2023-04-13 06:31:44

by Chengming Zhou

Subject: [PATCH v2 1/2] blk-stat: fix QUEUE_FLAG_STATS clear

From: Chengming Zhou <[email protected]>

We need to set QUEUE_FLAG_STATS for two cases:
1. blk_stat_enable_accounting()
2. blk_stat_add_callback()

So we should clear it only when ((q->stats->accounting == 0) &&
list_empty(&q->stats->callbacks)).

blk_stat_disable_accounting() only check if q->stats->accounting
is 0 before clear the flag, this patch fix it.

Also add list_empty(&q->stats->callbacks)) check when enable, or
the flag is already set.

The bug can be reproduced on kernel without BLK_DEV_THROTTLING
(since it unconditionally enable accounting, see the next patch).

# cat /sys/block/sr0/queue/scheduler
none mq-deadline [bfq]

# cat /sys/kernel/debug/block/sr0/state
SAME_COMP|IO_STAT|INIT_DONE|STATS|REGISTERED|NOWAIT|30

# echo none > /sys/block/sr0/queue/scheduler

# cat /sys/kernel/debug/block/sr0/state
SAME_COMP|IO_STAT|INIT_DONE|REGISTERED|NOWAIT

# cat /sys/block/sr0/queue/wbt_lat_usec
75000

We can see that after changing elevator from "bfq" to "none",
"STATS" flag is lost even though WBT callback still need it.

Fixes: 68497092bde9 ("block: make queue stat accounting a reference")
Cc: <[email protected]> # v5.17+
Signed-off-by: Chengming Zhou <[email protected]>
Acked-by: Tejun Heo <[email protected]>
---
block/blk-stat.c | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/block/blk-stat.c b/block/blk-stat.c
index 74a1a8c32d86..bc7e0ed81642 100644
--- a/block/blk-stat.c
+++ b/block/blk-stat.c
@@ -190,7 +190,7 @@ void blk_stat_disable_accounting(struct request_queue *q)
unsigned long flags;

spin_lock_irqsave(&q->stats->lock, flags);
- if (!--q->stats->accounting)
+ if (!--q->stats->accounting && list_empty(&q->stats->callbacks))
blk_queue_flag_clear(QUEUE_FLAG_STATS, q);
spin_unlock_irqrestore(&q->stats->lock, flags);
}
@@ -201,7 +201,7 @@ void blk_stat_enable_accounting(struct request_queue *q)
unsigned long flags;

spin_lock_irqsave(&q->stats->lock, flags);
- if (!q->stats->accounting++)
+ if (!q->stats->accounting++ && list_empty(&q->stats->callbacks))
blk_queue_flag_set(QUEUE_FLAG_STATS, q);
spin_unlock_irqrestore(&q->stats->lock, flags);
}
--
2.39.2

2023-04-13 06:41:41

by Chengming Zhou

Subject: [PATCH v2 2/2] blk-throttle: only enable blk-stat when BLK_DEV_THROTTLING_LOW

From: Chengming Zhou <[email protected]>

blk_throtl_register() will unconditionally enable blk-stat for gendisk
when register, even when we have no BLK_DEV_THROTTLING_LOW config.

Since the kernel always has only BLK_DEV_THROTTLING config and the
BLK_DEV_THROTTLING_LOW config is still in EXPERIMENTAL state, we can
just skip blk-stat when !BLK_DEV_THROTTLING_LOW.

Signed-off-by: Chengming Zhou <[email protected]>
Acked-by: Tejun Heo <[email protected]>
---
block/blk-throttle.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/block/blk-throttle.c b/block/blk-throttle.c
index 47e9d8be68f3..3c07c9cfa70a 100644
--- a/block/blk-throttle.c
+++ b/block/blk-throttle.c
@@ -2439,11 +2439,12 @@ void blk_throtl_register(struct gendisk *disk)
#ifndef CONFIG_BLK_DEV_THROTTLING_LOW
/* if no low limit, use previous default */
td->throtl_slice = DFL_THROTL_SLICE_HD;
-#endif

+#else
td->track_bio_latency = !queue_is_mq(q);
if (!td->track_bio_latency)
blk_stat_enable_accounting(q);
+#endif
}

#ifdef CONFIG_BLK_DEV_THROTTLING_LOW
--
2.39.2

2023-04-13 12:51:50

by Jens Axboe

Subject: Re: [PATCH v2 1/2] blk-stat: fix QUEUE_FLAG_STATS clear

On Thu, 13 Apr 2023 14:28:04 +0800, [email protected] wrote:
> We need to set QUEUE_FLAG_STATS for two cases:
> 1. blk_stat_enable_accounting()
> 2. blk_stat_add_callback()
>
> So we should clear it only when ((q->stats->accounting == 0) &&
> list_empty(&q->stats->callbacks)).
>
> [...]

Applied, thanks!

[1/2] blk-stat: fix QUEUE_FLAG_STATS clear
commit: 20de765f6d9da0c47b756429c60b41063b990a10
[2/2] blk-throttle: only enable blk-stat when BLK_DEV_THROTTLING_LOW
commit: 8e15dfbd9ae21e518979e3823e335073e725f445

Best regards,
--
Jens Axboe