Re: [PATCH] f2fs: fix long latency due to discard during umount

[Chao Yu]

On 2020/3/13 11:39, Sahitya Tummala wrote:
> On Fri, Mar 13, 2020 at 10:20:04AM +0800, Chao Yu wrote:
>> On 2020/3/12 19:14, Sahitya Tummala wrote:
>>> F2FS already has a default timeout of 5 secs for discards that
>>> can be issued during umount, but it can take more than the 5 sec
>>> timeout if the underlying UFS device queue is already full and there
>>> are no more available free tags to be used. In that case, submit_bio()
>>> will wait for the already queued discard requests to complete to get
>>> a free tag, which can potentially take way more than 5 sec.
>>>
>>> Fix this by submitting the discard requests with REQ_NOWAIT
>>> flags during umount. This will return -EAGAIN for UFS queue/tag full
>>> scenario without waiting in the context of submit_bio(). The FS can
>>> then handle these requests by retrying again within the stipulated
>>> discard timeout period to avoid long latencies.
>>>
>>> Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>
>>> ---
>>>  fs/f2fs/segment.c | 14 +++++++++++++-
>>>  1 file changed, 13 insertions(+), 1 deletion(-)
>>>
>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>> index fb3e531..a06bbac 100644
>>> --- a/fs/f2fs/segment.c
>>> +++ b/fs/f2fs/segment.c
>>> @@ -1124,10 +1124,13 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
>>>  	struct discard_cmd_control *dcc = SM_I(sbi)->dcc_info;
>>>  	struct list_head *wait_list = (dpolicy->type == DPOLICY_FSTRIM) ?
>>>  					&(dcc->fstrim_list) : &(dcc->wait_list);
>>> -	int flag = dpolicy->sync ? REQ_SYNC : 0;
>>> +	int flag;
>>>  	block_t lstart, start, len, total_len;
>>>  	int err = 0;
>>>  
>>> +	flag = dpolicy->sync ? REQ_SYNC : 0;
>>> +	flag |= dpolicy->type == DPOLICY_UMOUNT ? REQ_NOWAIT : 0;
>>> +
>>>  	if (dc->state != D_PREP)
>>>  		return 0;
>>>  
>>> @@ -1203,6 +1206,11 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
>>>  		bio->bi_end_io = f2fs_submit_discard_endio;
>>>  		bio->bi_opf |= flag;
>>>  		submit_bio(bio);
>>> +		if ((flag & REQ_NOWAIT) && (dc->error == -EAGAIN)) {
>>
>> If we want to update dc->state, we need to cover it with dc->lock.
> 
> Sure, will update it.
> 
>>
>>> +			dc->state = D_PREP;
>>
>> BTW, one dc can be referenced by multiple bios, so dc->state could be updated to
>> D_DONE later by f2fs_submit_discard_endio(), however we just relocate it to
>> pending list... which is inconsistent status.
> 
> In that case dc->bio_ref will reflect it and until it becomes 0, the dc->state
> will not be updated to D_DONE in f2fs_submit_discard_endio()?

__submit_discard_cmd()
 lock()
 dc->state = D_SUBMIT;
 dc->bio_ref++;
 unlock()
...
 submit_bio()
				f2fs_submit_discard_endio()
				 dc->error = -EAGAIN;
				 lock()
				 dc->bio_ref--;

 dc->state = D_PREP;

				 dc->state = D_DONE;
				 unlock()

So finally, dc's state is D_DONE, and it's in wait list, then will be relocated
to pending list.

> 
> Thanks,
> 
>>
>> Thanks,
>>
>>> +			err = dc->error;
>>> +			break;
>>> +		}
>>>  
>>>  		atomic_inc(&dcc->issued_discard);
>>>  
>>> @@ -1510,6 +1518,10 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>>>  			}
>>>  
>>>  			__submit_discard_cmd(sbi, dpolicy, dc, &issued);
>>> +			if (dc->error == -EAGAIN) {
>>> +				congestion_wait(BLK_RW_ASYNC, HZ/50);
>>> +				__relocate_discard_cmd(dcc, dc);
>>> +			}
>>>  
>>>  			if (issued >= dpolicy->max_requests)
>>>  				break;
>>>
>