On 2/7/18 11:00 AM, 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.
>
> [1] https://marc.info/?l=linux-block&m=151211117608676
>
> Reported-by: Ivan Kozik <i...@ludios.org>
> Reported-by: Alban Browaeys <alban.browa...@gmail.com>
> Tested-by: Mike Galbraith <efa...@gmx.de>
> Signed-off-by: Paolo Valente <paolo.vale...@linaro.org>
> Signed-off-by: Serena Ziviani <ziviani.ser...@gmail.com>
> ---
> block/bfq-iosched.c | 109
> ++++++++++++++++++++++++++++++++++++++++------------
> 1 file changed, 84 insertions(+), 25 deletions(-)
>
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 47e6ec7427c4..21e6b9e45638 100644
> --- a/block/bfq-iosched.c
> +++ b/block/bfq-iosched.c
> @@ -3823,24 +3823,26 @@ static struct request *__bfq_dispatch_request(struct
> blk_mq_hw_ctx *hctx)
> }
>
> /*
> - * We exploit the bfq_finish_request hook to decrement
> - * rq_in_driver, but bfq_finish_request will not be
> - * invoked on this request. So, to avoid unbalance,
> - * just start this request, without incrementing
> - * rq_in_driver. As a negative consequence,
> - * rq_in_driver is deceptively lower than it should be
> - * while this request is in service. This may cause
> - * bfq_schedule_dispatch to be invoked uselessly.
> + * We exploit the bfq_finish_requeue_request hook to
> + * decrement rq_in_driver, but
> + * bfq_finish_requeue_request will not be invoked on
> + * this request. So, to avoid unbalance, just start
> + * this request, without incrementing rq_in_driver. As
> + * a negative consequence, rq_in_driver is deceptively
> + * lower than it should be while this request is in
> + * service. This may cause bfq_schedule_dispatch to be
> + * invoked uselessly.
> *
> * As for implementing an exact solution, the
> - * bfq_finish_request hook, if defined, is probably
> - * invoked also on this request. So, by exploiting
> - * this hook, we could 1) increment rq_in_driver here,
> - * and 2) decrement it in bfq_finish_request. Such a
> - * solution would let the value of the counter be
> - * always accurate, but it would entail using an extra
> - * interface function. This cost seems higher than the
> - * benefit, being the frequency of non-elevator-private
> + * bfq_finish_requeue_request hook, if defined, is
> + * probably invoked also on this request. So, by
> + * exploiting this hook, we could 1) increment
> + * rq_in_driver here, and 2) decrement it in
> + * bfq_finish_requeue_request. Such a solution would
> + * let the value of the counter be always accurate,
> + * but it would entail using an extra interface
> + * function. This cost seems higher than the benefit,
> + * being the frequency of non-elevator-private
> * requests very low.
> */
> goto start_rq;
> @@ -4515,6 +4517,8 @@ static inline void bfq_update_insert_stats(struct
> request_queue *q,
> unsigned int cmd_flags) {}
> #endif
>
> +static void bfq_prepare_request(struct request *rq, struct bio *bio);
> +
> static void bfq_insert_request(struct blk_mq_hw_ctx *hctx, struct request
> *rq,
> bool at_head)
> {
> @@ -4541,6 +4545,20 @@ static void bfq_insert_request(struct blk_mq_hw_ctx
> *hctx, struct request *rq,
> else
> list_add_tail(&rq->queuelist, &bfqd->dispatch);
> } else {
> + if (!bfqq) {
> + /*
> + * This should never happen. Most likely rq is
> + * a requeued regular request, being
> + * re-inserted without being first
> + * re-prepared. Do a prepare, to avoid
> + * failure.
> + */
> + pr_warn("Regular request associated with no queue");
> + WARN_ON(1);
> + bfq_prepare_request(rq, rq->bio);
> + bfqq = RQ_BFQQ(rq);
This reads kind of strange. "Regular request not associated with a
queue" would be cleaner. Do we really need the message? Why not just
make the above:
if (WARN_ON_ONCE(!bfqq)) {
bfq_prepare_request(rq, rq->bio);
bfqq = RQ_BFQQ(rq);
}
which is much simpler, kills the useless message, and avoids constant
spew in case it does trigger.
--
Jens Axboe
