Re: [PATCH] blk-mq: fix io hung due to missing commit_rqs while scheduler is none

[Yu Kuai]

Hi, Ming

在 2022/07/26 10:48, Ming Lei 写道:
> On Tue, Jul 26, 2022 at 10:38:52AM +0800, Yu Kuai wrote:
> > From: Yu Kuai <yukuai3@xxxxxxxxxx>
> >
> > Currently, in virtio_scsi, if 'bd->last' is not set to true while
> > dispatching request, such io will stay in driver's queue, and driver
> > will wait for block layer to dispatch more rqs. However, if block
> > layer failed to dispatch more rq, it should trigger commit_rqs to
> > inform driver.
> >
> > There is a problem in blk_mq_try_issue_list_directly() that commit_rqs
> > won't be called:
> >
> > // assume that queue_depth is set to 1, list contains two rq
> > blk_mq_try_issue_list_directly
> >   blk_mq_request_issue_directly
> >   // dispatch first rq
> >   // last is false
> >    __blk_mq_try_issue_directly
> >     blk_mq_get_dispatch_budget
> >     // succeed to get first budget
> >     __blk_mq_issue_directly
> >      scsi_queue_rq
> >       cmd->flags |= SCMD_LAST
> >        virtscsi_queuecommand
> >         kick = (sc->flags & SCMD_LAST) != 0
> >         // kick is false, first rq won't issue to disk
> >   queued++
> >
> >   blk_mq_request_issue_directly
> >   // dispatch second rq
> >    __blk_mq_try_issue_directly
> >     blk_mq_get_dispatch_budget
> >     // failed to get second budget
> >   ret == BLK_STS_RESOURCE
> >    blk_mq_request_bypass_insert
> >   // errors is still 0
> >
> >   if (!list_empty(list) || errors && ...)
> >    // won't pass, commit_rqs won't be called
> >
> > In this situation, first rq relied on second rq to dispatch, while
> > second rq relied on first rq to complete, thus they will both hung.
> >
> > Fix the problem by also treat 'BLK_STS_RESOURCE' and
> > 'BLK_STS_DEV_RESOURCE' as 'errors' in blk_mq_try_issue_list_directly(),
> > so that 'commit_rqs' will be called when dispatch of last rq failed.
> >
> > Fixes: d666ba98f849 ("blk-mq: add mq_ops->commit_rqs()")
> > Signed-off-by: Yu Kuai <yukuai3@xxxxxxxxxx>
> > ---
> >   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 70177ee74295..752b0fe4c128 100644
> > --- a/block/blk-mq.c
> > +++ b/block/blk-mq.c
> > @@ -2680,6 +2680,7 @@ void blk_mq_try_issue_list_directly(struct blk_mq_hw_ctx *hctx,
> >   		list_del_init(&rq->queuelist);
> >   		ret = blk_mq_request_issue_directly(rq, list_empty(list));
> >   		if (ret != BLK_STS_OK) {
> > +			errors++;
>
> OK, it is because that list becomes empty, but .queue_rq(last_rq_in_list)
> returns BLK_STS_*RESOURCE, but scsi can't call ->queuecommand() for this
> real last rq. Then blk_mq_try_issue_list_directly() doesn't call
> ->commit_rqs() too.
>
> Here errors means that request not queued successfully, so this patch
> looks fine.
>
> Also I think blk_mq_dispatch_rq_list() needs similar handling too,
> right?

Yes, you're right, I'll handle blk_mq_dispatch_rq_list() in v2, and also
change the title.

Thanks,
Kuai