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

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Message-ID: <6983f8b3-a320-ce32-ef0d-273d11dd8648@opensource.wdc.com>
Date:   Tue, 6 Dec 2022 18:02:16 +0900
From:   Damien Le Moal <damien.lemoal@...nsource.wdc.com>
To:     Paolo Valente <paolo.valente@...aro.org>
Cc:     Jens Axboe <axboe@...nel.dk>,
        linux-block <linux-block@...r.kernel.org>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Arie van der Hoeven <arie.vanderhoeven@...gate.com>,
        Rory Chen <rory.c.chen@...gate.com>,
        Federico Gavioli <f.gavioli97@...il.com>
Subject: Re: [PATCH V6 6/8] block, bfq: retrieve independent access ranges
 from request queue

On 12/6/22 17:41, Paolo Valente wrote:
> 
> 
>> Il giorno 6 dic 2022, alle ore 09:29, Damien Le Moal <damien.lemoal@...nsource.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@...nsource.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)?

start = 0 and nr_sectors = bdev_nr_sectors(bdev).
No ia_ranges to read.

> 
>> 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?

Surely, you can add an "if (bfqd->num_actuators ==1)" optimization in
strategic places to optimize for regular devices with a single actuator,
which bfqd->num_actuators == 1 *exactly* describes. Having
"bfqd->num_actuators = 0" makes no sense to me.

But if you feel strongly about this, feel free to ignore this.

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

-- 
Damien Le Moal
Western Digital Research