Re: [PATCH RFC v2 02/24] scsi: allocate separate queue for reserved commands

Hannes Reinecke
2020-03-11T06:58:54Z

On 3/11/20 7:22 AM, Christoph Hellwig wrote:
> On Tue, Mar 10, 2020 at 09:08:56PM +0000, John Garry wrote:
>> On 10/03/2020 18:32, Christoph Hellwig wrote:
>>> On Wed, Mar 11, 2020 at 12:25:28AM +0800, John Garry wrote:
>>>> From: Hannes Reinecke <hare@suse.com>
>>>>
>>>> Allocate a separate 'reserved_cmd_q' for sending reserved commands.
>>>
>>> Why? Reserved command specifically are not in any way tied to queues.
>>> .
>>>
>>
>> So the v1 series used a combination of the sdev queue and the per-host
>> reserved_cmd_q. Back then you questioned using the sdev queue for virtio
>> scsi, and the unconfirmed conclusion was to use a common per-host q. This is
>> the best link I can find now:
>>
>> https://www.mail-archive.com/linux-scsi@vger.kernel.org/msg83177.html
>
> That was just a question on why virtio uses the per-device tags, which
> didn't look like it made any sense. What I'm worried about here is
> mixing up the concept of reserved tags in the tagset, and queues to use
> them. Note that we already have the scsi_get_host_dev to allocate
> a scsi_device and thus a request_queue for the host itself. That seems
> like the better interface to use a tag for a host wide command vs
> introducing a parallel path.
>
Ah. Right.
Will be looking into that, and convert the patchset over to it.

And the problem of the separate queue is the fact that I'll need a queue
to reserve tags from; trying to allocate a tag directly from the bitmap
turns out to be major surgery in the blocklayer with no immediate gain.
And I can't use per-device queues as for some drivers the reserved
commands are used to query the HBA itself to figure out how many devices
are present.

Cheers,

Hannes
--
Dr. Hannes Reinecke Kernel Storage Architect
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