Re: [PATCH] blk-mq: don't queue passthrough request into scheduler

[Jens Axboe <axboe@kernel.dk>]

On 5/12/23 9:19 AM, Ming Lei wrote:
> On Fri, May 12, 2023 at 09:08:54AM -0600, Jens Axboe wrote:
>> On 5/12/23 9:03?AM, Ming Lei wrote:
>>> Passthrough(pt) request shouldn't be queued to scheduler, especially some
>>> schedulers(such as bfq) supposes that req->bio is always available and
>>> blk-cgroup can be retrieved via bio.
>>>
>>> Sometimes pt request could be part of error handling, so it is better to always
>>> queue it into hctx->dispatch directly.
>>>
>>> Fix this issue by queuing pt request from plug list to hctx->dispatch
>>> directly.
>>
>> Why not just add the check to the BFQ insertion? That would be a lot
>> more trivial and would not be poluting the core with this stuff.
> 
> pt request is supposed to be issued to device directly, and we never
> queue it to scheduler before 1c2d2fff6dc0 ("block: wire-up support for
> passthrough plugging").
> 
> some pt request might be part of error handling, and adding it to
> scheduler could cause io hang.

I'm not suggesting adding it to the scheduler, just having the bypass
"add to dispatch" in a different spot.

Let me take a look at it... Do we have a reproducer for this issue?

-- 
Jens Axboe