Re: [PATCH RFC v3 2/7] ata: libata-scsi: Add ata_internal_queuecommand()

[Damien Le Moal <damien.lemoal@opensource.wdc.com>]

On 11/7/22 19:12, Hannes Reinecke wrote:
> On 11/2/22 12:25, Damien Le Moal wrote:
>> On 11/2/22 20:12, Hannes Reinecke wrote:
>>> On 11/2/22 11:07, Damien Le Moal wrote:
>>>> On 11/2/22 18:52, John Garry wrote:
>>>>> Hi Damien,
>>>>>
>>> [ .. ] >> So we only need to find a way of 're-using' that tag, then we won't have
>>> to set aside a reserved tag and everything would be dandy...
>>
>> I tried that. It is very ugly... Problem is that integration with EH in
>> case a real NCQ error happens when all that read-log-complete dance is
>> happening is hard. And don't get me started with the need to save/restore
>> the scsi command context of the command we are reusing the tag from.
>>
>> And given that the code is changing to use regular submission path for
>> internal commands, right now, we need a reserved tag. Or a way to "borrow"
>> the tag from a request that we need to check. Which means we need some
>> additional api to not always try to allocate a tag.
>>
>>>
>>> Maybe we can stop processing when we receive an error (should be doing
>>> that anyway as otherwise the log might be overwritten), then we should
>>> be having a pretty good chance of getting that tag.
>>
>> Hmmm.... that would be no better than using EH which does stop processing
>> until the internal house keeping is done.
>>
>>> Or, precisely, getting _any_ tag as at least one tag is free at that point.
>>> Hmm?
>>
>> See above. Not free, but usable as far as the device is concerned since we
>> have at least on command we need to check completed at the device level
>> (but not yet completed from scsi/block layer point of view).
>>
> So, having had an entire weekend pondering this issue why don't we 
> allocate an _additional_ set of requests?
> After all, we had been very generous with allocating queues and requests 
> (what with us doing a full provisioning of the requests for all queues 
> already for the non-shared tag case).
> 
> Idea would be to keep the single tag bitmap, but add eg a new rq state
> MQ_RQ_ERROR. Once that flag is set we'll fetch the error request instead 
> of the normal one:
> 
> @@ -761,6 +763,8 @@ static inline struct request 
> *blk_mq_tag_to_rq(struct blk_mq_tags *tags,
>   {
>          if (tag < tags->nr_tags) {
>                  prefetch(tags->rqs[tag]);
> +               if (unlikely(blk_mq_request_error(tags->rqs[tag])))
> +                       return tags->error_rqs[tag];
>                  return tags->rqs[tag];
>          }
> 
> and, of course, we would need to provision the error request first.
> 
> Rationale here is that this will be primarily for devices with a low 
> number of tags, so doubling the number of request isn't much of an 
> overhead (as we'll be doing it essentially anyway in the error case as 
> we'll have to save the original request _somewhere_), and that it would 
> remove quite some cruft from the subsystem; look at SCSI EH trying to 
> store the original request contents and then after EH restoring them again.

Interesting idea. I like it. It is essentially a set of reserved requests
without reserved tags: the tag to use for these requests would be provided
"manually" by the user. Right ?

That should allow simplifying any processing that needs to reuse a tag,
and currently its request. That is, CDL, but also usb-scsi, scsi EH and
the few scsi LLDs using scsi_eh_prep_cmnd()+scsi_eh_restore_cmnd().
Ideally, these 2 functions could go away too.

> 
> Hmm?
> 
> Cheers,
> 
> Hannes

-- 
Damien Le Moal
Western Digital Research