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

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Message-ID: <39bc2d82-2676-e329-5d32-8acb99b0a204@suse.de>
Date:   Tue, 7 Apr 2020 16:00:10 +0200
From:   Hannes Reinecke <hare@...e.de>
To:     John Garry <john.garry@...wei.com>,
        Christoph Hellwig <hch@...radead.org>
Cc:     axboe@...nel.dk, jejb@...ux.ibm.com, martin.petersen@...cle.com,
        ming.lei@...hat.com, bvanassche@....org,
        linux-block@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-scsi@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        esc.storagedev@...rosemi.com, chenxiang66@...ilicon.com,
        Hannes Reinecke <hare@...e.com>
Subject: Re: [PATCH RFC v2 02/24] scsi: allocate separate queue for reserved
 commands

On 4/7/20 1:54 PM, John Garry wrote:
> On 06/04/2020 10:05, Hannes Reinecke wrote:
>> 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@...e.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.
>>>
>> Thinking about it some more, I don't think that scsi_get_host_dev() is
>> the best way of handling it.
>> Problem is that it'll create a new scsi_device with <hostno:this_id:0>,
>> which will then show up via eg 'lsscsi'.
> 
> are you sure? Doesn't this function just allocate the sdev, but do 
> nothing with it, like probing it?
> 
> I bludgeoned it in here for PoC:
> 
> https://github.com/hisilicon/kernel-dev/commit/ef0ae8540811e32776f64a5b42bd76cbed17ba47 
> 
> 
> And then still:
> 
> john@...ntu:~$ lsscsi
> [0:0:0:0] disk SEAGATE  ST2000NM0045  N004  /dev/sda
> [0:0:1:0] disk SEAGATE  ST2000NM0045  N004  /dev/sdb
> [0:0:2:0] disk ATASAMSUNG HM320JI  0_01  /dev/sdc
> [0:0:3:0] disk SEAGATE  ST1000NM0023  0006  /dev/sdd
> [0:0:4:0] enclosu HUAWEIExpander 12Gx16  128-
> john@...ntu:~$
> 
> Some proper plumbing would be needed, though.
> 
>> This would be okay if 'this_id' would have been defined by the driver;
>> sadly, most drivers which are affected here do set 'this_id' to -1.
>> So we wouldn't have a nice target ID to allocate the device from, let
>> alone the problem that we would have to emulate a complete scsi device
>> with all required minimal command support etc.
>> And I'm not quite sure how well that would play with the exising SCSI
>> host template; the device we'll be allocating would have basically
>> nothing in common with the 'normal' SCSI devices.
>>
>> What we could do, though, is to try it the other way round:
>> Lift the request queue from scsi_get_host_dev() into the scsi host
>> itself, so that scsi_get_host_dev() can use that queue, but we also
>> would be able to use it without a SCSI device attached.
> 
> wouldn't that limit 1x scsi device per host, not that I know if any more 
> would ever be required? But it does still seem better to use the request 
> queue in the scsi device.
> 
My concern is this:

struct scsi_device *scsi_get_host_dev(struct Scsi_Host *shost)
{
	[ .. ]
	starget = scsi_alloc_target(&shost->shost_gendev, 0, shost->this_id);
	[ .. ]

and we have typically:

drivers/scsi/hisi_sas/hisi_sas_v3_hw.c: .this_id                = -1,

It's _very_ uncommon to have a negative number as the SCSI target 
device; in fact, it _is_ an unsigned int already.

But alright, I'll give it a go; let's see what I'll end up with.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke            Teamlead Storage & Networking
hare@...e.de                               +49 911 74053 688
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