Re: commit 01e99aeca397 causes longer runtime of block/004

[Ming Lei <ming.lei@redhat.com>]

On Sat, Mar 07, 2020 at 01:02:23AM +0000, Shinichiro Kawasaki wrote:
> On Mar 06, 2020 / 16:13, Ming Lei wrote:
> > On Fri, Mar 06, 2020 at 06:06:23AM +0000, Shinichiro Kawasaki wrote:
> > > On Mar 05, 2020 / 10:48, Ming Lei wrote:
> > > > Hi Shinichiro,
> > > > 
> > > > On Thu, Mar 05, 2020 at 01:19:02AM +0000, Shinichiro Kawasaki wrote:
> > > > > On Mar 04, 2020 / 17:53, Ming Lei wrote:
> > > > > > On Wed, Mar 04, 2020 at 06:11:37AM +0000, Shinichiro Kawasaki wrote:
> > > > > > > On Mar 04, 2020 / 11:46, Ming Lei wrote:
> > > > > > > > On Wed, Mar 04, 2020 at 02:38:43AM +0000, Shinichiro Kawasaki wrote:
> > > > > > > > > I noticed that blktests block/004 takes longer runtime with 5.6-rc4 than
> > > > > > > > > 5.6-rc3, and found that the commit 01e99aeca397 ("blk-mq: insert passthrough
> > > > > > > > > request into hctx->dispatch directly") triggers it.
> > > > > > > > > 
> > > > > > > > > The longer runtime was observed with dm-linear device which maps SATA SMR HDD
> > > > > > > > > connected via AHCI. It was not observed with dm-linear on SAS/SATA SMR HDDs
> > > > > > > > > connected via SAS-HBA. Not observed with dm-linear on non-SMR HDDs either.
> > > > > > > > > 
> > > > > > > > > Before the commit, block/004 took around 130 seconds. After the commit, it takes
> > > > > > > > > around 300 seconds. I need to dig in further details to understand why the
> > > > > > > > > commit makes the test case longer.
> > > > > > > > > 
> > > > > > > > > The test case block/004 does "flush intensive workload". Is this longer runtime
> > > > > > > > > expected?
> > > > > > > > 
> > > > > > > > The following patch might address this issue:
> > > > > > > > 
> > > > > > > > https://lore.kernel.org/linux-block/20200207190416.99928-1-sqazi@google.com/#t
> > > > > > > > 
> > > > > > > > Please test and provide us the result.
> > > > > > > > 
> > > > > > > > thanks,
> > > > > > > > Ming
> > > > > > > >
> > > > > > > 
> > > > > > > Hi Ming,
> > > > > > > 
> > > > > > > I applied the patch to 5.6-rc4 but I observed the longer runtime of block/004.
> > > > > > > Still it takes around 300 seconds.
> > > > > > 
> > > > > > Hello Shinichiro,
> > > > > > 
> > > > > > block/004 only sends 1564 sync randwrite, and seems 130s has been slow
> > > > > > enough.
> > > > > > 
> > > > > > There are two related effect in that commit for your issue:
> > > > > > 
> > > > > > 1) 'at_head' is applied in blk_mq_sched_insert_request() for flush
> > > > > > request
> > > > > > 
> > > > > > 2) all IO is added back to tail of hctx->dispatch after .queue_rq()
> > > > > > returns STS_RESOURCE
> > > > > > 
> > > > > > Seems it is more related with 2) given you can't reproduce the issue on 
> > > > > > SAS.
> > > > > > 
> > > > > > So please test the following two patches, and see which one makes a
> > > > > > difference for you.
> > > > > > 
> > > > > > BTW, both two looks not reasonable, just for narrowing down the issue.
> > > > > > 
> > > > > > 1) patch 1
> > > > > > 
> > > > > > diff --git a/block/blk-mq-sched.c b/block/blk-mq-sched.c
> > > > > > index 856356b1619e..86137c75283c 100644
> > > > > > --- a/block/blk-mq-sched.c
> > > > > > +++ b/block/blk-mq-sched.c
> > > > > > @@ -398,7 +398,7 @@ void blk_mq_sched_insert_request(struct request *rq, bool at_head,
> > > > > >  	WARN_ON(e && (rq->tag != -1));
