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

Paolo Valente
2018-02-08T07:17:01Z

> 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?

Thanks,
Paolo

> --
> Jens Axboe