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Date:   Fri, 27 Mar 2020 08:35:42 +0530
From:   Sahitya Tummala <stummala@codeaurora.org>
To:     Chao Yu <yuchao0@huawei.com>
Cc:     Jaegeuk Kim <jaegeuk@kernel.org>,
        linux-f2fs-devel@lists.sourceforge.net,
        linux-kernel@vger.kernel.org, stummala@codeaurora.org
Subject: Re: [PATCH] f2fs: fix long latency due to discard during umount
Message-ID: <20200327030542.GS20234@codeaurora.org>
References: <1584506689-5041-1-git-send-email-stummala@codeaurora.org>
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On Fri, Mar 27, 2020 at 09:51:43AM +0800, Chao Yu wrote:
> On 2020/3/26 21:37, Sahitya Tummala wrote:
> > Hi Chao,
> > 
> > On Thu, Mar 26, 2020 at 05:00:18PM +0800, Chao Yu wrote:
> >> Hi Sahitya,
> >>
> >> On 2020/3/18 12:44, 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@codeaurora.org>
> >>> ---
> >>> v2:
> >>> - Handle the case where a dc can have multiple bios associated with it
> >>>
> >>>  fs/f2fs/f2fs.h    |  1 +
> >>>  fs/f2fs/segment.c | 83 ++++++++++++++++++++++++++++++++++++++++++++++++-------
> >>>  2 files changed, 74 insertions(+), 10 deletions(-)
> >>>
> >>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> >>> index 12a197e..67b8dcc 100644
> >>> --- a/fs/f2fs/f2fs.h
> >>> +++ b/fs/f2fs/f2fs.h
> >>> @@ -340,6 +340,7 @@ struct discard_cmd_control {
> >>>  	struct list_head pend_list[MAX_PLIST_NUM];/* store pending entries */
> >>>  	struct list_head wait_list;		/* store on-flushing entries */
> >>>  	struct list_head fstrim_list;		/* in-flight discard from fstrim */
> >>> +	struct list_head retry_list;		/* list of cmds to retry */
> >>>  	wait_queue_head_t discard_wait_queue;	/* waiting queue for wake-up */
> >>>  	unsigned int discard_wake;		/* to wake up discard thread */
> >>>  	struct mutex cmd_lock;
> >>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> >>> index fb3e531..4162c76 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;
> >>> -	block_t lstart, start, len, total_len;
> >>> +	int flag;
> >>> +	block_t lstart, start, len, total_len, orig_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;
> >>>  
> >>> @@ -1139,7 +1145,7 @@ static int __submit_discard_cmd(struct f2fs_sb_info *sbi,
> >>>  	lstart = dc->lstart;
> >>>  	start = dc->start;
> >>>  	len = dc->len;
> >>> -	total_len = len;
> >>> +	orig_len = total_len = len;
> >>>  
> >>>  	dc->len = 0;
> >>>  
> >>> @@ -1203,6 +1209,14 @@ 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) {
> >>> +				dc->len = orig_len;
> >>> +				list_move(&dc->list, &dcc->retry_list);
> >>> +				err = dc->error;
> >>
> >> I encounter lots of dmesg, which should be printed by __remove_discard_cmd()
> >>
> >> F2FS-fs (dm-0): Issue discard(23552, 23552, 2) failed, ret: -11
> >>
> >> This should happen only if we didn't handle all discard in 5 seconds during
> >> umount.
> >>
> >> So I doubt we failed to move dc to retry_list, because after submit_bio(),
> >> end_io() is not called synchronously as the bio was just pluged?
> >>
> > This can happen if a discard cmd has multiple bios and at least 1 bio is 
> > already submitted and when submitting other bios, we encounter -EAGAIN.
> > In this case, this dc will be moved to retry_list and will be moved back
> > to dcc->pend_list only if the dc->bio_ref becomes 0 within 5 sec timeout.
> > If it doesn't become zero, then it will be left in retry_list itself, which
> > will be later removed from retry_list. Before removing from retry_list we 
> > will however ensure that submitted bio is done i.e., dc->bio_ref is 0, but
> > dc->error will be -EAGAIN as this dc could not be requeued/retried.
> > 
> > So this is expected, where it only means that this dc could not be
> > submitted/retried again within timeout. I think we can ignore
> > this -EAGAIN error in __remove_discard_cmd().
> 
> With this patch, most of xfstest cases cost 5 * n second longer than before.
> 
> E.g. generic/003, during umount(), we looped into retrying one bio
> submission.
> 
> [61279.829724] F2FS-fs (zram1): Found nat_bits in checkpoint
> [61279.885337] F2FS-fs (zram1): Mounted with checkpoint version = 5cf3cb8e
> [61281.912832] submit discard bio start [23555,1]
> [61281.912835] f2fs_submit_discard_endio [23555,1] err:-11
> [61281.912836] submit discard bio end [23555,1]
> [61281.912836] move dc to retry list [23555,1]
> 
> ...
> 
> [61286.881212] submit discard bio start [23555,1]
> [61286.881217] f2fs_submit_discard_endio [23555,1] err:-11
> [61286.881223] submit discard bio end [23555,1]
> [61286.881224] move dc to retry list [23555,1]
> [61286.905198] submit discard bio start [23555,1]
> [61286.905203] f2fs_submit_discard_endio [23555,1] err:-11
> [61286.905205] submit discard bio end [23555,1]
> [61286.905206] move dc to retry list [23555,1]
> [61286.929157] F2FS-fs (zram1): Issue discard(23555, 23555, 1) failed, ret: -11
> 
> Could you take a look at this issue?

