Re: stalling IO regression since linux 5.12, through 5.18

[Jan Kara <jack@suse.cz>]

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
?

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
-- 
Jan Kara <jack@suse.com>
SUSE Labs, CR