Re: dm-rq: do not update rq partially in each ending bio

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

On Fri, Aug 25, 2017 at 11:48:39AM -0400, Mike Snitzer wrote:
> On Fri, Aug 25 2017 at 11:27am -0400,
> Ming Lei <ming.lei@redhat.com> wrote:
> 
> > We don't need to update orignal dm request partially
> > when ending each cloned bio, and this patch just
> > updates orignal dm request once when the whole
> > cloned request is finished.
> > 
> > Partial request update can be a bit expensive, so
> > we should try to avoid it, especially it is run
> > in softirq context.
> > 
> > After this patch is applied, both hard lockup and
> > soft lockup aren't reproduced any more in one hour
> > of running Laurence's test[1] on IB/SRP. Without
> > this patch, the lockup can be reproduced in several
> > minutes.
> > 
> > BTW, after d4acf3650c7c(block: Make blk_mq_delay_kick_requeue_list()
> > rerun the queue at a quiet time), we need to make the
> > test more aggressive for reproducing the lockup:
> > 
> > 	1) run hammer_write.sh 32 or 64 concurrently.
> > 	2) write 8M each time
> > 
> > [1] https://marc.info/?l=linux-block&m=150220185510245&w=2
> 
> Bart said he cannot reproduce the lockups with his patchset applied.
> Have you tested using Bart's patchset?

d4acf3650c7c(block: Make blk_mq_delay_kick_requeue_list() rerun the
queue at a quiet time) has been in linus tree.

For other patches, I didn't test it yet. Because every time
when the lockup is triggered, it is always in blk_recalc_rq_segments(),
and not see any patch is dealing with that.

> 
> Comments inlined below.
> 
> > ---
> >  drivers/md/dm-rq.c | 15 +++++----------
> >  drivers/md/dm-rq.h |  1 +
> >  2 files changed, 6 insertions(+), 10 deletions(-)
> > 
> > diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
> > index c6ebc5b1e00e..50cd96c7de45 100644
> > --- a/drivers/md/dm-rq.c
> > +++ b/drivers/md/dm-rq.c
> > @@ -117,9 +117,9 @@ static void end_clone_bio(struct bio *clone)
> >  	struct dm_rq_clone_bio_info *info =
> >  		container_of(clone, struct dm_rq_clone_bio_info, clone);
> >  	struct dm_rq_target_io *tio = info->tio;
> > -	struct bio *bio = info->orig;
> >  	unsigned int nr_bytes = info->orig->bi_iter.bi_size;
> >  	blk_status_t error = clone->bi_status;
> > +	bool is_last = !clone->bi_next;
> >  
> >  	bio_put(clone);
> >  
> > @@ -144,21 +144,15 @@ static void end_clone_bio(struct bio *clone)
> >  	 * I/O for the bio successfully completed.
> >  	 * Notice the data completion to the upper layer.
> >  	 */
> > -
> > -	/*
> > -	 * bios are processed from the head of the list.
> > -	 * So the completing bio should always be rq->bio.
> > -	 * If it's not, something wrong is happening.
> > -	 */
> > -	if (tio->orig->bio != bio)
> > -		DMERR("bio completion is going in the middle of the request");
> 
> Why did you remove this check?

The check isn't valid any more because this patch only update
original dm rq in .end_bio() of the last cloned bio.

> 
> > +	tio->completed += nr_bytes;
> >  
> >  	/*
> >  	 * Update the original request.
> >  	 * Do not use blk_end_request() here, because it may complete
> >  	 * the original request before the clone, and break the ordering.
> >  	 */
> > -	blk_update_request(tio->orig, BLK_STS_OK, nr_bytes);
> > +	if (is_last)
> > +		blk_update_request(tio->orig, BLK_STS_OK, tio->completed);
> >  }
> 
> Partial completion support is important given the potential for path
> failures interrupting requests.  Why do you think it is OK to avoid it
> by switching to an all or nothing approach?

If the cloned rq is partially completed, this dm rq is still partially
completed. This patch only update dm rq in the last cloned bio's
.end_io().

Also if one middle cloned bio is completed with error, the current
implementation doesn't update dm rq any more from that bio, so
looks the following patch is consistent with current
implementation, what do you think of it?

