Re: [PATCH v3] f2fs: fix long latency due to discard during umount

[Chao Yu]

On 2020/4/1 17:22, Sahitya Tummala wrote:
> Hi Jaegeuk,
> 
> On Tue, Mar 31, 2020 at 11:46:55AM -0700, Jaegeuk Kim wrote:
>> On 03/30, 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.
>>
>> Sorry, Sahitya, but, do we really need to do like this? How about just
>> controlling # of outstanding discarding bios in __issue_discard_cmd()?
> 
> Do you mean something like this?
> 
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index 1a62b27..860dd43 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -1099,7 +1099,7 @@ static void __init_discard_policy(struct f2fs_sb_info *sbi,
>         } else if (discard_type == DPOLICY_FSTRIM) {
>                 dpolicy->io_aware = false;
>         } else if (discard_type == DPOLICY_UMOUNT) {
> -               dpolicy->max_requests = UINT_MAX;
> +               dpolicy->max_requests = 30;

8 or 16?

It looks more simple than previous implementation.

Thanks,


>                 dpolicy->io_aware = false;
>                 /* we need to issue all to keep CP_TRIMMED_FLAG */
>                 dpolicy->granularity = 1;
> @@ -1470,12 +1470,14 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>         struct list_head *pend_list;
>         struct discard_cmd *dc, *tmp;
>         struct blk_plug plug;
> -       int i, issued = 0;
> +       int i, issued;
>         bool io_interrupted = false;
> 
>         if (dpolicy->timeout != 0)
>                 f2fs_update_time(sbi, dpolicy->timeout);
> 
> +retry:
> +       issued = 0;
>         for (i = MAX_PLIST_NUM - 1; i >= 0; i--) {
>                 if (dpolicy->timeout != 0 &&
>                                 f2fs_time_over(sbi, dpolicy->timeout))
> @@ -1522,6 +1524,11 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>                         break;
>         }
> 
> +       if (dpolicy->type == DPOLICY_UMOUNT && issued) {
> +               __wait_all_discard_cmd(sbi, dpolicy);
> +               goto retry;
> +       }
> +
>         if (!issued && io_interrupted)
>                 issued = -1;
> 
> Thanks,
> 
>>
>>>
>>> Signed-off-by: Sahitya Tummala <stummala@xxxxxxxxxxxxxx>
>>> ---
>>> v3:
>>> -Handle the regression reported by Chao with v2.
>>> -simplify the logic to split the dc with multiple bios incase any bio returns
>>>  EAGAIN and retry those new dc within 5 sec timeout.
>>>
>>>  fs/f2fs/segment.c | 65 +++++++++++++++++++++++++++++++++++++++++++------------
>>>  1 file changed, 51 insertions(+), 14 deletions(-)
>>>
>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>> index fb3e531..55d18c7 100644
>>> --- a/fs/f2fs/segment.c
>>> +++ b/fs/f2fs/segment.c
>>> @@ -1029,13 +1029,16 @@ static void f2fs_submit_discard_endio(struct bio *bio)
>>>  	struct discard_cmd *dc = (struct discard_cmd *)bio->bi_private;
>>>  	unsigned long flags;
>>>  
>>> -	dc->error = blk_status_to_errno(bio->bi_status);
>>> -
>>>  	spin_lock_irqsave(&dc->lock, flags);
>>> +	if (!dc->error)
>>> +		dc->error = blk_status_to_errno(bio->bi_status);
>>> +
>>>  	dc->bio_ref--;
>>> -	if (!dc->bio_ref && dc->state == D_SUBMIT) {
>>> -		dc->state = D_DONE;
>>> -		complete_all(&dc->wait);
