Re: [PATCH] f2fs: let fstrim issue discard commands in lower priority

[Jaegeuk Kim]

On 05/25, Chao Yu wrote:
> Hi Jaegeuk,
> 
> On 2018/5/25 13:10, Jaegeuk Kim wrote:
> > The fstrim gathers huge number of large discard commands, and tries to issue
> > without IO awareness, which results in long user-perceive IO latencies on
> > READ, WRITE, and FLUSH in UFS. We've observed some of commands take several
> > seconds due to long discard latency.
> > 
> > This patch limits the maximum size to 2MB per candidate, and check IO congestion
> > when issuing them to disk.
> > 
> > Signed-off-by: Jaegeuk Kim <jaegeuk@xxxxxxxxxx>
> > ---
> >  fs/f2fs/f2fs.h    |   4 +-
> >  fs/f2fs/segment.c | 123 +++++++++++++++++++++++-----------------------
> >  2 files changed, 64 insertions(+), 63 deletions(-)
> > 
> > diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> > index 3bddf13794d9..75ae7fc86ae8 100644
> > --- a/fs/f2fs/f2fs.h
> > +++ b/fs/f2fs/f2fs.h
> > @@ -178,6 +178,7 @@ enum {
> >  
> >  #define MAX_DISCARD_BLOCKS(sbi)		BLKS_PER_SEC(sbi)
> >  #define DEF_MAX_DISCARD_REQUEST		8	/* issue 8 discards per round */
> > +#define DEF_MAX_DISCARD_LEN		512	/* Max. 2MB per discard */
> >  #define DEF_MIN_DISCARD_ISSUE_TIME	50	/* 50 ms, if exists */
> >  #define DEF_MID_DISCARD_ISSUE_TIME	500	/* 500 ms, if device busy */
> >  #define DEF_MAX_DISCARD_ISSUE_TIME	60000	/* 60 s, if no candidates */
> > @@ -698,7 +699,8 @@ static inline void set_extent_info(struct extent_info *ei, unsigned int fofs,
> >  static inline bool __is_discard_mergeable(struct discard_info *back,
> >  						struct discard_info *front)
> >  {
> > -	return back->lstart + back->len == front->lstart;
> > +	return (back->lstart + back->len == front->lstart) &&
> > +		(back->len + front->len < DEF_MAX_DISCARD_LEN);
> >  }
> >  
> >  static inline bool __is_discard_back_mergeable(struct discard_info *cur,
> > diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> > index 843fc2e6d41c..ba996d4091bc 100644
> > --- a/fs/f2fs/segment.c
> > +++ b/fs/f2fs/segment.c
> > @@ -1139,68 +1139,6 @@ static int __queue_discard_cmd(struct f2fs_sb_info *sbi,
> >  	return 0;
> >  }
> >  
> > -static void __issue_discard_cmd_range(struct f2fs_sb_info *sbi,
> > -					struct discard_policy *dpolicy,
> > -					unsigned int start, unsigned int end)
> > -{
> > -	struct discard_cmd_control *dcc = SM_I(sbi)->dcc_info;
> > -	struct discard_cmd *prev_dc = NULL, *next_dc = NULL;
> > -	struct rb_node **insert_p = NULL, *insert_parent = NULL;
> > -	struct discard_cmd *dc;
> > -	struct blk_plug plug;
> > -	int issued;
> > -
> > -next:
> > -	issued = 0;
> > -
> > -	mutex_lock(&dcc->cmd_lock);
> > -	f2fs_bug_on(sbi, !__check_rb_tree_consistence(sbi, &dcc->root));
> > -
> > -	dc = (struct discard_cmd *)__lookup_rb_tree_ret(&dcc->root,
> > -					NULL, start,
> > -					(struct rb_entry **)&prev_dc,
> > -					(struct rb_entry **)&next_dc,
> > -					&insert_p, &insert_parent, true);
> > -	if (!dc)
> > -		dc = next_dc;
> > -
> > -	blk_start_plug(&plug);
> > -
> > -	while (dc && dc->lstart <= end) {
> > -		struct rb_node *node;
> > -
> > -		if (dc->len < dpolicy->granularity)
> > -			goto skip;
> > -
> > -		if (dc->state != D_PREP) {
