Recently, we encountered the following hungtask:
INFO: task kworker/11:2:2981147 blocked for more than 6266 seconds "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. kworker/11:2 D 0 2981147 2 0x80004000 Workqueue: cgroup_destroy css_free_rwork_fn Call Trace: __schedule+0x934/0xe10 schedule+0x40/0xb0 wb_wait_for_completion+0x52/0x80 ? finish_wait+0x80/0x80 mem_cgroup_css_free+0x3a/0x1b0 css_free_rwork_fn+0x42/0x380 process_one_work+0x1a2/0x360 worker_thread+0x30/0x390 ? create_worker+0x1a0/0x1a0 kthread+0x110/0x130 ? __kthread_cancel_work+0x40/0x40 ret_from_fork+0x1f/0x30
This is because the writeback thread has been continuously and repeatedly throttled by wbt, but at the same time, the writes of another thread proceed quite smoothly. After debugging, I believe it is caused by the following reasons.
When thread A is blocked by wbt, the I/O issued by thread B will use a deeper queue depth(rwb->rq_depth.max_depth) because it meets the conditions of wb_recent_wait(), thus allowing thread B's I/O to be issued smoothly and resulting in the inflight I/O of wbt remaining relatively high.
However, when I/O completes, due to the high inflight I/O of wbt, the condition "limit - inflight >= rwb->wb_background / 2" in wbt_rqw_done() cannot be satisfied, causing thread A's I/O to remain unable to be woken up.
Some on-site information:
rwb.rq_depth.max_depth
(unsigned int)48
rqw.inflight.counter.value_()
44
rqw.inflight.counter.value_()
35
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)3
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)2
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)20
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)12
cat wb_normal 24 cat wb_background 12
To fix this issue, we can use max_depth in wbt_rqw_done(), so that the handling of wb_recent_wait by wbt_rqw_done() and get_limit() will also be consistent, which is more reasonable.
Signed-off-by: Julian Sun sunjunchao@bytedance.com Fixes: e34cbd307477 ("blk-wbt: add general throttling mechanism") --- block/blk-wbt.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/block/blk-wbt.c b/block/blk-wbt.c index a50d4cd55f41..d6a2782d442f 100644 --- a/block/blk-wbt.c +++ b/block/blk-wbt.c @@ -210,6 +210,8 @@ static void wbt_rqw_done(struct rq_wb *rwb, struct rq_wait *rqw, else if (blk_queue_write_cache(rwb->rqos.disk->queue) && !wb_recent_wait(rwb)) limit = 0; + else if (wb_recent_wait(rwb)) + limit = rwb->rq_depth.max_depth; else limit = rwb->wb_normal;
Hi,
Thanks for your patch.
FYI: kernel test robot notices the stable kernel rule is not satisfied.
The check is based on https://www.kernel.org/doc/html/latest/process/stable-kernel-rules.html#opti...
Rule: add the tag "Cc: stable@vger.kernel.org" in the sign-off area to have the patch automatically included in the stable tree. Subject: [PATCH] blk-wbt: Fix io starvation in wbt_rqw_done() Link: https://lore.kernel.org/stable/20250731123319.1271527-1-sunjunchao%40bytedan...
Hi Julian,
On Thu, Jul 31, 2025 at 8:33 PM Julian Sun sunjunchao2870@gmail.com wrote:
Recently, we encountered the following hungtask:
INFO: task kworker/11:2:2981147 blocked for more than 6266 seconds "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. kworker/11:2 D 0 2981147 2 0x80004000 Workqueue: cgroup_destroy css_free_rwork_fn Call Trace: __schedule+0x934/0xe10 schedule+0x40/0xb0 wb_wait_for_completion+0x52/0x80
I don’t see __wbt_wait() or rq_qos_wait() here, so I suspect this call stack is not directly related to wbt.
? finish_wait+0x80/0x80 mem_cgroup_css_free+0x3a/0x1b0 css_free_rwork_fn+0x42/0x380 process_one_work+0x1a2/0x360 worker_thread+0x30/0x390 ? create_worker+0x1a0/0x1a0 kthread+0x110/0x130 ? __kthread_cancel_work+0x40/0x40 ret_from_fork+0x1f/0x30
This is because the writeback thread has been continuously and repeatedly throttled by wbt, but at the same time, the writes of another thread proceed quite smoothly. After debugging, I believe it is caused by the following reasons.
When thread A is blocked by wbt, the I/O issued by thread B will use a deeper queue depth(rwb->rq_depth.max_depth) because it meets the conditions of wb_recent_wait(), thus allowing thread B's I/O to be issued smoothly and resulting in the inflight I/O of wbt remaining relatively high.
However, when I/O completes, due to the high inflight I/O of wbt, the condition "limit - inflight >= rwb->wb_background / 2" in wbt_rqw_done() cannot be satisfied, causing thread A's I/O to remain unable to be woken up.
