Re: [PATCH] f2fs: avoid hungtask problem caused by losing wake_up

[Shawn Lin <shawn.lin@kernel-upstream.org>]

On 2016/2/23 15:02, He YunLei wrote:
> On 2016/2/23 13:44, Chao Yu wrote:
>> Hi Yunlei,
> Hi Chao,
>>
>>> -----Original Message-----
>>> From: Yunlei He [mailto:heyunlei@huawei.com]
>>> Sent: Tuesday, February 23, 2016 12:08 PM
>>> To: chao2.yu@samsung.com; jaegeuk@kernel.org; linux-f2fs-devel@lists.sourceforge.net
>>> Cc: bintian.wang@huawei.com; Yunlei He; Biao He
>>> Subject: [f2fs-dev] [PATCH] f2fs: avoid hungtask problem caused by losing wake_up
>>>
>>> The D state of wait_on_all_pages_writeback should be waken by
>>> function f2fs_write_end_io when all writeback pages have been
>>> succesfully written to device. It's possible that wake_up comes
>>> between get_pages and io_schedule. Maybe in this case it will
>>> lost wake_up and still in D state even if all pages have been
>>> write back to device, and finally, the whole system will be into
>>> the hungtask state.
>>
>> I haven't encountered such issue so far, do you suffer this in real
>> world?
>>
> yes, I have encounter it, the whole file system is blocked at function
> wait_on_all_pages_writeback beyond 120s when write cp, and no error reported
> by storage device driver.

Hi YunLei

Interesting...
How to prove the page isn't been locked by storage block layer which
doesn't finish the related bio?

I suffer that case before, but then I solve it by seting a timer to
break my mmc driver while missing some important interrupts(chip
problem) for data-transfer to finish the io queue by mmcqd.

Another thing,
why making 5HZ for timeout? why not 10HZ ,15HZ...
io_schedule_timeout(5*HZ);

>>>
>>>                   if (!get_pages(sbi, F2FS_WRITEBACK))
>>>                            break;
>>> 					<---------  wake_up
>>
>> wake_up will put all tasks linked in sbi->cp_wait on run-queue, so
>> here it should be save to call io_schedule, after being rescheduled,
>> it will get the chance to check above condition to break out.
>>
>> Thanks,
>
> Here, we just doubt something weird may cause wait_on_all_pages_writeback
> could not be waken. Wake_up trigger only one time by last bio's end_io
> function, if the thread happen to miss it, the thread will be in D state
> forever. So we change the code to make wait_on_all_pages_writeback awaken
> periodically, then check the condition.
>
>>
>>>                   io_schedule();
>>>
>>> Signed-off-by: Yunlei He <heyunlei@huawei.com>
>>> Signed-off-by: Biao He <hebiao6@huawei.com>
>>> ---
>>>    fs/f2fs/checkpoint.c | 2 +-
>>>    1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/fs/f2fs/checkpoint.c b/fs/f2fs/checkpoint.c
>>> index 2bac8a1..f55355d 100644
>>> --- a/fs/f2fs/checkpoint.c
>>> +++ b/fs/f2fs/checkpoint.c
>>> @@ -920,7 +920,7 @@ static void wait_on_all_pages_writeback(struct f2fs_sb_info *sbi)
>>>    		if (!get_pages(sbi, F2FS_WRITEBACK))
>>>    			break;
>>>
>>> -		io_schedule();
>>> +		io_schedule_timeout(5*HZ);
>>>    	}
>>>    	finish_wait(&sbi->cp_wait, &wait);
>>>    }
>>> --
>>> 1.9.1
>>
>>
>>
>> .
>>
>
>
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