Re: [PATCH V9 7/8] block, bfq: inject I/O to underutilized actuators

[Damien Le Moal]

On 12/8/22 19:43, Paolo Valente wrote:
> From: Davide Zini <davidezini2@xxxxxxxxx>
> 
> The main service scheme of BFQ for sync I/O is serving one sync
> bfq_queue at a time, for a while. In particular, BFQ enforces this
> scheme when it deems the latter necessary to boost throughput or
> to preserve service guarantees. Unfortunately, when BFQ enforces
> this policy, only one actuator at a time gets served for a while,
> because each bfq_queue contains I/O only for one actuator. The
> other actuators may remain underutilized.
> 
> Actually, BFQ may serve (inject) extra I/O, taken from other
> bfq_queues, in parallel with that of the in-service queue. This
> injection mechanism may provide the ground for dealing also with
> the above actuator-underutilization problem. Yet BFQ does not take
> the actuator load into account when choosing which queue to pick
> extra I/O from. In addition, BFQ may happen to inject extra I/O
> only when the in-service queue is temporarily empty.
> 
> In view of these facts, this commit extends the
> injection mechanism in such a way that the latter:
> (1) takes into account also the actuator load;
> (2) checks such a load on each dispatch, and injects I/O for an
>     underutilized actuator, if there is one and there is I/O for it.
> 
> To perform the check in (2), this commit introduces a load
> threshold, currently set to 4.  A linear scan of each actuator is
> performed, until an actuator is found for which the following two
> conditions hold: the load of the actuator is below the threshold,
> and there is at least one non-in-service queue that contains I/O
> for that actuator. If such a pair (actuator, queue) is found, then
> the head request of that queue is returned for dispatch, instead
> of the head request of the in-service queue.
> 
> We have set the threshold, empirically, to the minimum possible
> value for which an actuator is fully utilized, or close to be
> fully utilized. By doing so, injected I/O 'steals' as few
> drive-queue slots as possibile to the in-service queue. This
> reduces as much as possible the probability that the service of
> I/O from the in-service bfq_queue gets delayed because of slot
> exhaustion, i.e., because all the slots of the drive queue are
> filled with I/O injected from other queues (NCQ provides for 32
> slots).
> 
> This new mechanism also counters actuator underutilization in the
> case of asymmetric configurations of bfq_queues. Namely if there
> are few bfq_queues containing I/O for some actuators and many
> bfq_queues containing I/O for other actuators. Or if the
> bfq_queues containing I/O for some actuators have lower weights
> than the other bfq_queues.
> 
> Reviewed-by: Damien Le Moal <damien.lemoal@xxxxxxxxxxxxxxxxxx>
> Signed-off-by: Paolo Valente <paolo.valente@xxxxxxxxxx>
> Signed-off-by: Davide Zini <davidezini2@xxxxxxxxx>

[...]

> @@ -4792,22 +4799,69 @@ bfq_choose_bfqq_for_injection(struct bfq_data *bfqd)
>  			else
>  				limit = in_serv_bfqq->inject_limit;
>  
> -			if (bfqd->rq_in_driver < limit) {
> +			if (bfqd->tot_rq_in_driver < limit) {
>  				bfqd->rqs_injected = true;
>  				return bfqq;
>  			}
>  		}
> +	}
> +
> +	return NULL;
> +}
> +
> +static struct bfq_queue *
> +bfq_find_active_bfqq_for_actuator(struct bfq_data *bfqd, int idx)
> +{
> +	struct bfq_queue *bfqq = NULL;

I do not think that you need the NULL initialization here.

> +
> +	if (bfqd->in_service_queue &&
> +	    bfqd->in_service_queue->actuator_idx == idx)
> +		return bfqd->in_service_queue;
> +
> +	list_for_each_entry(bfqq, &bfqd->active_list[idx], bfqq_list) {
> +		if (!RB_EMPTY_ROOT(&bfqq->sort_list) &&
> +			bfq_serv_to_charge(bfqq->next_rq, bfqq) <=
> +				bfq_bfqq_budget_left(bfqq)) {
> +			return bfqq;
> +		}
> +	}
>  
>  	return NULL;
>  }

Otherwise looks OK.

Reviewed-by: Damien Le Moal <damien.lemoal@xxxxxxxxxxxxxxxxxx>

-- 
Damien Le Moal
Western Digital Research