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

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Date:   Wed, 11 Mar 2020 07:51:18 +0000
From:   John Garry <john.garry@...wei.com>
To:     Hannes Reinecke <hare@...e.de>,
        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 11/03/2020 06:58, 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.
>>
> Ah. Right.
> Will be looking into that, and convert the patchset over to it.
> 

Hannes,

I assume that you will take back over this series now. Let me know. The 
patches are here, if not in patchwork:
https://github.com/hisilicon/kernel-dev/tree/private-topic-sas-5.6-resv-commands-v2

> 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.

Right, we still need to be able to allocate somewhere apart from sdev queue

> 


Thanks