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

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

On 25/10/2022 10:16, Ming Lei wrote:
>>>> I mentioned before that if no hctx->cpumask is online then we don't need
>>>> to allocate a request. That is because if no hctx->cpumask is online,
>>>> this means that original erroneous IO must be completed due to nature of
>>>> how blk-mq cpu hotplug handler works, i.e. drained, and then we don't
>>>> actually need to abort it any longer, so ok to not get a request.
>>> No, it is really not OK, if all cpus in hctx->cpumask are offline, you
>>> can't allocate
>>> request on the specified hw queue, then the erroneous IO can't be handled,
>>> then cpu hotplug handler may hang for ever.
>> If the erroneous IO is still in-flight from blk-mq perspective, then how can
>> hctx->cpumask still be offline? I thought that we guarantee that
>> hctx->cpumask cannot go offline until drained.
> Yeah, the draining is done before the cpu is offline. But the drain is
> simply waiting for the inflight IO to be completed. If the IO is failed
> during the waiting, you can't allocate such reserved request for error
> handling, then hang ever in blk_mq_hctx_notify_offline().

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.

> 
> 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.

Thanks,
John