Re: [PATCH V6 4/8] block, bfq: turn bfqq_data into an array in bfq_io_cq

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 11/4/22 01:26, Paolo Valente wrote:
> When a bfq_queue Q is merged with another queue, several pieces of
> information are saved about Q. These pieces are stored in the
> bfqq_data field in the bfq_io_cq data structure of the process
> associated with Q.
> 
> Yet, with a multi-actuator drive, a process may get associated with
> multiple bfq_queues: one queue for each of the N actuators. Each of
> these queues may undergo a merge. So, the bfq_io_cq data structure
> must be able to accommodate the above information for N queues.
> 
> This commit solves this problem by turning the bfqq_data scalar field
> into an array of N elements (and by changing code so as to handle
> this array).
> 
> This solution is written under the assumption that bfq_queues
> associated with different actuators cannot be cross-merged. This
> assumption holds naturally with basic queue merging: the latter is
> triggered by spatial locality, and sectors for different actuators are
> not close to each other. As for stable cross-merging, the assumption

The last sector served by actuator N is very close to the first sector
served by actuator N+1 :)

So I am not sure this argument is valid. Better explanation required here
I think.

> here is that it is disabled.
> 
> Signed-off-by: Gabriele Felici <felicigb@gmail.com>
> Signed-off-by: Gianmarco Lusvardi <glusvardi@posteo.net>
> Signed-off-by: Giulio Barabino <giuliobarabino99@gmail.com>
> Signed-off-by: Emiliano Maccaferri <inbox@emilianomaccaferri.com>
> Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
> ---
>  block/bfq-iosched.c | 72 ++++++++++++++++++++++++---------------------
>  block/bfq-iosched.h | 12 +++++---
>  2 files changed, 47 insertions(+), 37 deletions(-)
> 
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 01528182c0c5..f44bac054aaf 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -404,7 +404,7 @@ void bic_set_bfqq(struct bfq_io_cq *bic,
>  	 * we cancel the stable merge if
>  	 * bic->stable_merge_bfqq == bfqq.
>  	 */
> -	struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data;
> +	struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data[actuator_idx];
>  	bic->bfqq[is_sync][actuator_idx] = bfqq;
>  
>  	if (bfqq && bfqq_data->stable_merge_bfqq == bfqq) {
> @@ -1175,9 +1175,10 @@ static void
>  bfq_bfqq_resume_state(struct bfq_queue *bfqq, struct bfq_data *bfqd,
>  		      struct bfq_io_cq *bic, bool bfq_already_existing)
>  {
> -	struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data;
>  	unsigned int old_wr_coeff = 1;
>  	bool busy = bfq_already_existing && bfq_bfqq_busy(bfqq);
> +	unsigned int a_idx = bfqq->actuator_idx;
> +	struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data[a_idx];
>  
>  	if (bfqq_data->saved_has_short_ttime)
>  		bfq_mark_bfqq_has_short_ttime(bfqq);
> @@ -1827,6 +1828,16 @@ 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.
> +	 */
> +	return 0;
> +}

??? This was added in patch 1. Why again here ?

> +
>  static bool bfq_better_to_idle(struct bfq_queue *bfqq);
>  
>  static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd,
> @@ -1881,7 +1892,9 @@ static void bfq_bfqq_handle_idle_busy_switch(struct bfq_data *bfqd,
>  	wr_or_deserves_wr = bfqd->low_latency &&
>  		(bfqq->wr_coeff > 1 ||
>  		 (bfq_bfqq_sync(bfqq) &&
> -		  (bfqq->bic || RQ_BIC(rq)->bfqq_data.stably_merged) &&
> +		  (bfqq->bic ||
> +		   RQ_BIC(rq)->bfqq_data[bfq_actuator_index(bfqd, rq->bio)]
> +		   .stably_merged) &&
>  		   (*interactive || soft_rt)));
>  
>  	/*
> @@ -2469,16 +2482,6 @@ 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;
> -}
> -

ah. You moved it... May be add it in the right place in patch 1 then to
avoid that ? That may be due to some git/diff artifacts though.