From your description above, it seems you're suggesting that if A is throttled by wbt, then a writer B on the same device could continuously starve A. This situation is not possible — please refer to rq_qos_wait(): if A is already sleeping, then when B calls wq_has_sleeper(), it will detect A’s presence, meaning B will also be throttled.
Thanks, Yi
Some on-site information:
rwb.rq_depth.max_depth
(unsigned int)48
rqw.inflight.counter.value_()
44
rqw.inflight.counter.value_()
35
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)3
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)2
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)20
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)12
cat wb_normal 24 cat wb_background 12
To fix this issue, we can use max_depth in wbt_rqw_done(), so that the handling of wb_recent_wait by wbt_rqw_done() and get_limit() will also be consistent, which is more reasonable.
Signed-off-by: Julian Sun sunjunchao@bytedance.com Fixes: e34cbd307477 ("blk-wbt: add general throttling mechanism")
block/blk-wbt.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/block/blk-wbt.c b/block/blk-wbt.c index a50d4cd55f41..d6a2782d442f 100644 --- a/block/blk-wbt.c +++ b/block/blk-wbt.c @@ -210,6 +210,8 @@ static void wbt_rqw_done(struct rq_wb *rwb, struct rq_wait *rqw, else if (blk_queue_write_cache(rwb->rqos.disk->queue) && !wb_recent_wait(rwb)) limit = 0;
else if (wb_recent_wait(rwb))limit = rwb->rq_depth.max_depth; else limit = rwb->wb_normal;-- 2.20.1
Hi,
在 2025/7/31 23:40, Yizhou Tang 写道:
Hi Julian,
On Thu, Jul 31, 2025 at 8:33 PM Julian Sun sunjunchao2870@gmail.com wrote:
Recently, we encountered the following hungtask:
INFO: task kworker/11:2:2981147 blocked for more than 6266 seconds "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. kworker/11:2 D 0 2981147 2 0x80004000 Workqueue: cgroup_destroy css_free_rwork_fn Call Trace: __schedule+0x934/0xe10 schedule+0x40/0xb0 wb_wait_for_completion+0x52/0x80
I don’t see __wbt_wait() or rq_qos_wait() here, so I suspect this call stack is not directly related to wbt.
? finish_wait+0x80/0x80 mem_cgroup_css_free+0x3a/0x1b0 css_free_rwork_fn+0x42/0x380 process_one_work+0x1a2/0x360 worker_thread+0x30/0x390 ? create_worker+0x1a0/0x1a0 kthread+0x110/0x130 ? __kthread_cancel_work+0x40/0x40 ret_from_fork+0x1f/0x30
This is writeback cgroup is waiting for writeback to be done, if you figured out they are throttled by wbt, you need to explain clearly, and it's very important to provide evidence to support your analysis. However, the following analysis is a mess :(
This is because the writeback thread has been continuously and repeatedly throttled by wbt, but at the same time, the writes of another thread proceed quite smoothly. After debugging, I believe it is caused by the following reasons.
When thread A is blocked by wbt, the I/O issued by thread B will use a deeper queue depth(rwb->rq_depth.max_depth) because it meets the conditions of wb_recent_wait(), thus allowing thread B's I/O to be issued smoothly and resulting in the inflight I/O of wbt remaining relatively high.
However, when I/O completes, due to the high inflight I/O of wbt, the condition "limit - inflight >= rwb->wb_background / 2" in wbt_rqw_done() cannot be satisfied, causing thread A's I/O to remain unable to be woken up.
From your description above, it seems you're suggesting that if A is throttled by wbt, then a writer B on the same device could continuously starve A. This situation is not possible — please refer to rq_qos_wait(): if A is already sleeping, then when B calls wq_has_sleeper(), it will detect A’s presence, meaning B will also be throttled.
Yes, there are three rq_wait in wbt, and each one is FIFO. It will be possible if A is backgroup, and B is swap.
Thanks, Yi
Some on-site information:
rwb.rq_depth.max_depth
(unsigned int)48
rqw.inflight.counter.value_()
44
rqw.inflight.counter.value_()
35
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)3
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)2
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)20
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)12
cat wb_normal 24 cat wb_background 12
To fix this issue, we can use max_depth in wbt_rqw_done(), so that the handling of wb_recent_wait by wbt_rqw_done() and get_limit() will also be consistent, which is more reasonable.
Are you able to reproduce this problem, and give this patch a test before you send it?