>>> +	if (!dc->bio_ref) {
>>> +		if (dc->error || dc->state == D_SUBMIT) {
>>> +			dc->state = D_DONE;
>>> +			complete_all(&dc->wait);
>>> +		}
>>>  	}
>>>  	spin_unlock_irqrestore(&dc->lock, flags);
>>>  	bio_put(bio);
>>> @@ -1124,10 +1127,13 @@ 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;
>>>  
>>> @@ -1192,10 +1198,6 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
>>>  		dc->bio_ref++;
>>>  		spin_unlock_irqrestore(&dc->lock, flags);
>>>  
>>> -		atomic_inc(&dcc->queued_discard);
>>> -		dc->queued++;
>>> -		list_move_tail(&dc->list, wait_list);
>>> -
>>>  		/* sanity check on discard range */
>>>  		__check_sit_bitmap(sbi, lstart, lstart + len);
>>>  
>>> @@ -1203,6 +1205,29 @@ 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) {
>>> +			if (dc->error == -EAGAIN) {
>>> +				spin_lock_irqsave(&dc->lock, flags);
>>> +				dc->len -= len;
>>> +				if (!dc->len) {
>>> +					dc->len = total_len;
>>> +					dc->state = D_PREP;
>>> +					reinit_completion(&dc->wait);
>>> +				} else {
>>> +					dcc->undiscard_blks -= total_len;
>>> +					if (dc->state == D_PARTIAL)
>>> +						dc->state = D_SUBMIT;
>>> +				}
>>> +				err = dc->error;
>>> +				dc->error = 0;
>>> +				spin_unlock_irqrestore(&dc->lock, flags);
>>> +				break;
>>> +			}
>>> +		}
>>> +
>>> +		atomic_inc(&dcc->queued_discard);
>>> +		dc->queued++;
>>> +		list_move_tail(&dc->list, wait_list);
>>>  
>>>  		atomic_inc(&dcc->issued_discard);
>>>  
>>> @@ -1214,8 +1239,9 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
>>>  		len = total_len;
>>>  	}
>>>  
>>> -	if (!err && len)
>>> -		__update_discard_tree_range(sbi, bdev, lstart, start, len);
>>> +	if ((!err || err == -EAGAIN) && total_len && dc->start != start)
>>> +		__update_discard_tree_range(sbi, bdev, lstart, start,
>>> +					total_len);
>>>  	return err;
>>>  }
>>>  
>>> @@ -1470,12 +1496,15 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>>>  	struct list_head *pend_list;
>>>  	struct discard_cmd *dc, *tmp;
>>>  	struct blk_plug plug;
>>> -	int i, issued = 0;
>>> +	int i, err, issued = 0;
>>>  	bool io_interrupted = false;
>>> +	bool retry;
>>>  
>>>  	if (dpolicy->timeout != 0)
>>>  		f2fs_update_time(sbi, dpolicy->timeout);
>>>  
>>> +retry:
>>> +	retry = false;
>>>  	for (i = MAX_PLIST_NUM - 1; i >= 0; i--) {
>>>  		if (dpolicy->timeout != 0 &&
>>>  				f2fs_time_over(sbi, dpolicy->timeout))
>>> @@ -1509,7 +1538,12 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>>>  				break;
>>>  			}
>>>  
>>> -			__submit_discard_cmd(sbi, dpolicy, dc, &issued);
>>> +			err = __submit_discard_cmd(sbi, dpolicy, dc, &issued);
>>> +			if (err == -EAGAIN) {
>>> +				congestion_wait(BLK_RW_ASYNC,
>>> +						DEFAULT_IO_TIMEOUT);
>>> +				retry = true;
>>> +			}
>>>  
>>>  			if (issued >= dpolicy->max_requests)
>>>  				break;
>>> @@ -1522,6 +1556,9 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>>>  			break;
>>>  	}
>>>  
>>> +	if (retry)
>>> +		goto retry;
>>> +
>>>  	if (!issued && io_interrupted)
>>>  		issued = -1;
>>>  
>>> -- 
>>> Qualcomm India Private Limited, on behalf of Qualcomm Innovation Center, Inc.
>>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project.
>