>  static bool bfq_bio_merge(struct request_queue *q, struct bio *bio,
>  		unsigned int nr_segs)
>  {
> @@ -2905,7 +2908,8 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>  		     void *io_struct, bool request, struct bfq_io_cq *bic)
>  {
>  	struct bfq_queue *in_service_bfqq, *new_bfqq;
> -	struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data;
> +	unsigned int a_idx = bfqq->actuator_idx;
> +	struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data[a_idx];
>  
>  	/* if a merge has already been setup, then proceed with that first */
>  	if (bfqq->new_bfqq)
> @@ -2952,8 +2956,9 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>  				if (new_bfqq) {
>  					bfqq_data->stably_merged = true;
>  					if (new_bfqq->bic)
> -						new_bfqq->bic->bfqq_data.stably_merged =
> -							true;
> +						new_bfqq->bic->bfqq_data
> +							[new_bfqq->actuator_idx]
> +							.stably_merged = true;

Aouch. This is really hard to read as that is really not supposed to be
split like this.

>  				}
>  				return new_bfqq;
>  			} else
> @@ -3052,7 +3057,9 @@ bfq_setup_cooperator(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>  static void bfq_bfqq_save_state(struct bfq_queue *bfqq)
>  {
>  	struct bfq_io_cq *bic = bfqq->bic;
> -	struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data;
> +	/* State must be saved for the right queue index. */

Drop this comment. It serves no purpose in my opinion.