Thanks, Kuai
Signed-off-by: Julian Sun sunjunchao@bytedance.com Fixes: e34cbd307477 ("blk-wbt: add general throttling mechanism")
block/blk-wbt.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/block/blk-wbt.c b/block/blk-wbt.c index a50d4cd55f41..d6a2782d442f 100644 --- a/block/blk-wbt.c +++ b/block/blk-wbt.c @@ -210,6 +210,8 @@ static void wbt_rqw_done(struct rq_wb *rwb, struct rq_wait *rqw, else if (blk_queue_write_cache(rwb->rqos.disk->queue) && !wb_recent_wait(rwb)) limit = 0;
else if (wb_recent_wait(rwb))limit = rwb->rq_depth.max_depth; else limit = rwb->wb_normal;-- 2.20.1
Hi,
On Fri, Aug 1, 2025 at 1:13 AM Yu Kuai yukuai@kernel.org wrote:
Hi,
在 2025/7/31 23:40, Yizhou Tang 写道:
Hi Julian,
On Thu, Jul 31, 2025 at 8:33 PM Julian Sun sunjunchao2870@gmail.com wrote:
Recently, we encountered the following hungtask:
INFO: task kworker/11:2:2981147 blocked for more than 6266 seconds "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message. kworker/11:2 D 0 2981147 2 0x80004000 Workqueue: cgroup_destroy css_free_rwork_fn Call Trace: __schedule+0x934/0xe10 schedule+0x40/0xb0 wb_wait_for_completion+0x52/0x80
I don’t see __wbt_wait() or rq_qos_wait() here, so I suspect this call stack is not directly related to wbt.
? finish_wait+0x80/0x80 mem_cgroup_css_free+0x3a/0x1b0 css_free_rwork_fn+0x42/0x380 process_one_work+0x1a2/0x360 worker_thread+0x30/0x390 ? create_worker+0x1a0/0x1a0 kthread+0x110/0x130 ? __kthread_cancel_work+0x40/0x40 ret_from_fork+0x1f/0x30
This is writeback cgroup is waiting for writeback to be done, if you figured out they are throttled by wbt, you need to explain clearly, and it's very important to provide evidence to support your analysis. However, the following analysis is a mess :(
Thanks for the detailed review. Yes, the description is a bit confusing. I will take a more detailed look at the on-site information.
This is because the writeback thread has been continuously and repeatedly throttled by wbt, but at the same time, the writes of another thread proceed quite smoothly. After debugging, I believe it is caused by the following reasons.
When thread A is blocked by wbt, the I/O issued by thread B will use a deeper queue depth(rwb->rq_depth.max_depth) because it meets the conditions of wb_recent_wait(), thus allowing thread B's I/O to be issued smoothly and resulting in the inflight I/O of wbt remaining relatively high.
However, when I/O completes, due to the high inflight I/O of wbt, the condition "limit - inflight >= rwb->wb_background / 2" in wbt_rqw_done() cannot be satisfied, causing thread A's I/O to remain unable to be woken up.
From your description above, it seems you're suggesting that if A is throttled by wbt, then a writer B on the same device could continuously starve A. This situation is not possible — please refer to rq_qos_wait(): if A is already sleeping, then when B calls wq_has_sleeper(), it will detect A’s presence, meaning B will also be throttled.
Yes, there are three rq_wait in wbt, and each one is FIFO. It will be possible if A is backgroup, and B is swap.
Thanks, Yi
Some on-site information:
rwb.rq_depth.max_depth
(unsigned int)48
rqw.inflight.counter.value_()
44
rqw.inflight.counter.value_()
35
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)3
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)2
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)20
prog['jiffies'] - rwb.rqos.q.backing_dev_info.last_bdp_sleep
(unsigned long)12
cat wb_normal 24 cat wb_background 12
To fix this issue, we can use max_depth in wbt_rqw_done(), so that the handling of wb_recent_wait by wbt_rqw_done() and get_limit() will also be consistent, which is more reasonable.
Are you able to reproduce this problem, and give this patch a test before you send it?
Thanks, Kuai
Signed-off-by: Julian Sun sunjunchao@bytedance.com Fixes: e34cbd307477 ("blk-wbt: add general throttling mechanism")
block/blk-wbt.c | 2 ++ 1 file changed, 2 insertions(+)
diff --git a/block/blk-wbt.c b/block/blk-wbt.c index a50d4cd55f41..d6a2782d442f 100644 --- a/block/blk-wbt.c +++ b/block/blk-wbt.c @@ -210,6 +210,8 @@ static void wbt_rqw_done(struct rq_wb *rwb, struct rq_wait *rqw, else if (blk_queue_write_cache(rwb->rqos.disk->queue) && !wb_recent_wait(rwb)) limit = 0;
else if (wb_recent_wait(rwb))limit = rwb->rq_depth.max_depth; else limit = rwb->wb_normal;-- 2.20.1
Thanks,
linux-stable-mirror@lists.linaro.org
-
Julian Sun -
kernel test robot -
Yizhou Tang -
Yu Kuai