Re: [PATCH BUGFIX V3] block, bfq: add requeue-request hook

[Oleksandr Natalenko <oleksandr@natalenko.name>]

Hi.

08.02.2018 08:16, Paolo Valente wrote:
>> Il giorno 07 feb 2018, alle ore 23:18, Jens Axboe <axboe@kernel.dk> ha 
>> scritto:
>> 
>> On 2/7/18 2:19 PM, Paolo Valente wrote:
>>> Commit 'a6a252e64914 ("blk-mq-sched: decide how to handle flush rq 
>>> via
>>> RQF_FLUSH_SEQ")' makes all non-flush re-prepared requests for a 
>>> device
>>> be re-inserted into the active I/O scheduler for that device. As a
>>> consequence, I/O schedulers may get the same request inserted again,
>>> even several times, without a finish_request invoked on that request
>>> before each re-insertion.
>>> 
>>> This fact is the cause of the failure reported in [1]. For an I/O
>>> scheduler, every re-insertion of the same re-prepared request is
>>> equivalent to the insertion of a new request. For schedulers like
>>> mq-deadline or kyber, this fact causes no harm. In contrast, it
>>> confuses a stateful scheduler like BFQ, which keeps state for an I/O
>>> request, until the finish_request hook is invoked on the request. In
>>> particular, BFQ may get stuck, waiting forever for the number of
>>> request dispatches, of the same request, to be balanced by an equal
>>> number of request completions (while there will be one completion for
>>> that request). In this state, BFQ may refuse to serve I/O requests
>>> from other bfq_queues. The hang reported in [1] then follows.
>>> 
>>> However, the above re-prepared requests undergo a requeue, thus the
>>> requeue_request hook of the active elevator is invoked for these
>>> requests, if set. This commit then addresses the above issue by
>>> properly implementing the hook requeue_request in BFQ.
>> 
>> Thanks, applied.
>> 
> 
> I Jens,
> I forgot to add
> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name>
> in the patch.
> 
> Is it still possible to add it?
> 

In addition to this I think it should be worth considering CC'ing Greg 
to pull this fix into 4.15 stable tree.

Oleksandr