> +	unsigned int a_idx = bfqq->actuator_idx;
> +	struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data[a_idx];
>  
>  	/*
>  	 * If !bfqq->bic, the queue is already shared or its requests
> @@ -3063,7 +3070,7 @@ static void bfq_bfqq_save_state(struct bfq_queue *bfqq)
>  		return;
>  
>  	bfqq_data->saved_last_serv_time_ns = bfqq->last_serv_time_ns;
> -	bfqq_data->saved_inject_limit = bfqq->inject_limit;
> +	bfqq_data->saved_inject_limit =	bfqq->inject_limit;
>  	bfqq_data->saved_decrease_time_jif = bfqq->decrease_time_jif;
>  
>  	bfqq_data->saved_weight = bfqq->entity.orig_weight;
> @@ -5425,7 +5432,7 @@ static void bfq_exit_icq(struct io_cq *icq)
>  	unsigned long flags;
>  	unsigned int act_idx;
>  	unsigned int num_actuators;
> -	struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data;
> +	struct bfq_iocq_bfqq_data *bfqq_data = bic->bfqq_data;
>  
>  	/*
>  	 * bfqd is NULL if scheduler already exited, and in that case
> @@ -5445,10 +5452,10 @@ static void bfq_exit_icq(struct io_cq *icq)
>  		num_actuators = BFQ_MAX_ACTUATORS;
>  	}
>  
> -	if (bfqq_data->stable_merge_bfqq)
> -		bfq_put_stable_ref(bfqq_data->stable_merge_bfqq);
> -
>  	for (act_idx = 0; act_idx < num_actuators; act_idx++) {
> +		if (bfqq_data[act_idx].stable_merge_bfqq)
> +			bfq_put_stable_ref(bfqq_data[act_idx].stable_merge_bfqq);
> +
>  		bfq_exit_icq_bfqq(bic, true, act_idx);
>  		bfq_exit_icq_bfqq(bic, false, act_idx);
>  	}
> @@ -5635,16 +5642,16 @@ bfq_do_early_stable_merge(struct bfq_data *bfqd, struct bfq_queue *bfqq,
>  			  struct bfq_io_cq *bic,
>  			  struct bfq_queue *last_bfqq_created)
>  {
> +	unsigned int a_idx = last_bfqq_created->actuator_idx;
>  	struct bfq_queue *new_bfqq =
>  		bfq_setup_merge(bfqq, last_bfqq_created);
> -	struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data;
>  
>  	if (!new_bfqq)
>  		return bfqq;
>  
>  	if (new_bfqq->bic)
> -		new_bfqq->bic->bfqq_data.stably_merged = true;
> -	bfqq_data->stably_merged = true;
> +		new_bfqq->bic->bfqq_data[a_idx].stably_merged = true;
> +	bic->bfqq_data[a_idx].stably_merged = true;
>  
>  	/*
>  	 * Reusing merge functions. This implies that
> @@ -5713,7 +5720,6 @@ static struct bfq_queue *bfq_do_or_sched_stable_merge(struct bfq_data *bfqd,
>  		&bfqd->last_bfqq_created;
>  
>  	struct bfq_queue *last_bfqq_created = *source_bfqq;
> -	struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data;
>  
>  	/*
>  	 * If last_bfqq_created has not been set yet, then init it. If
> @@ -5775,7 +5781,8 @@ static struct bfq_queue *bfq_do_or_sched_stable_merge(struct bfq_data *bfqd,
>  			/*
>  			 * Record the bfqq to merge to.
>  			 */
> -			bfqq_data->stable_merge_bfqq = last_bfqq_created;
> +			bic->bfqq_data[last_bfqq_created->actuator_idx].stable_merge_bfqq =
> +				last_bfqq_created;
>  		}
>  	}
>  
> @@ -6696,7 +6703,7 @@ static struct bfq_queue *bfq_get_bfqq_handle_split(struct bfq_data *bfqd,
>  {
>  	unsigned int act_idx = bfq_actuator_index(bfqd, bio);
>  	struct bfq_queue *bfqq = bic_to_bfqq(bic, is_sync, act_idx);
> -	struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data;
> +	struct bfq_iocq_bfqq_data *bfqq_data = &bic->bfqq_data[act_idx];
>  
>  	if (likely(bfqq && bfqq != &bfqd->oom_bfqq))
>  		return bfqq;
> @@ -6804,7 +6811,7 @@ static struct bfq_queue *bfq_init_rq(struct request *rq)
>  	struct bfq_queue *bfqq;
>  	bool new_queue = false;
>  	bool bfqq_already_existing = false, split = false;
> -	struct bfq_iocq_bfqq_data *bfqq_data;
> +	unsigned int a_idx = bfq_actuator_index(bfqd, bio);
>  
>  	if (unlikely(!rq->elv.icq))
>  		return NULL;
> @@ -6828,17 +6835,16 @@ static struct bfq_queue *bfq_init_rq(struct request *rq)
>  	bfqq = bfq_get_bfqq_handle_split(bfqd, bic, bio, false, is_sync,
>  					 &new_queue);
>  
> -	bfqq_data = &bic->bfqq_data;
> -
>  	if (likely(!new_queue)) {
>  		/* If the queue was seeky for too long, break it apart. */
>  		if (bfq_bfqq_coop(bfqq) && bfq_bfqq_split_coop(bfqq) &&
> -			!bfqq_data->stably_merged) {
> +			!bic->bfqq_data[a_idx].stably_merged) {
>  			struct bfq_queue *old_bfqq = bfqq;
>  
>  			/* Update bic before losing reference to bfqq */
>  			if (bfq_bfqq_in_large_burst(bfqq))
> -				bfqq_data->saved_in_large_burst = true;
> +				bic->bfqq_data[a_idx].saved_in_large_burst =
> +					true;
>  
>  			bfqq = bfq_split_bfqq(bic, bfqq);
>  			split = true;
> diff --git a/block/bfq-iosched.h b/block/bfq-iosched.h
> index f2e8ab91951c..e27897d66a0f 100644
> --- a/block/bfq-iosched.h
> +++ b/block/bfq-iosched.h
> @@ -416,7 +416,7 @@ struct bfq_queue {
>  struct bfq_iocq_bfqq_data {
>  	/*
>  	 * Snapshot of the has_short_time flag before merging; taken
> -	 * to remember its value while the queue is merged, so as to
> +	 * to remember its values while the queue is merged, so as to
>  	 * be able to restore it in case of split.
>  	 */
>  	bool saved_has_short_ttime;
> @@ -430,7 +430,7 @@ struct bfq_iocq_bfqq_data {
>  	u64 saved_tot_idle_time;
>  
>  	/*
> -	 * Same purpose as the previous fields for the value of the
> +	 * Same purpose as the previous fields for the values of the
>  	 * field keeping the queue's belonging to a large burst
>  	 */
>  	bool saved_in_large_burst;
> @@ -493,8 +493,12 @@ struct bfq_io_cq {
>  	uint64_t blkcg_serial_nr; /* the current blkcg serial */
>  #endif
>  
> -	/* persistent data for associated synchronous process queue */
> -	struct bfq_iocq_bfqq_data bfqq_data;
> +	/*
> +	 * Persistent data for associated synchronous process queues
> +	 * (one queue per actuator, see field bfqq above). In
> +	 * particular, each of these queues may undergo a merge.
> +	 */
> +	struct bfq_iocq_bfqq_data bfqq_data[BFQ_MAX_ACTUATORS];

I wonder if packing this together with struct bfq_queue would be cleaner.
That would avoid the 2 arrays you have in this struct. Something like this:

struct bfq_queue_data {
	struct bfq_queue 	*bfqq[2];
	struct bfq_iocq_bfqq_data iocq_data;
}

struct bfq_io_cq {
	...
	struct bfq_queue_data bfqqd[BFQ_MAX_ACTUATORS];
	...
}

Thinking aloud here. That may actually make the code more complicated.

>  
>  	unsigned int requests;	/* Number of requests this process has in flight */
>  };

-- 
Damien Le Moal
Western Digital Research