[dm-devel] dm-rq: don't call blk_mq_queue_stopped in dm_stop_queue()
Ming Lei
ming.lei at redhat.com
Fri Jun 19 23:37:40 UTC 2020
On Sat, Jun 20, 2020 at 07:14:51AM +0800, Ming Lei wrote:
> On Fri, Jun 19, 2020 at 07:04:05PM -0400, Mike Snitzer wrote:
> > On Fri, Jun 19 2020 at 6:52pm -0400,
> > Ming Lei <ming.lei at redhat.com> wrote:
> >
> > > On Sat, Jun 20, 2020 at 06:37:44AM +0800, Ming Lei wrote:
> > > > On Fri, Jun 19, 2020 at 01:40:41PM -0400, Mike Snitzer wrote:
> > > > > On Fri, Jun 19 2020 at 12:06pm -0400,
> > > > > Mike Snitzer <snitzer at redhat.com> wrote:
> > > > >
> > > > > > On Fri, Jun 19 2020 at 6:11am -0400,
> > > > > > Ming Lei <ming.lei at redhat.com> wrote:
> > > > > >
> > > > > > > Hi Mike,
> > > > > > >
> > > > > > > On Fri, Jun 19, 2020 at 05:42:50AM -0400, Mike Snitzer wrote:
> > > > > > > > Hi Ming,
> > > > > > > >
> > > > > > > > Thanks for the patch! But I'm having a hard time understanding what
> > > > > > > > you've written in the patch header,
> > > > > > > >
> > > > > > > > On Fri, Jun 19 2020 at 4:42am -0400,
> > > > > > > > Ming Lei <ming.lei at redhat.com> wrote:
> > > > > > > >
> > > > > > > > > dm-rq won't stop queue, meantime blk-mq won't stop one queue too, so
> > > > > > > > > remove the check.
> > > > > > > >
> > > > > > > > It'd be helpful if you could unpack this with more detail before going on
> > > > > > > > to explain why using blk_queue_quiesced, despite dm-rq using
> > > > > > > > blk_mq_queue_stopped, would also be ineffective.
> > > > > > > >
> > > > > > > > SO:
> > > > > > > >
> > > > > > > > > dm-rq won't stop queue
> > > > > > > >
> > > > > > > > 1) why won't dm-rq stop the queue? Do you mean it won't reliably
> > > > > > > > _always_ stop the queue because of the blk_mq_queue_stopped() check?
> > > > > > >
> > > > > > > device mapper doesn't call blk_mq_stop_hw_queue or blk_mq_stop_hw_queues.
> > > > > > >
> > > > > > > >
> > > > > > > > > meantime blk-mq won't stop one queue too, so remove the check.
> > > > > > > >
> > > > > > > > 2) Meaning?: blk_mq_queue_stopped() will return true even if only one hw
> > > > > > > > queue is stopped, given blk-mq must stop all hw queues a positive return
> > > > > > > > from this blk_mq_queue_stopped() check is incorrectly assuming it meanss
> > > > > > > > all hw queues are stopped.
> > > > > > >
> > > > > > > blk-mq won't call blk_mq_stop_hw_queue or blk_mq_stop_hw_queues for
> > > > > > > dm-rq's queue too, so dm-rq's hw queue won't be stopped.
> > > > > > >
> > > > > > > BTW blk_mq_stop_hw_queue or blk_mq_stop_hw_queues are supposed to be
> > > > > > > used for throttling queue.
> > > > > >
> > > > > > I'm going to look at actually stopping the queue (using one of these
> > > > > > interfaces). I didn't realize I wasn't actually stopping the queue.
> > > > > > The intent was to do so.
> > > > > >
> > > > > > In speaking with Jens yesterday about freeze vs stop: it is clear that
> > > > > > dm-rq needs to still be able to allocate new requests, but _not_ call
> > > > > > the queue_rq to issue the requests, while "stopped" (due to dm-mpath
> > > > > > potentially deferring retries of failed requests because of path failure
> > > > > > while quiescing the queue during DM device suspend). But that freezing
> > > > > > the queue goes too far because it won't allow such request allocation.
