Re: [PATCH BUGFIX RFC 0/2] reverting two commits causing freezes

From: Jens Axboe
Date: Fri Jan 18 2019 - 08:35:35 EST

On 1/18/19 4:52 AM, Paolo Valente wrote:
> Hi Jens,
> a user reported a warning, followed by freezes, in case he increases
> nr_requests to more than 64 [1]. After reproducing the issues, I
> reverted the commit f0635b8a416e ("bfq: calculate shallow depths at
> init time"), plus the related commit bd7d4ef6a4c9 ("bfq-iosched:
> remove unused variable"). The problem went away.

For reverts, please put the justification into the actual revert
commit. With this series, if applied as-is, we'd have two patches
in the tree that just says "revert X" without any hint as to why
that was done.

> Maybe the assumption in commit f0635b8a416e ("bfq: calculate shallow
> depths at init time") does not hold true?

It apparently doesn't! But let's try and figure this out instead of
blindly reverting it. OK, I think I see it. For the sched_tags
case, when we grow the requests, we allocate a new set. Hence any
old cache would be stale at that point.

How about something like this? It still keeps the code of having
to update this out of the hot IO path, and only calls it when we
actually change the depths.

Totally untested...

diff --git a/block/bfq-iosched.c b/block/bfq-iosched.c
index cd307767a134..b09589915667 100644
--- a/block/bfq-iosched.c
+++ b/block/bfq-iosched.c
@@ -5342,7 +5342,7 @@ static unsigned int bfq_update_depths(struct bfq_data *bfqd,
return min_shallow;

-static int bfq_init_hctx(struct blk_mq_hw_ctx *hctx, unsigned int index)
+static void bfq_depth_updated(struct blk_mq_hw_ctx *hctx)
struct bfq_data *bfqd = hctx->queue->elevator->elevator_data;
struct blk_mq_tags *tags = hctx->sched_tags;
@@ -5350,6 +5350,11 @@ static int bfq_init_hctx(struct blk_mq_hw_ctx *hctx, unsigned int index)

min_shallow = bfq_update_depths(bfqd, &tags->bitmap_tags);
sbitmap_queue_min_shallow_depth(&tags->bitmap_tags, min_shallow);
+static int bfq_init_hctx(struct blk_mq_hw_ctx *hctx, unsigned int index)
+ bfq_depth_updated(hctx);
return 0;

@@ -5772,6 +5777,7 @@ static struct elevator_type iosched_bfq_mq = {
.requests_merged = bfq_requests_merged,
.request_merged = bfq_request_merged,
.has_work = bfq_has_work,
+ .depth_updated = bfq_depth_updated,
.init_hctx = bfq_init_hctx,
.init_sched = bfq_init_queue,
.exit_sched = bfq_exit_queue,
diff --git a/block/blk-mq.c b/block/blk-mq.c
index 3ba37b9e15e9..a047b297ade5 100644
--- a/block/blk-mq.c
+++ b/block/blk-mq.c
@@ -3101,6 +3101,8 @@ int blk_mq_update_nr_requests(struct request_queue *q, unsigned int nr)
if (ret)
+ if (q->elevator && q->elevator->type->ops.depth_updated)
+ q->elevator->type->ops.depth_updated(hctx);

if (!ret)
diff --git a/include/linux/elevator.h b/include/linux/elevator.h
index 2e9e2763bf47..6e8bc53740f0 100644
--- a/include/linux/elevator.h
+++ b/include/linux/elevator.h
@@ -31,6 +31,7 @@ struct elevator_mq_ops {
void (*exit_sched)(struct elevator_queue *);
int (*init_hctx)(struct blk_mq_hw_ctx *, unsigned int);
void (*exit_hctx)(struct blk_mq_hw_ctx *, unsigned int);
+ void (*depth_updated)(struct blk_mq_hw_ctx *);

bool (*allow_merge)(struct request_queue *, struct request *, struct bio *);
bool (*bio_merge)(struct blk_mq_hw_ctx *, struct bio *);

Jens Axboe