> > > > > >  
> > > > > >  	if (blk_mq_sched_bypass_insert(hctx, !!e, rq)) {
> > > > > > -		blk_mq_request_bypass_insert(rq, at_head, false);
> > > > > > +		blk_mq_request_bypass_insert(rq, true, false);
> > > > > >  		goto run;
> > > > > >  	}
> > > > > 
> > > > > Ming, thank you for the trial patches.
> > > > > This "patch 1" reduced the runtime, as short as rc3.
> > > > > 
> > > > > > 
> > > > > > 
> > > > > > 2) patch 2
> > > > > > diff --git a/block/blk-mq.c b/block/blk-mq.c
> > > > > > index d92088dec6c3..447d5cb39832 100644
> > > > > > --- a/block/blk-mq.c
> > > > > > +++ b/block/blk-mq.c
> > > > > > @@ -1286,7 +1286,7 @@ bool blk_mq_dispatch_rq_list(struct request_queue *q, struct list_head *list,
> > > > > >  			q->mq_ops->commit_rqs(hctx);
> > > > > >  
> > > > > >  		spin_lock(&hctx->lock);
> > > > > > -		list_splice_tail_init(list, &hctx->dispatch);
> > > > > > +		list_splice_init(list, &hctx->dispatch);
> > > > > >  		spin_unlock(&hctx->lock);
> > > > > >  
> > > > > >  		/*
> > > > > 
> > > > > This patch 2 didn't reduce the runtime.
> > > > > 
> > > > > Wish this report helps.
> > > > 
> > > > Your feedback does help, then please test the following patch:
> > > 
> > > Hi Ming, thank you for the patch. I applied it on top of rc4 and confirmed
> > > it reduces the runtime as short as rc3. Good.
> > 
> > Hi Shinichiro,
> > 
> > Thanks for your test!
> > 
> > Then I think the following change should make the difference actually,
> > you may double check that and confirm if it is that.
> > 
> > > @@ -334,7 +334,7 @@ static void blk_kick_flush(struct request_queue *q, struct blk_flush_queue *fq,
> > >  	flush_rq->rq_disk = first_rq->rq_disk;
> > >  	flush_rq->end_io = flush_end_io;
> > >  
> > > -	blk_flush_queue_rq(flush_rq, false);
> > > +	blk_flush_queue_rq(flush_rq, true);
> 
> Yes, with this the one line change above only, the runtime was reduced.
> 
> > 
> > However, the flush request is added to tail of dispatch queue[1] for long time.
> > 0cacba6cf825 ("blk-mq-sched: bypass the scheduler for flushes entirely")
> > and its predecessor(all mq scheduler start patches) changed to add flush request
> > to front of dispatch queue for blk-mq by ignoring 'add_queue' parameter of
> > blk_mq_sched_insert_flush(). That change may be by accident, and not sure it is
> > correct.
> > 
> > I guess once flush rq is added to tail of dispatch queue in block/004,
> > in which lots of FS request may stay in hctx->dispatch because of low
> > AHCI queue depth, then we may take a bit long for flush rq to be
> > submitted to LLD.
> > 
> > I'd suggest to root cause/understand the issue given it isn't obvious
> > correct to queue flush rq at front of dispatch queue, so could you collect
> > the following trace via the following script with/without the single line
> > patch?
> 
> Thank you for the thoughts for the correct design. I have taken the two traces,
> with and without the one liner patch above. The gzip archived trace files have
> 1.6MB size. It looks too large to post to the list. Please let me know how you
> want the trace files shared.

I didn't thought the trace can be so big given the ios should be just
256 * 64(1564).

You may put the log somewhere in Internet, cloud storage, web, or
whatever. Then just provides us the link.

Or if you can't find a place to hold it, just send to me, and I will put
it in my RH people web link.


Thank, 
Ming