> > > > >
> > > > > Seems I'm damned if I do (stop) or damned if I don't (new reports of
> > > > > requests completing after DM device suspend's
> > > > > blk_mq_quiesce_queue()+dm_wait_for_completion()).
> > > >
> > > > request(but not new) completing is possible after blk_mq_quiesce_queue()+
> > > > dm_wait_for_completion, because blk_mq_rq_inflight() only checks INFLIGHT
> > > > request. If all requests are marked as MQ_RQ_COMPLETE, blk_mq_rq_inflight()
> > > > still may return false. However, MQ_RQ_COMPLETE is one transient state.
> > > >
> > > > So what does dm-rq expect from dm_wait_for_completion()?
> > > >
> > > > If it is just no new request entering dm_queue_rq(), there shouldn't be
> > > > issue.
> > > >
> > > > If dm-rq hopes there aren't any real inflight request(MQ_RQ_COMPLETE &
> > > > MQ_RQ_INFLIGHT), we can change blk_mq_rq_inflight to support that.
> > >
> > > Hi Mike,
> > >
> > > Please test the following patch and see if the issue can be fixed:
> > >
> > > From faf0f9f15627446e8c35db518e37a4a2e4323eb2 Mon Sep 17 00:00:00 2001
> > > From: Ming Lei <ming.lei at redhat.com>
> > > Date: Sat, 20 Jun 2020 06:45:49 +0800
> > > Subject: [PATCH] blk-mq: cover request of MQ_RQ_COMPLETE as inflight in
> > > blk_mq_rq_inflight
> > >
> > > When request is marked as MQ_RQ_COMPLETE, ->complete isn't called & done
> > > yet, and driver may expect that there isn't any driver related activity since
> > > blk_mq_queue_inflight() returns.
> > >
> > > Fixes it by covering request of MQ_RQ_COMPLETE as inflight in blk_mq_rq_inflight().
> > >
> > > Cc: Mike Snitzer <snitzer at redhat.com>
> > > Signed-off-by: Ming Lei <ming.lei at redhat.com>
> > > ---
> > > block/blk-mq.c | 2 +-
> > > 1 file changed, 1 insertion(+), 1 deletion(-)
> > >
> > > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > > index 4f57d27bfa73..7bc084b5bc37 100644
> > > --- a/block/blk-mq.c
> > > +++ b/block/blk-mq.c
> > > @@ -831,7 +831,7 @@ static bool blk_mq_rq_inflight(struct blk_mq_hw_ctx *hctx, struct request *rq,
> > > * If we find a request that is inflight and the queue matches,
> > > * we know the queue is busy. Return false to stop the iteration.
> > > */
> > > - if (rq->state == MQ_RQ_IN_FLIGHT && rq->q == hctx->queue) {
> > > + if (rq->state != MQ_RQ_IDLE && rq->q == hctx->queue) {
> > > bool *busy = priv;
> > >
> > > *busy = true;
> > > --
> > > 2.25.2
> > >
> >
> > I was going to ask if being more explit would be better:
> >
> > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > index 4f57d27bfa73..96816ce57eb1 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -828,10 +828,11 @@ static bool blk_mq_rq_inflight(struct blk_mq_hw_ctx *hctx, struct request *rq,
> > void *priv, bool reserved)
> > {
> > /*
> > - * If we find a request that is inflight and the queue matches,
> > + * If we find a request that is inflight or complete and the queue matches,
> > * we know the queue is busy. Return false to stop the iteration.
> > */
> > - if (rq->state == MQ_RQ_IN_FLIGHT && rq->q == hctx->queue) {
> > + if ((rq->state == MQ_RQ_IN_FLIGHT || rq->state == MQ_RQ_COMPLETE) &&
> > + rq->q == hctx->queue) {
> > bool *busy = priv;
> >
> > *busy = true;
> >
> > But is your patch more forgiving of any future blk-mq states that might
> > also consistitute outstanding work? Seems likely.
>
> I am fine with either way since it is called in slow path.
BTW, another candidate is to use blk_mq_request_started().
Thanks,
Ming
More information about the dm-devel
mailing list