> > -			list_move_tail(&dc->list, &dcc->fstrim_list);
> > -			goto skip;
> > -		}
> > -
> > -		__submit_discard_cmd(sbi, dpolicy, dc);
> > -
> > -		if (++issued >= dpolicy->max_requests) {
> > -			start = dc->lstart + dc->len;
> > -
> > -			blk_finish_plug(&plug);
> > -			mutex_unlock(&dcc->cmd_lock);
> > -
> > -			schedule();
> > -
> > -			goto next;
> > -		}
> > -skip:
> > -		node = rb_next(&dc->rb_node);
> > -		dc = rb_entry_safe(node, struct discard_cmd, rb_node);
> > -
> > -		if (fatal_signal_pending(current))
> > -			break;
> > -	}
> > -
> > -	blk_finish_plug(&plug);
> > -	mutex_unlock(&dcc->cmd_lock);
> > -}
> > -
> >  static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
> >  					struct discard_policy *dpolicy)
> >  {
> > @@ -2397,6 +2335,67 @@ bool exist_trim_candidates(struct f2fs_sb_info *sbi, struct cp_control *cpc)
> >  	return has_candidate;
> >  }
> >  
> > +static void __issue_discard_cmd_range(struct f2fs_sb_info *sbi,
> > +					struct discard_policy *dpolicy,
> > +					unsigned int start, unsigned int end)
> > +{
> > +	struct discard_cmd_control *dcc = SM_I(sbi)->dcc_info;
> > +	struct discard_cmd *prev_dc = NULL, *next_dc = NULL;
> > +	struct rb_node **insert_p = NULL, *insert_parent = NULL;
> > +	struct discard_cmd *dc;
> > +	struct blk_plug plug;
> > +	int issued;
> 
> unsigned int cur = start;
> 
> > +
> > +next:
> > +	issued = 0;
> > +
> > +	mutex_lock(&dcc->cmd_lock);
> > +	f2fs_bug_on(sbi, !__check_rb_tree_consistence(sbi, &dcc->root));
> > +
> > +	dc = (struct discard_cmd *)__lookup_rb_tree_ret(&dcc->root,
> > +					NULL, start,
> 
> NULL, cur,
> 
> > +					(struct rb_entry **)&prev_dc,
> > +					(struct rb_entry **)&next_dc,
> > +					&insert_p, &insert_parent, true);
> > +	if (!dc)
> > +		dc = next_dc;
> > +
> > +	blk_start_plug(&plug);
> > +
> > +	while (dc && dc->lstart <= end) {
> > +		struct rb_node *node;
> > +
> > +		if (dc->len < dpolicy->granularity)
> > +			goto skip;
> > +
> > +		if (dc->state != D_PREP) {
> > +			list_move_tail(&dc->list, &dcc->fstrim_list);
> > +			goto skip;
> > +		}
> > +
> > +		__submit_discard_cmd(sbi, dpolicy, dc);
> > +
> > +		if (++issued >= dpolicy->max_requests) {
> > +			start = dc->lstart + dc->len;
> 
> cur = dc->lstart + dc->len;
> 
> > +
> > +			blk_finish_plug(&plug);
> > +			mutex_unlock(&dcc->cmd_lock);
> > +			__wait_all_discard_cmd(sbi, dpolicy);
> 
> How about moving __wait_discard_cmd_range(sbi, &dpolicy, start, end) from
> f2fs_trim_fs here, this can avoid wait non-fstrim discard commands.

The key here is to avoid any outstanding discard commands.

> 
> > +			congestion_wait(BLK_RW_ASYNC, HZ/50);
> 
> What about using msleep(dpolicy->min_interval) instead?

We need to detect congestion in block layer.

Thanks,

> 
> Thanks,
> 
> > +			goto next;
> > +		}
> > +skip:
> > +		node = rb_next(&dc->rb_node);
> > +		dc = rb_entry_safe(node, struct discard_cmd, rb_node);
> > +
> > +		if (fatal_signal_pending(current))
> > +			break;
> > +	}
> > +
> > +	blk_finish_plug(&plug);
> > +	mutex_unlock(&dcc->cmd_lock);
> > +}
> > +
> >  int f2fs_trim_fs(struct f2fs_sb_info *sbi, struct fstrim_range *range)
> >  {
> >  	__u64 start = F2FS_BYTES_TO_BLK(range->start);
> >