Let me check and get back on this.

Thanks,

> 
> Thanks,
> 
> > 
> > Thanks,
> > 
> >> Thanks,
> >>
> >>> +				break;
> >>> +			}
> >>> +		}
> >>>  
> >>>  		atomic_inc(&dcc->issued_discard);
> >>>  
> >>> @@ -1463,6 +1477,40 @@ static unsigned int __issue_discard_cmd_orderly(struct f2fs_sb_info *sbi,
> >>>  	return issued;
> >>>  }
> >>>  
> >>> +static bool __should_discard_retry(struct f2fs_sb_info *sbi,
> > s> > +		struct discard_policy *dpolicy)
> >>> +{
> >>> +	struct discard_cmd_control *dcc = SM_I(sbi)->dcc_info;
> >>> +	struct discard_cmd *dc, *tmp;
> >>> +	bool retry = false;
> >>> +	unsigned long flags;
> >>> +
> >>> +	if (dpolicy->type != DPOLICY_UMOUNT)
> >>> +		f2fs_bug_on(sbi, 1);
> >>> +
> >>> +	mutex_lock(&dcc->cmd_lock);
> >>> +	list_for_each_entry_safe(dc, tmp, &(dcc->retry_list), list) {
> >>> +		if (dpolicy->timeout != 0 &&
> >>> +			f2fs_time_over(sbi, dpolicy->timeout)) {
> >>> +			retry = false;
> >>> +			break;
> >>> +		}
> >>> +
> >>> +		spin_lock_irqsave(&dc->lock, flags);
> >>> +		if (!dc->bio_ref) {
> >>> +			dc->state = D_PREP;
> >>> +			dc->error = 0;
> >>> +			reinit_completion(&dc->wait);
> >>> +			__relocate_discard_cmd(dcc, dc);
> >>> +			retry = true;
> >>> +		}
> >>> +		spin_unlock_irqrestore(&dc->lock, flags);
> >>> +	}
> >>> +	mutex_unlock(&dcc->cmd_lock);
> >>> +
> >>> +	return retry;
> >>> +}
> >>> +
> >>>  static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
> >>>  					struct discard_policy *dpolicy)
> >>>  {
> >>> @@ -1470,12 +1518,13 @@ 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;
> >>>  
> >>>  	if (dpolicy->timeout != 0)
> >>>  		f2fs_update_time(sbi, dpolicy->timeout);
> >>>  
> >>> +retry:
> >>>  	for (i = MAX_PLIST_NUM - 1; i >= 0; i--) {
> >>>  		if (dpolicy->timeout != 0 &&
> >>>  				f2fs_time_over(sbi, dpolicy->timeout))
> >>> @@ -1509,7 +1558,10 @@ 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);
> >>>  
> >>>  			if (issued >= dpolicy->max_requests)
> >>>  				break;
> >>> @@ -1522,6 +1574,10 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
> >>>  			break;
> >>>  	}
> >>>  
> >>> +	if (!list_empty(&dcc->retry_list) &&
> >>> +		__should_discard_retry(sbi, dpolicy))
> >>> +		goto retry;
> >>> +
> >>>  	if (!issued && io_interrupted)
> >>>  		issued = -1;
> >>>  
> >>> @@ -1613,6 +1669,12 @@ static unsigned int __wait_discard_cmd_range(struct f2fs_sb_info *sbi,
> >>>  		goto next;
> >>>  	}
> >>>  
> >>> +	if (dpolicy->type == DPOLICY_UMOUNT &&
> >>> +		!list_empty(&dcc->retry_list)) {
> >>> +		wait_list = &dcc->retry_list;
> >>> +		goto next;
> >>> +	}
> >>> +
> >>>  	return trimmed;
> >>>  }
> >>>  
> >>> @@ -2051,6 +2113,7 @@ static int create_discard_cmd_control(struct f2fs_sb_info *sbi)
> >>>  	for (i = 0; i < MAX_PLIST_NUM; i++)
> >>>  		INIT_LIST_HEAD(&dcc->pend_list[i]);
> >>>  	INIT_LIST_HEAD(&dcc->wait_list);
> >>> +	INIT_LIST_HEAD(&dcc->retry_list);
> >>>  	INIT_LIST_HEAD(&dcc->fstrim_list);
> >>>  	mutex_init(&dcc->cmd_lock);
> >>>  	atomic_set(&dcc->issued_discard, 0);
> >>>
> > 

-- 
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