Re: [PATCH] writeback: fix hung_task alarm when sync block

[Wanpeng Li <liwp.linux@gmail.com>]

On Wed, Jun 13, 2012 at 08:59:46AM +0800, Fengguang Wu wrote:
>On Wed, Jun 13, 2012 at 08:52:53AM +0800, Wanpeng Li wrote:
>> From: Wanpeng Li <liwp@linux.vnet.ibm.com>
>> 
>> I use several dd processes to write a slow SD card
>> dd if=/dev/sda1 of=/dev/sdc4 bs=1M count=4000
>> and several sync commands(maybe > 10),dmesg show this:
>> 
>> [  366.888741] INFO: task sync:3518 blocked for more than 120 seconds.
>> [  366.888742] "echo 0 > /proc/sys/kernel/hung_task_timeout_secs" disables this message.
>> [  366.888746] sync            D 00000201     0  3518   3462 0x00000000
>
>> Too many similar messages flood the logs. So I use a present method to 
>> fix this issue.
>> ------------------------------------------
>> Author: Mark Lord <kernel@teksavvy.com>
>> Date:   Fri Sep 24 09:51:13 2010 -0400
>> 
>>     block: Prevent hang_check firing during long I/O
>> ------------------------------------------
>> 
>> Signed-off-by: Wanpeng Li <liwp.linux@gmail.com>
>
>Yeah that's also what I'd like to do. So you've tested it?

Not yet, I will test it today.

>
>> ---
>>  fs/fs-writeback.c |    8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>> 
>> diff --git a/fs/fs-writeback.c b/fs/fs-writeback.c
>> index f2d0109..5d403a1 100644
>> --- a/fs/fs-writeback.c
>> +++ b/fs/fs-writeback.c
>> @@ -1300,6 +1300,7 @@ void writeback_inodes_sb_nr(struct super_block *sb,
>>  			    enum wb_reason reason)
>>  {
>>  	DECLARE_COMPLETION_ONSTACK(done);
>> +	unsigned long hangcheck;
>>  	struct wb_writeback_work work = {
>>  		.sb			= sb,
>>  		.sync_mode		= WB_SYNC_NONE,
>> @@ -1311,7 +1312,12 @@ void writeback_inodes_sb_nr(struct super_block *sb,
>>  
>>  	WARN_ON(!rwsem_is_locked(&sb->s_umount));
>>  	bdi_queue_work(sb->s_bdi, &work);
>> -	wait_for_completion(&done);
>> +	hangcheck = sysctl_hung_task_timeout_secs;
>> +	if (hangcheck)
>
>The hangcheck variable looks redundant.

if sysctl_hung_task_timeout_secs is equal to ZERO, it means infinite
timeout -- no checking done. So I think wait_for_completion_timeout 
makes no sense this time.

Regards,
Wanpeng Li

>
>> +		while (!wait_for_completion_timeout(&done, HZ/2))
>> +			;
>> +	else
>> +		wait_for_completion(&done);
>>  }
>>  EXPORT_SYMBOL(writeback_inodes_sb_nr);
>>  
>> -- 
>> 1.7.9.5