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

[Chao Yu]

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@xxxxxxxxxxxxxx>
> ---
> 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?

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,
> +		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);
>