Re: [PATCH] write-back: Wake up waiting tasks when finishing the writeback of a chunk.

[Julian Sun <sunjunchao@bytedance.com>]

On 9/26/25 1:25 AM, Mateusz Guzik wrote:
> On Thu, Sep 25, 2025 at 09:22:39PM +0800, Julian Sun wrote:
>> Writing back a large number of pages can take a lots of time.
>> This issue is exacerbated when the underlying device is slow or
>> subject to block layer rate limiting, which in turn triggers
>> unexpected hung task warnings.
>>
>> We can trigger a wake-up once a chunk has been written back and the
>> waiting time for writeback exceeds half of
>> sysctl_hung_task_timeout_secs.
>> This action allows the hung task detector to be aware of the writeback
>> progress, thereby eliminating these unexpected hung task warnings.
>>
> 
> If I'm reading correctly this is also messing with stats how long the
> thread was stuck to begin with.

IMO, it will not mess up the time. Since it only updates the time when 
we can see progress (which is not a hang). If the task really hangs for 
a long time, then we can't perform the time update—so it will not mess 
up the time.

cc Lance and Andrew.
> 
> Perhaps it would be better to have a var in task_struct which would
> serve as an indicator of progress being made (e.g., last time stamp of
> said progress).
> 
> task_struct already has numerous holes so this would not have to grow it
> above what it is now.
> 
> 
>> This patch has passed the xfstests 'check -g quick' test based on ext4,
>> with no additional failures introduced.
>>
>> Signed-off-by: Julian Sun <sunjunchao@bytedance.com>
>> Suggested-by: Peter Zijlstra <peterz@infradead.org>
>> ---
>>   fs/fs-writeback.c                | 13 +++++++++++--
>>   include/linux/backing-dev-defs.h |  1 +
>>   2 files changed, 12 insertions(+), 2 deletions(-)
>>
>> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
>> index a07b8cf73ae2..475d52abfb3e 100644
>> --- a/fs/fs-writeback.c
>> +++ b/fs/fs-writeback.c
>> @@ -14,6 +14,7 @@
>>    *		Additions for address_space-based writeback
>>    */
>>   
>> +#include <linux/sched/sysctl.h>
>>   #include <linux/kernel.h>
>>   #include <linux/export.h>
>>   #include <linux/spinlock.h>
>> @@ -174,9 +175,12 @@ static void finish_writeback_work(struct wb_writeback_work *work)
>>   		kfree(work);
>>   	if (done) {
>>   		wait_queue_head_t *waitq = done->waitq;
>> +		/* Report progress to inform the hung task detector of the progress. */
>> +		bool force_wake = (jiffies - done->stamp) >
>> +				   sysctl_hung_task_timeout_secs * HZ / 2;
>>   
>>   		/* @done can't be accessed after the following dec */
>> -		if (atomic_dec_and_test(&done->cnt))
>> +		if (atomic_dec_and_test(&done->cnt) || force_wake)
>>   			wake_up_all(waitq);
>>   	}
>>   }
>> @@ -213,7 +217,7 @@ static void wb_queue_work(struct bdi_writeback *wb,
>>   void wb_wait_for_completion(struct wb_completion *done)
>>   {
>>   	atomic_dec(&done->cnt);		/* put down the initial count */
>> -	wait_event(*done->waitq, !atomic_read(&done->cnt));
>> +	wait_event(*done->waitq, ({ done->stamp = jiffies; !atomic_read(&done->cnt); }));
>>   }
>>   
>>   #ifdef CONFIG_CGROUP_WRITEBACK
>> @@ -1975,6 +1979,11 @@ static long writeback_sb_inodes(struct super_block *sb,
>>   		 */
>>   		__writeback_single_inode(inode, &wbc);
>>   
>> +		/* Report progress to inform the hung task detector of the progress. */
>> +		if (work->done && (jiffies - work->done->stamp) >
>> +		    HZ * sysctl_hung_task_timeout_secs / 2)
>> +			wake_up_all(work->done->waitq);
>> +
>>   		wbc_detach_inode(&wbc);
>>   		work->nr_pages -= write_chunk - wbc.nr_to_write;
>>   		wrote = write_chunk - wbc.nr_to_write - wbc.pages_skipped;
>> diff --git a/include/linux/backing-dev-defs.h b/include/linux/backing-dev-defs.h
>> index 2ad261082bba..c37c6bd5ef5c 100644
>> --- a/include/linux/backing-dev-defs.h
>> +++ b/include/linux/backing-dev-defs.h
>> @@ -63,6 +63,7 @@ enum wb_reason {
>>   struct wb_completion {
>>   	atomic_t		cnt;
>>   	wait_queue_head_t	*waitq;
>> +	unsigned long stamp;
>>   };
>>   
>>   #define __WB_COMPLETION_INIT(_waitq)	\
>> -- 
>> 2.39.5
>>

Thanks,
-- 
Julian Sun <sunjunchao@bytedance.com>