Re: [PATCH V6 6/8] block, bfq: retrieve independent access ranges from request queue

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

On 12/6/22 17:06, Paolo Valente wrote:
> 
> 
>> Il giorno 21 nov 2022, alle ore 02:01, Damien Le Moal <damien.lemoal@opensource.wdc.com> ha scritto:
>>
> 
> ...
> 
>>
>>> }
>>>
>>> static bool bfq_bio_merge(struct request_queue *q, struct bio *bio,
>>> @@ -7144,6 +7159,8 @@ static int bfq_init_queue(struct request_queue *q, struct elevator_type *e)
>>> {
>>> 	struct bfq_data *bfqd;
>>> 	struct elevator_queue *eq;
>>> +	unsigned int i;
>>> +	struct blk_independent_access_ranges *ia_ranges = q->disk->ia_ranges;
>>>
>>> 	eq = elevator_alloc(q, e);
>>> 	if (!eq)
>>> @@ -7187,10 +7204,31 @@ static int bfq_init_queue(struct request_queue *q, struct elevator_type *e)
>>> 	bfqd->queue = q;
>>>
>>> 	/*
>>> -	 * Multi-actuator support not complete yet, default to single
>>> -	 * actuator for the moment.
>>> +	 * If the disk supports multiple actuators, we copy the independent
>>> +	 * access ranges from the request queue structure.
>>> 	 */
>>> -	bfqd->num_actuators = 1;
>>> +	spin_lock_irq(&q->queue_lock);
>>> +	if (ia_ranges) {
>>> +		/*
>>> +		 * Check if the disk ia_ranges size exceeds the current bfq
>>> +		 * actuator limit.
>>> +		 */
>>> +		if (ia_ranges->nr_ia_ranges > BFQ_MAX_ACTUATORS) {
>>> +			pr_crit("nr_ia_ranges higher than act limit: iars=%d, max=%d.\n",
>>> +				ia_ranges->nr_ia_ranges, BFQ_MAX_ACTUATORS);
>>> +			pr_crit("Falling back to single actuator mode.\n");
>>> +			bfqd->num_actuators = 0;
>>> +		} else {
>>> +			bfqd->num_actuators = ia_ranges->nr_ia_ranges;
>>> +
>>> +			for (i = 0; i < bfqd->num_actuators; i++)
>>> +				bfqd->ia_ranges[i] = ia_ranges->ia_range[i];
>>> +		}
>>> +	} else {
>>> +		bfqd->num_actuators = 0;
>>
>> That is very weird. The default should be 1 actuator.
>> ia_ranges->nr_ia_ranges is 0 when the disk does not provide any range
>> information, meaning it is a regular disk with a single actuator.
> 
> Actually, IIUC this assignment to 0 seems to be done exactly when you
> say that it should be done, i.e., when the disk does not provide any
> range information (ia_ranges is NULL). Am I missing something else?

No ranges reported means no extra actuators, so a single actuator an
single LBA range for the entire device. In that case, bfq should process
all IOs using bfqd->ia_ranges[0]. The get range function will always
return that range. That makes the code clean and avoids different path for
nr_ranges == 1 and nr_ranges > 1. No ?

> 
> Once again, all other suggestions applied. I'm about to submit a V7.
> 
> Thanks,
> Paolo
> 

-- 
Damien Le Moal
Western Digital Research