Re: [PATCH 6/6] scsi: hisi_sas: Expose multiple hw queues for v3 as experimental

[John Garry <john.garry@huawei.com>]

On 12/11/2019 13:54, John Garry wrote:
>>>
>>> I mentioned in the thread "blk-mq: improvement on handling IO during CPU
>>> hotplug" that I was using this series to test that patchset.
>>>
>>> So just with this patchset (and without yours), I get what looks like 
>>> some
>>> IO errors in the LLDD. The error is an underflow error. I can't 
>>> figure out
>>> what is the cause.
>>
> 
> Hi Ming,
> 
>> Can you post the error log? Or interpret the 'underflow error' from hisi
>> sas or scsi viewpoint?
> 
> The check here fails:
> https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/drivers/scsi/libsas/sas_scsi_host.c?h=v5.4-rc7#n57 
> 
> 
> Indeed, no data is received.
> 
>>
>>>
>>> I'm wondering if the SCSI command is getting corrupted someway.
>>
>> Why do you think the command is corrupted?
> 
> I considered that the underflow may occur if we were to clobber a SCSI 
> command/request from another hctx and zero some fields, which is 
> detected as an underflow. But that's just guessing.
> 
> However do I find if I set shost->can_queue = HISI_SAS_MAX_COMMANDS / 
> #queues, then no issue  But maybe that's a coincidence. For this, total
> queue depth = HISI_SAS_MAX_COMMANDS. I don't see the impact of that.
> 

Scratch that. I have seen the issue here also.

Thanks,
John

> I need to test that more.
> 
>>
>>>
>>>>> +    if (expose_mq_experimental) {
>>>>> +        shost->can_queue = HISI_SAS_MAX_COMMANDS;
>>>>> +        shost->cmd_per_lun = HISI_SAS_MAX_COMMANDS;
>>>> The above is contradictory with current 'nr_hw_queues''s meaning,
>>>> see commit on Scsi_Host.nr_hw_queues.
>>>>
>>>
>>> Right, so I am generating the hostwide tag in the LLDD. And the Scsi
>>> host-wide host_busy counter should ensure that we don't pump too much 
>>> IO to
>>> the HBA.
>>
>> Even without the host-wide host_busy, your approach should work if you
>> build the hisi sas tag correctly(uniquely), just not efficiently.
> 
> Yes, I do that.
> 
>   I'd
>> suggest you to collect trace and observe if request with expected hisi 
>> sas
>> tag is sent to hardware.
>>
> 
> I can add some debug for that. What trace do you mean?
> 
>> BTW, the patch of 'scsi: core: avoid host-wide host_busy counter for 
>> scsi_mq'
>> will be merged to v5.5 if everything is fine.
>>
>> https://git.kernel.org/pub/scm/linux/kernel/git/mkp/scsi.git/commit/?h=5.5/scsi-queue&id=6eb045e092efefafc6687409a6fa6d1dabf0fb69 
>>
> 
> Yeah, it seems a good change.
> 
> Thanks,
> John
> .