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

[Sahitya Tummala]

Hi Chao,

On Mon, Mar 16, 2020 at 08:52:25AM +0800, Chao Yu wrote:
> On 2020/3/13 19:08, Sahitya Tummala wrote:
> > On Fri, Mar 13, 2020 at 02:30:55PM +0800, Chao Yu wrote:
> >> 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.
> > 
> > In case of queue full, f2fs_submit_discard_endio() will not be called
> 
> I guess the case is there are multiple bios related to one dc and partially callback
> of bio is called asynchronously and the other is called synchronously, so the race
> condition could happen.

You are right. Let me review that case and try to fix it.

Thanks,

> 
> Thanks,
> 
> > asynchronously. It will be called in the context of submit_bio() itself.
> > So by the time, submit_bio returns dc->error will be -EAGAIN and dc->state
> > will be D_DONE. 
> > 
> > submit_bio()
> > ->blk_mq_make_request
> > ->blk_mq_get_request()
> >   ->bio_wouldblock_error() (called due to queue full)
> >     ->bio_endio()
> >     
> > Thanks,
> >>
> >>>
> >>> 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;
> >>>>>
> >>>
> > 

-- 
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