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

[Paolo Valente <paolo.valente@linaro.org>]

> Il giorno 6 dic 2022, alle ore 09:29, Damien Le Moal <damien.lemoal@opensource.wdc.com> ha scritto:
> 
> 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.

I'm still confused, sorry.  Where will I read sector ranges from, if
no sector range information is available (ia_ranges is NULL)?

> 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 ?

Apart from the above point, for which maybe there is some other
source of information for getting ranges, I see the following issue.

What you propose is to save sector information and trigger the
range-checking for loop also for the above single-actuator case.  Yet
txecuting (one iteration of) that loop will will always result in
getting a 0 as index.  So, what's the point is saving data and
executing code on each IO, for getting a static result that we already
know we will get?

Thanks,
Paolo

> 
>> 
>> Once again, all other suggestions applied. I'm about to submit a V7.
>> 
>> Thanks,
>> Paolo
>> 
> 
> -- 
> Damien Le Moal
> Western Digital Research