linux-kernel - Re: [PATCH 10/14] blk-mq: initial support for multiple queue maps

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Date:   Mon, 29 Oct 2018 14:09:11 -0600
From:   Jens Axboe <axboe@...nel.dk>
To:     Bart Van Assche <bvanassche@....org>, linux-block@...r.kernel.org,
        linux-scsi@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 10/14] blk-mq: initial support for multiple queue maps

On 10/29/18 2:00 PM, Bart Van Assche wrote:
> On Mon, 2018-10-29 at 13:53 -0600, Jens Axboe wrote:
>> On 10/29/18 1:40 PM, Bart Van Assche wrote:
>>> On Mon, 2018-10-29 at 10:37 -0600, Jens Axboe wrote:
>>>> -static int cpu_to_queue_index(unsigned int nr_queues, const int cpu)
>>>> +static int cpu_to_queue_index(struct blk_mq_queue_map *qmap,
>>>> +			      unsigned int nr_queues, const int cpu)
>>>>  {
>>>> -	return cpu % nr_queues;
>>>> +	return qmap->queue_offset + (cpu % nr_queues);
>>>>  }
>>>>
>>>> [ ... ]
>>>>  
>>>> --- a/include/linux/blk-mq.h
>>>> +++ b/include/linux/blk-mq.h
>>>> @@ -78,10 +78,11 @@ struct blk_mq_hw_ctx {
>>>>  struct blk_mq_queue_map {
>>>>  	unsigned int *mq_map;
>>>>  	unsigned int nr_queues;
>>>> +	unsigned int queue_offset;
>>>>  };
>>>
>>> I think it's unfortunate that the blk-mq core uses the .queue_offset member but
>>> that mapping functions in block drivers are responsible for setting that member.
>>> Since the block driver mapping functions have to set blk_mq_queue_map.nr_queues,
>>> how about adding a loop in blk_mq_update_queue_map() that derives .queue_offset
>>> from .nr_queues from previous array entries?
>>
>> It's not a simple increment, so the driver has to be the one setting it. If
>> we end up sharing queues, for instance, then the driver will need to set
>> it to the start offset of that set. If you go two patches forward you
>> can see that exact construct.
>>
>> IOW, it's the driver that controls the offset, not the core.
> 
> If sharing of hardware queues between hardware queue types is supported,
> what should hctx->type be set to? Additionally, patch 5 adds code that uses
> hctx->type as an array index. How can that code work if a single hardware
> queue can be shared by multiple hardware queue types?

hctx->type will be set to the value of the first type. This is all driver
private, blk-mq could not care less what the value of the type means.

As to the other question, it works just fine since that is the queue
that is being accessed. There's no confusion there. I think you're
misunderstanding how it's seutp. To use nvme as the example, type 0
would be reads, 1 writes, and 2 pollable queues. If reads and writes
share the same set of hardware queues, then type 1 simply doesn't
exist in terms of ->flags_to_type() return value. This is purely
driven by the driver. That hook is the only decider of where something
will go. If we share hctx sets, we share the same hardware queue as
well. There is just the one set for that case.

-- 
Jens Axboe