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

Ming Lei
2018-02-23T15:07:35Z

Hi Paolo,

On Wed, Feb 07, 2018 at 10:19:20PM +0100, 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

No, this behaviour isn't related with commit a6a252e64914, and
it has been there since blk_mq_requeue_request() is introduced.

And you can see blk_mq_requeue_request() is called by lots of drivers,
especially it is often used in error handler, see SCSI's example.

> 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 <ivan@ludios.org>
> Reported-by: Alban Browaeys <alban.browaeys@gmail.com>
> Tested-by: Mike Galbraith <efault@gmx.de>
> Signed-off-by: Paolo Valente <paolo.valente@linaro.org>
> Signed-off-by: Serena Ziviani <ziviani.serena@gmail.com>
> ---
> V2: contains fix to bug reported in [2]
> V3: implements the improvement suggested in [3]
>
> [2] https://lkml.org/lkml/2018/2/5/599
> [3] https://lkml.org/lkml/2018/2/7/532
>
> block/bfq-iosched.c | 107 ++++++++++++++++++++++++++++++++++++++++------------
> 1 file changed, 82 insertions(+), 25 deletions(-)
>
> diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
> index 47e6ec7427c4..aeca22d91101 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 +4545,18 @@ static void bfq_insert_request(struct blk_mq_hw_ctx *hctx, struct request *rq,
> else
> list_add_tail(&rq->queuelist, &bfqd->dispatch);
> } else {
> + if (WARN_ON_ONCE(!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.
> + */
> + bfq_prepare_request(rq, rq->bio);
> + bfqq = RQ_BFQQ(rq);
> + }
> +
> idle_timer_disabled = __bfq_insert_request(bfqd, rq);
> /*
> * Update bfqq, because, if a queue merge has occurred
> @@ -4697,22 +4713,44 @@ static void bfq_completed_request(struct bfq_queue *bfqq, struct bfq_data *bfqd)
> bfq_schedule_dispatch(bfqd);
> }
>
> -static void bfq_finish_request_body(struct bfq_queue *bfqq)
> +static void bfq_finish_requeue_request_body(struct bfq_queue *bfqq)
> {
> bfqq->allocated--;
>
> bfq_put_queue(bfqq);
> }
>
> -static void bfq_finish_request(struct request *rq)
> +/*
> + * Handle either a requeue or a finish for rq. The things to do are
> + * the same in both cases: all references to rq are to be dropped. In
> + * particular, rq is considered completed from the point of view of
> + * the scheduler.
> + */
> +static void bfq_finish_requeue_request(struct request *rq)
> {
> - struct bfq_queue *bfqq;
> + struct bfq_queue *bfqq = RQ_BFQQ(rq);
> struct bfq_data *bfqd;
>
> - if (!rq->elv.icq)
> + /*
> + * Requeue and finish hooks are invoked in blk-mq without
> + * checking whether the involved request is actually still
> + * referenced in the scheduler. To handle this fact, the
> + * following two checks make this function exit in case of
> + * spurious invocations, for which there is nothing to do.
> + *
> + * First, check whether rq has nothing to do with an elevator.
> + */
> + if (unlikely(!(rq->rq_flags & RQF_ELVPRIV)))
> + return;
> +
> + /*
> + * rq either is not associated with any icq, or is an already
> + * requeued request that has not (yet) been re-inserted into
> + * a bfq_queue.
> + */
> + if (!rq->elv.icq || !bfqq)
> return;
>
> - bfqq = RQ_BFQQ(rq);
> bfqd = bfqq->bfqd;
>
> if (rq->rq_flags & RQF_STARTED)
> @@ -4727,13 +4765,14 @@ static void bfq_finish_request(struct request *rq)
> spin_lock_irqsave(&bfqd->lock, flags);
>
> bfq_completed_request(bfqq, bfqd);
> - bfq_finish_request_body(bfqq);
> + bfq_finish_requeue_request_body(bfqq);
>
> spin_unlock_irqrestore(&bfqd->lock, flags);
> } else {
> /*
> * Request rq may be still/already in the scheduler,
> - * in which case we need to remove it. And we cannot
> + * in which case we need to remove it (this should
> + * never happen in case of requeue). And we cannot
> * defer such a check and removal, to avoid
> * inconsistencies in the time interval from the end
> * of this function to the start of the deferred work.
> @@ -4748,9 +4787,26 @@ static void bfq_finish_request(struct request *rq)
> bfqg_stats_update_io_remove(bfqq_group(bfqq),
> rq->cmd_flags);
> }
> - bfq_finish_request_body(bfqq);
> + bfq_finish_requeue_request_body(bfqq);
> }
>
> + /*
> + * Reset private fields. In case of a requeue, this allows
> + * this function to correctly do nothing if it is spuriously
> + * invoked again on this same request (see the check at the
> + * beginning of the function). Probably, a better general
> + * design would be to prevent blk-mq from invoking the requeue
> + * or finish hooks of an elevator, for a request that is not
> + * referred by that elevator.
> + *
> + * Resetting the following fields would break the
> + * request-insertion logic if rq is re-inserted into a bfq
> + * internal queue, without a re-preparation. Here we assume
> + * that re-insertions of requeued requests, without
> + * re-preparation, can happen only for pass_through or at_head
> + * requests (which are not re-inserted into bfq internal
> + * queues).
> + */
> rq->elv.priv[0] = NULL;
> rq->elv.priv[1] = NULL;
> }
> @@ -5426,7 +5482,8 @@ static struct elevator_type iosched_bfq_mq = {
> .ops.mq = {
> .limit_depth = bfq_limit_depth,
> .prepare_request = bfq_prepare_request,
> - .finish_request = bfq_finish_request,
> + .requeue_request = bfq_finish_requeue_request,
> + .finish_request = bfq_finish_requeue_request,
> .exit_icq = bfq_exit_icq,
> .insert_requests = bfq_insert_requests,
> .dispatch_request = bfq_dispatch_request,

This way may not be correct since blk_mq_sched_requeue_request() can be
called for one request which won't enter io scheduler.

__blk_mq_requeue_request() is called for two cases:

- one is that the requeued request is added to hctx->dispatch, such
as blk_mq_dispatch_rq_list()
- another case is that the request is requeued to io scheduler, such as
blk_mq_requeue_request().

For the 1st case, blk_mq_sched_requeue_request() shouldn't be called
since it is nothing to do with scheduler, seems we only need to do that
for 2nd case.

So looks we need the following patch:

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 23de7fd8099a..a216f3c3c3ce 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -712,7 +714,6 @@ static void __blk_mq_requeue_request(struct request *rq)

trace_block_rq_requeue(q, rq);
wbt_requeue(q->rq_wb, &rq->issue_stat);
- blk_mq_sched_requeue_request(rq);

if (blk_mq_rq_state(rq) != MQ_RQ_IDLE) {
blk_mq_rq_update_state(rq, MQ_RQ_IDLE);
@@ -725,6 +726,9 @@ void blk_mq_requeue_request(struct request *rq, bool kick_requeue_list)
{
__blk_mq_requeue_request(rq);

+ /* this request will be re-inserted to io scheduler queue */
+ blk_mq_sched_requeue_request(rq);
+
BUG_ON(blk_queued_rq(rq));
blk_mq_add_to_requeue_list(rq, true, kick_requeue_list);
}

Thanks,
Ming