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

[Sahitya Tummala]

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()?

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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