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

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Date:   Fri, 13 Mar 2020 16:38:46 +0530
From:   Sahitya Tummala <stummala@...eaurora.org>
To:     Chao Yu <yuchao0@...wei.com>
Cc:     Jaegeuk Kim <jaegeuk@...nel.org>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org, stummala@...eaurora.org
Subject: Re: [PATCH] f2fs: fix long latency due to discard during umount

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@...eaurora.org>
> >>> ---
> >>>  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
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;
> >>>
> > 

-- 
--
Sent by a consultant of the Qualcomm Innovation Center, Inc.
The Qualcomm Innovation Center, Inc. is a member of the Code Aurora Forum.