>From 2b5ef7e64ba8595c0c58790d14fd77b10a28afee Mon Sep 17 00:00:00 2001
From: Ming Lei <ming.lei@redhat.com>
Date: Thu, 24 Aug 2017 20:19:52 +0800
Subject: [PATCH] dm-rq: do not update rq partially in each ending bio

We don't need to update orignal dm request partially
when ending each cloned bio, and this patch just
updates orignal dm request once when the whole
cloned request is finished.

Partial request update can be a bit expensive, so
we should try to avoid it, especially it is run
in softirq context.

After this patch is applied, both hard lockup and
soft lockup aren't reproduced any more in one hour
of running Laurence's test[1] on IB/SRP. Without
this patch, the lockup can be reproduced in several
minutes.

BTW, after d4acf3650c7c(block: Make blk_mq_delay_kick_requeue_list()
rerun the queue at a quiet time), we need to make the
test more aggressive for reproducing the lockup:

	1) run hammer_write.sh 32 or 64 concurrently.
	2) write 8M each time

[1] https://marc.info/?l=linux-block&m=150220185510245&w=2

Cc: Laurence Oberman <loberman@redhat.com>
Cc: Bart Van Assche <Bart.VanAssche@wdc.com>
Cc: "dm-devel@redhat.com" <dm-devel@redhat.com>
Cc: Mike Snitzer <snitzer@redhat.com>
Cc: Alasdair Kergon <agk@redhat.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
---
 drivers/md/dm-rq.c | 18 +++++++-----------
 drivers/md/dm-rq.h |  1 +
 2 files changed, 8 insertions(+), 11 deletions(-)

diff --git a/drivers/md/dm-rq.c b/drivers/md/dm-rq.c
index c6ebc5b1e00e..72396b0ce485 100644
--- a/drivers/md/dm-rq.c
+++ b/drivers/md/dm-rq.c
@@ -117,9 +117,9 @@ static void end_clone_bio(struct bio *clone)
 	struct dm_rq_clone_bio_info *info =
 		container_of(clone, struct dm_rq_clone_bio_info, clone);
 	struct dm_rq_target_io *tio = info->tio;
-	struct bio *bio = info->orig;
 	unsigned int nr_bytes = info->orig->bi_iter.bi_size;
 	blk_status_t error = clone->bi_status;
+	bool is_last = !clone->bi_next;
 
 	bio_put(clone);
 
@@ -137,28 +137,23 @@ static void end_clone_bio(struct bio *clone)
 		 * when the request is completed.
 		 */
 		tio->error = error;
-		return;
+		goto exit;
 	}
 
 	/*
 	 * I/O for the bio successfully completed.
 	 * Notice the data completion to the upper layer.
 	 */
-
-	/*
-	 * bios are processed from the head of the list.
-	 * So the completing bio should always be rq->bio.
-	 * If it's not, something wrong is happening.
-	 */
-	if (tio->orig->bio != bio)
-		DMERR("bio completion is going in the middle of the request");
+	tio->completed += nr_bytes;
 
 	/*
 	 * Update the original request.
 	 * Do not use blk_end_request() here, because it may complete
 	 * the original request before the clone, and break the ordering.
 	 */
-	blk_update_request(tio->orig, BLK_STS_OK, nr_bytes);
+	if (is_last)
+ exit:
+		blk_update_request(tio->orig, BLK_STS_OK, tio->completed);
 }
 
 static struct dm_rq_target_io *tio_from_request(struct request *rq)
@@ -455,6 +450,7 @@ static void init_tio(struct dm_rq_target_io *tio, struct request *rq,
 	tio->clone = NULL;
 	tio->orig = rq;
 	tio->error = 0;
+	tio->completed = 0;
 	/*
 	 * Avoid initializing info for blk-mq; it passes
 	 * target-specific data through info.ptr
diff --git a/drivers/md/dm-rq.h b/drivers/md/dm-rq.h
index 9813922e4fe5..f43c45460aac 100644
--- a/drivers/md/dm-rq.h
+++ b/drivers/md/dm-rq.h
@@ -29,6 +29,7 @@ struct dm_rq_target_io {
 	struct dm_stats_aux stats_aux;
 	unsigned long duration_jiffies;
 	unsigned n_sectors;
+	unsigned completed;
 };
 
 /*
-- 
2.9.5