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Date:   Wed, 1 Apr 2020 14:52:01 +0530
From:   Sahitya Tummala <stummala@codeaurora.org>
To:     Jaegeuk Kim <jaegeuk@kernel.org>
Cc:     Chao Yu <yuchao0@huawei.com>,
        linux-f2fs-devel@lists.sourceforge.net,
        linux-kernel@vger.kernel.org, stummala@codeaurora.org
Subject: Re: [PATCH v3] f2fs: fix long latency due to discard during umount
Message-ID: <20200401092201.GB20234@codeaurora.org>
References: <1585550730-1858-1-git-send-email-stummala@codeaurora.org>
 <20200331184655.GB198665@google.com>
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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;
                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@codeaurora.org>
> > ---
> > 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.

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