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Subject: Re: [PATCH 1/2] scsi_host: add support for request batching
To:     Paolo Bonzini <pbonzini@redhat.com>, linux-kernel@vger.kernel.org,
        kvm@vger.kernel.org
Cc:     jejb@linux.ibm.com, martin.petersen@oracle.com,
        linux-scsi@vger.kernel.org, stefanha@redhat.com
References: <20190530112811.3066-1-pbonzini@redhat.com>
 <20190530112811.3066-2-pbonzini@redhat.com>
 <ad0578b0-ce73-85ed-b67d-70c5d8176a23@acm.org>
 <461fe0cd-c5bc-a612-6013-7c002b92dcdc@redhat.com>
From:   Bart Van Assche <bvanassche@acm.org>
Message-ID: <740d2f33-004e-7a37-1f6e-cf29480439b1@acm.org>
Date:   Thu, 30 May 2019 10:54:12 -0700
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On 5/30/19 8:54 AM, Paolo Bonzini wrote:
> On 30/05/19 17:36, Bart Van Assche wrote:
>> On 5/30/19 4:28 AM, Paolo Bonzini wrote:
>>> +static const struct blk_mq_ops scsi_mq_ops_no_commit = {
>>> +    .get_budget    = scsi_mq_get_budget,
>>> +    .put_budget    = scsi_mq_put_budget,
>>> +    .queue_rq    = scsi_queue_rq,
>>> +    .complete    = scsi_softirq_done,
>>> +    .timeout    = scsi_timeout,
>>> +#ifdef CONFIG_BLK_DEBUG_FS
>>> +    .show_rq    = scsi_show_rq,
>>> +#endif
>>> +    .init_request    = scsi_mq_init_request,
>>> +    .exit_request    = scsi_mq_exit_request,
>>> +    .initialize_rq_fn = scsi_initialize_rq,
>>> +    .busy        = scsi_mq_lld_busy,
>>> +    .map_queues    = scsi_map_queues,
>>> +};
>>> +
>>> +static void scsi_commit_rqs(struct blk_mq_hw_ctx *hctx)
>>> +{
>>> +    struct request_queue *q = hctx->queue;
>>> +    struct scsi_device *sdev = q->queuedata;
>>> +    struct Scsi_Host *shost = sdev->host;
>>> +
>>> +    shost->hostt->commit_rqs(shost, hctx->queue_num);
>>> +}
>>> +
>>>   static const struct blk_mq_ops scsi_mq_ops = {
>>>       .get_budget    = scsi_mq_get_budget,
>>>       .put_budget    = scsi_mq_put_budget,
>>>       .queue_rq    = scsi_queue_rq,
>>> +    .commit_rqs    = scsi_commit_rqs,
>>>       .complete    = scsi_softirq_done,
>>>       .timeout    = scsi_timeout,
>>>   #ifdef CONFIG_BLK_DEBUG_FS
>>
>> Hi Paolo,
>>
>> Have you considered to modify the block layer such that a single
>> scsi_mq_ops structure can be used for all SCSI LLD types?
> 
> Yes, but I don't think it's possible to do it in a nice way.
> Any adjustment we make to the block layer to fit the SCSI subsystem's
> desires would make all other block drivers uglier, so I chose to confine
> the ugliness here.
> 
> The root issue is that the SCSI subsystem is unique in how it sits on
> top of the block layer; this is the famous "adapter" (or "midlayer",
> though that is confusing when talking about SCSI) design that Linux
> usually tries to avoid.

As far as I can see the only impact of defining an empty commit_rqs
callback on the queueing behavior is that blk_mq_make_request() will
queue requests for multiple hwqs on the plug list instead of requests
for a single hwq. The plug list is sorted by hwq before it is submitted
to a block driver. If that helps NVMe performance it should also help
SCSI performance. How about always setting commit_rqs = scsi_commit_rqs
in scsi_mq_ops?

Thanks,

Bart.