Re: stalling IO regression since linux 5.12, through 5.18

[Yu Kuai]

在 2022/09/01 16:03, Jan Kara 写道:
> On Thu 01-09-22 15:02:03, Yu Kuai wrote:
> > Hi, Chris
> >
> > 在 2022/08/20 15:00, Ming Lei 写道:
> > > On Fri, Aug 19, 2022 at 03:20:25PM -0400, Chris Murphy wrote:
> > > > On Thu, Aug 18, 2022, at 1:24 AM, Ming Lei wrote:
> > > > > On Thu, Aug 18, 2022 at 12:27:04AM -0400, Chris Murphy wrote:
> > > > > > On Thu, Aug 18, 2022, at 12:18 AM, Chris Murphy wrote:
> > > > > > > On Thu, Aug 18, 2022, at 12:12 AM, Chris Murphy wrote:
> > > > > > > > On Wed, Aug 17, 2022, at 11:41 PM, Ming Lei wrote:
> > > > > > > >
> > > > > > > > > OK, can you post the blk-mq debugfs log after you trigger it on v5.17?
> > > > > >
> > > > > > Same boot, 3rd log. But the load is above 300 so I kinda need to sysrq+b soon.
> > > > > >
> > > > > > https://drive.google.com/file/d/1375H558kqPTdng439rvG6LuXXWPXLToo/view?usp=sharing
> > > > >
> > > > > Also please test the following one too:
> > > > >
> > > > >
> > > > > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > > > > index 5ee62b95f3e5..d01c64be08e2 100644
> > > > > --- a/block/blk-mq.c
> > > > > +++ b/block/blk-mq.c
> > > > > @@ -1991,7 +1991,8 @@ bool blk_mq_dispatch_rq_list(struct blk_mq_hw_ctx
> > > > > *hctx, struct list_head *list,
> > > > >    		if (!needs_restart ||
> > > > >    		    (no_tag && list_empty_careful(&hctx->dispatch_wait.entry)))
> > > > >    			blk_mq_run_hw_queue(hctx, true);
> > > > > -		else if (needs_restart && needs_resource)
> > > > > +		else if (needs_restart && (needs_resource ||
> > > > > +					blk_mq_is_shared_tags(hctx->flags)))
> > > > >    			blk_mq_delay_run_hw_queue(hctx, BLK_MQ_RESOURCE_DELAY);
> > > > >
> > > > >    		blk_mq_update_dispatch_busy(hctx, true);
> > > >
> > > >
> > > > With just this patch on top of 5.17.0, it still hangs. I've captured block debugfs log:
> > > > https://drive.google.com/file/d/1ic4YHxoL9RrCdy_5FNdGfh_q_J3d_Ft0/view?usp=sharing
> > >
> > > The log is similar with before, and the only difference is RESTART not
> > > set.
> > >
> > > Also follows another patch merged to v5.18 and it fixes io stall too, feel free to test it:
> > >
> > > 8f5fea65b06d blk-mq: avoid extending delays of active hctx from blk_mq_delay_run_hw_queues
> >
> > Have you tried this patch?
> >
> > We meet a similar problem in our test, and I'm pretty sure about the
> > situation at the scene,
> >
> > Our test environment：nvme with bfq ioscheduler,
> >
> > How io is stalled:
> >
> > 1. hctx1 dispatch rq from bfq in service queue, bfqq becomes empty,
> > dispatch somehow fails and rq is inserted to hctx1->dispatch, new run
> > work is queued.
> >
> > 2. other hctx tries to dispatch rq, however, in service bfqq is
> > empty, bfq_dispatch_request return NULL, thus
> > blk_mq_delay_run_hw_queues is called.
> >
> > 3. for the problem described in above patch，run work from "hctx1"
> > can be stalled.
> >
> > Above patch should fix this io stall, however, it seems to me bfq do
> > have some problems that in service bfqq doesn't expire under following
> > situation:
> >
> > 1. dispatched rqs don't complete
> > 2. no new rq is issued to bfq
>
> And I guess:
> 3. there are requests queued in other bfqqs
> ?

Yes, of course, other bfqqs still have requests, but current
implementation have flaws that even if other bfqqs doesn't have
requests, bfq_asymmetric_scenario() can still return true because
num_groups_with_pending_reqs > 0. We tried to fix this, however, there
seems to be some misunderstanding with Paolo, and it's not applied to
mainline yet...

Thanks,
Kuai
> Otherwise I don't see a point in expiring current bfqq because there's
> nothing bfq could do anyway. But under normal circumstances the request
> completion should not take so long so I don't think it would be really
> worth it to implement some special mechanism for this in bfq.
>
> 								Honza