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

[Oleksandr Natalenko]

Hi.

08.02.2018 08:16, Paolo Valente wrote:
> > Il giorno 07 feb 2018, alle ore 23:18, Jens Axboe <axboe@xxxxxxxxx> 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@xxxxxxxxxxxxxx>
> 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