Re: block: neutralize blk_insert_cloned_request IO stall regression (was: Re: [RFC PATCH] blk-mq: fixup RESTART when queue becomes idle)

[Mike Snitzer]

On Tue, Jan 23 2018 at  5:53am -0500,
Ming Lei <ming.lei@xxxxxxxxxx> wrote:

> Hi Mike,
> 
> On Tue, Jan 23, 2018 at 10:22:04AM +0100, Mike Snitzer wrote:
> > On Thu, Jan 18 2018 at  5:20pm -0500,
> > Bart Van Assche <Bart.VanAssche@xxxxxxx> wrote:
> > 
> > > On Thu, 2018-01-18 at 17:01 -0500, Mike Snitzer wrote:
> > > > And yet Laurence cannot reproduce any such lockups with your test...
> > > 
> > > Hmm ... maybe I misunderstood Laurence but I don't think that Laurence has
> > > already succeeded at running an unmodified version of my tests. In one of the
> > > e-mails Laurence sent me this morning I read that he modified these scripts
> > > to get past a kernel module unload failure that was reported while starting
> > > these tests. So the next step is to check which changes were made to the test
> > > scripts and also whether the test results are still valid.
> > > 
> > > > Are you absolutely certain this patch doesn't help you?
> > > > https://patchwork.kernel.org/patch/10174037/
> > > > 
> > > > If it doesn't then that is actually very useful to know.
> > > 
> > > The first I tried this morning is to run the srp-test software against a merge
> > > of Jens' for-next branch and your dm-4.16 branch. Since I noticed that the dm
> > > queue locked up I reinserted a blk_mq_delay_run_hw_queue() call in the dm code.
> > > Since even that was not sufficient I tried to kick the queues via debugfs (for
> > > s in /sys/kernel/debug/block/*/state; do echo kick >$s; done). Since that was
> > > not sufficient to resolve the queue stall I reverted the following tree patches
> > > that are in Jens' tree:
> > > * "blk-mq: improve DM's blk-mq IO merging via blk_insert_cloned_request feedback"
> > > * "blk-mq-sched: remove unused 'can_block' arg from blk_mq_sched_insert_request"
> > > * "blk-mq: don't dispatch request in blk_mq_request_direct_issue if queue is busy"
> > > 
> > > Only after I had done this the srp-test software ran again without triggering
> > > dm queue lockups.
> > 
> > Given that Ming's notifier-based patchset needs more development time I
> > think we're unfortunately past the point where we can comfortably wait
> > for that to be ready.
> > 
> > So we need to explore alternatives to fixing this IO stall regression.
> 
> The fix for IO stall doesn't need the notifier-based patchset, and only
> the 1st patch is enough for fixing the IO stall. And it is a generic
> issue, which need generic solution, that is the conclusion made by
> Jens and me.
> 
> 	https://marc.info/?l=linux-kernel&m=151638176727612&w=2

That's fine if Bart verifies it.

> And the notifier-based patchset is for solving the performance issue
> reported by Jens:
> 
> 	- run IO on dm-mpath
> 	- run background IO on low depth underlying queue
> 	- then IO performance on dm-mpath is extremely slow
> 
> I will send out the 1st patch of 'blk-mq: introduce BLK_STS_DEV_RESOURCE'
> soon, but the notifier-based patchset shouldn't be very urgent, since
> the above test case isn't usual in reality.
> 
> > Rather than attempt the above block reverts (which is an incomplete
> > listing given newer changes): might we develop a more targeted code
> > change to neutralize commit 396eaf21ee ("blk-mq: improve DM's blk-mq IO
> > merging via blk_insert_cloned_request feedback")? -- which, given Bart's
> > findings above, seems to be the most problematic block commit.
> 
> The stall isn't related with commit 396eaf21ee too.
> 
> > 
> > To that end, assuming I drop this commit from dm-4.16:
> > https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.16&id=316a795ad388e0c3ca613454851a28079d917a92
> > 
> > Here is my proposal for putting this regression behind us for 4.16
> > (Ming's line of development would continue and hopefully be included in
> > 4.17):
> 
> Actually notifier based approach is ready, even cache for clone is ready
> too, but the test result isn't good enough on random IO on Jens's above
> case, and sequential IO is much better with both cache clone and
> notifier based allocation(much better than non-mq). And follows the tree
> if anyone is interested:
> 
> https://github.com/ming1/linux/commits/v4.15-rc-block-dm-mpath
> 
> Now looks there is still one issue: the notifier can come early, just
> before the request is added to hctx->dispatch_list, and performance
> still gets hurt, especially on random IO in Jens's case. But queue
> won't stall, :-)
> 
> > 
> > From: Mike Snitzer <snitzer@xxxxxxxxxx>
> > Date: Tue, 23 Jan 2018 09:40:22 +0100
> > Subject: [PATCH] block: neutralize blk_insert_cloned_request IO stall regression
> > 
> > The series of blk-mq changes intended to improve sequential IO
> > performace (through improved merging with dm-mapth blk-mq stacked on
> > underlying blk-mq device).  Unfortunately these changes have caused
> > dm-mpath blk-mq IO stalls when blk_mq_request_issue_directly()'s call to
> > q->mq_ops->queue_rq() fails (due to device-specific resource
> > unavailability).
> > 
> > Fix this by reverting back to how blk_insert_cloned_request() functioned
> > prior to commit 396eaf21ee -- by using blk_mq_request_bypass_insert()
> > instead of blk_mq_request_issue_directly().
> > 
> > In the future, this commit should be reverted as the first change in a
> > followup series of changes that implements a comprehensive solution to
> > allowing an underlying blk-mq queue's resource limitation to trigger the
> > upper blk-mq queue to run once that underlying limited resource is
> > replenished.
> > 
> > Fixes: 396eaf21ee ("blk-mq: improve DM's blk-mq IO merging via blk_insert_cloned_request feedback")
> > Signed-off-by: Mike Snitzer <snitzer@xxxxxxxxxx>
> > ---
> >  block/blk-core.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> > 
> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index cdae69be68e9..a224f282b4a6 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -2520,7 +2520,8 @@ blk_status_t blk_insert_cloned_request(struct request_queue *q, struct request *
> >  		 * bypass a potential scheduler on the bottom device for
> >  		 * insert.
> >  		 */
> > -		return blk_mq_request_issue_directly(rq);
> > +		blk_mq_request_bypass_insert(rq, true);
> > +		return BLK_STS_OK;
> >  	}
> 
> If this patch is for fixing IO stall on DM, it isn't needed, and actually
> it can't fix the IO stall issue.

It should "fix it" in conjunction with this:

https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-4.16&id=87b7e73546b55f4a3a37d4726daedd9a17a20509

(Bart already said as much, I just effectively enabled the equivalent of
his reverts)