599d87 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -377,14 +377,19 @@ 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];
> +	return bic->bfqq[is_sync][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 +404,7 @@ 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;
> +	bic->bfqq[is_sync][actuator_idx] = bfqq;
>  
>  	if (bfqq && bic->stable_merge_bfqq == bfqq) {
>  		/*
> @@ -672,9 +677,9 @@ 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 +689,21 @@ 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 < BFQ_NUM_ACTUATORS; act_idx++) {
> +		struct bfq_queue *bfqq =
> +			bic ? bic_to_bfqq(bic, op_is_sync(opf), act_idx) : NULL;
>  
> +		/*
> +		 * 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)
> @@ -2142,7 +2154,7 @@ 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 ||
> @@ -2454,6 +2466,16 @@ static void bfq_remove_request(struct request_queue *q,
>  
>  }
>  
> +/* 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.
> +	 */
> +	return 0;
> +}
> +
>  static bool bfq_bio_merge(struct request_queue *q, struct bio *bio,
>  		unsigned int nr_segs)
>  {
> @@ -2478,7 +2500,8 @@ 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));
>  	} else {
>  		bfqd->bio_bfqq = NULL;
>  	}
> @@ -3174,7 +3197,7 @@ 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);
>  	bfq_mark_bfqq_coop(new_bfqq);
>  	/*
>  	 * new_bfqq now belongs to at least two bics (it is a shared queue):
> @@ -4808,11 +4831,12 @@ 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))) {
> +		unsigned int act_idx = bfqq->actuator_idx;
>  		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;
> +			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 ?
> +			bfqq->bic->bfqq[0][act_idx] : NULL;
>  		struct bfq_queue *blocked_bfqq =
>  			!hlist_empty(&bfqq->woken_list) ?
>  			container_of(bfqq->woken_list.first,
> @@ -4904,7 +4928,7 @@ 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 &&
> @@ -5367,49 +5391,47 @@ 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 (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 bfqq is accessed.
> -		 */
> -		if (bfqd) {
> -			unsigned long flags;
> +	/*
> +	 * 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);
>  
> -			spin_lock_irqsave(&bfqd->lock, flags);
> -			bfq_put_stable_ref(bic->stable_merge_bfqq);
> -			spin_unlock_irqrestore(&bfqd->lock, flags);
> -		} else {
> +	for (act_idx = 0; act_idx < BFQ_NUM_ACTUATORS; act_idx++) {
> +		if (bic->stable_merge_bfqq)
>  			bfq_put_stable_ref(bic->stable_merge_bfqq);
> -		}
> +
> +		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);
>  }
>  
>  /*
> @@ -5486,23 +5508,25 @@ 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);
> @@ -5741,6 +5765,7 @@ static struct bfq_queue *bfq_get_queue(struct bfq_data *bfqd,
>  	struct bfq_group *bfqg;
>  
>  	bfqg = bfq_bio_bfqg(bfqd, bio);
> +
>  	if (!is_sync) {
>  		async_bfqq = bfq_async_queue_prio(bfqd, bfqg, ioprio_class,
>  						  ioprio);
> @@ -5755,7 +5780,7 @@ 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 {
> @@ -6070,7 +6095,8 @@ 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)
>  			bfq_merge_bfqqs(bfqd, RQ_BIC(rq),
>  					bfqq, new_bfqq);
>  
> @@ -6624,7 +6650,7 @@ 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);
>  
>  	bfq_put_cooperator(bfqq);
>  
> @@ -6638,7 +6664,8 @@ 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;
> @@ -6650,7 +6677,7 @@ 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)
> @@ -7092,8 +7119,10 @@ 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;
> diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
> index ad8e513d7e87..8b5225a9e080 100644
> --- a/block/bfq-iosched.h
> +++ b/block/bfq-iosched.h
> @@ -33,6 +33,8 @@
>   */
>  #define BFQ_SOFTRT_WEIGHT_FACTOR	100
>  
> +#define BFQ_NUM_ACTUATORS 2
> +
>  struct bfq_entity;
>  
>  /**
> @@ -225,12 +227,14 @@ 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 */
> @@ -399,6 +403,9 @@ 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;
>  };
>  
>  /**
> @@ -407,8 +414,17 @@ 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_NUM_ACTUATORS];
>  	/* per (request_queue, blkcg) ioprio */
>  	int ioprio;
>  #ifdef CONFIG_BFQ_GROUP_IOSCHED
> @@ -968,8 +984,10 @@ 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

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