Re: [PATCH] blk-mq: Properly init bios from blk_mq_alloc_request_hctx()

John Garry
2022-10-25T11:36:17Z

On 25/10/2022 12:21, Ming Lei wrote:
>> Actually if final cpu in hctx->cpumask is going offline, then hctx won't
>> queue any more requests, right? In this case I don't think we can queue on
>> that hctx anyway. I need to think about this more.
> It can be queued actually, but interrupt may not be delivered if managed
> irq is used.

Yes, I think it will be queued elsewhere. I would need to check the code
again.

>
>>> If you just make it one driver private command, there can't be such
>>> issue.
>> Well we're trying to use reserved requests for EH commands, which that goes
>> against.
>>
>>> Block layer is supposed for handling common case(normal io and pt io),
>>> I'd suggest to not put such special cases into block layer.
>> It also supports reserved commands, which I would assume would be suitable
>> for EH scenarios.
> Then you have to be careful, as I mentioned, EH has to provide forward
> progress, if you let blk-mq allocate & submit EH request, the implied
> dependency from blk-mq has to be payed attention.

OK, thanks, I know that this carries risk, but it seems right approach.

I have been thinking about my HW queue allocation requirement and maybe
we can solve in low-level driver instead.

The requirement is to send this abort command on same queue as erroneous
command to ensure that they do not race in HW submission, even though
chance of this is really tiny. Maybe we can make low-level driver wait
until erroneous command is really submitted to HW by checking HW
register, etc. before issuing abort on any HW queue (and so would not
need blk_mq_alloc_request_hctx() or similar).

BTW, I would still like to fix blk_mq_alloc_request_hctx() to properly
init ->bio and other fields - ok?

Thanks,
John