Re: [PATCH 2/2] f2fs: issue small discard by LBA order

[Jaegeuk Kim]

On 07/05, Chao Yu wrote:
> For small granularity discard which size is smaller than 64KB, if we
> issue those kind of discards orderly by size, their IOs will be spread
> into entire logical address, so that in FTL, L2P table will be updated
> randomly, result bad wear rate in the table.
> 
> In this patch, we choose to issue small discard by LBA order, by this
> way, we can expect that L2P table updates from adjacent discard IOs can
> be merged in the cache, so it can reduce lifetime wearing of flash.
> 
> Signed-off-by: Chao Yu <yuchao0@xxxxxxxxxx>
> ---
>  fs/f2fs/f2fs.h    |  2 ++
>  fs/f2fs/segment.c | 71 +++++++++++++++++++++++++++++++++++++++++++++++
>  2 files changed, 73 insertions(+)
> 
> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
> index 47ac0a9b022f..8d592029328a 100644
> --- a/fs/f2fs/f2fs.h
> +++ b/fs/f2fs/f2fs.h
> @@ -290,6 +290,7 @@ struct discard_policy {
>  	unsigned int io_aware_gran;	/* minimum granularity discard not be aware of I/O */
>  	bool io_aware;			/* issue discard in idle time */
>  	bool sync;			/* submit discard with REQ_SYNC flag */
> +	bool ordered;			/* issue discard by lba order */
>  	unsigned int granularity;	/* discard granularity */
>  };
>  
> @@ -306,6 +307,7 @@ struct discard_cmd_control {
>  	unsigned int max_discards;		/* max. discards to be issued */
>  	unsigned int discard_granularity;	/* discard granularity */
>  	unsigned int undiscard_blks;		/* # of undiscard blocks */
> +	unsigned int next_pos;			/* next discard position */
>  	atomic_t issued_discard;		/* # of issued discard */
>  	atomic_t issing_discard;		/* # of issing discard */
>  	atomic_t discard_cmd_cnt;		/* # of cached cmd count */
> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
> index f95bf618bc1e..df0d91dfb8ac 100644
> --- a/fs/f2fs/segment.c
> +++ b/fs/f2fs/segment.c
> @@ -18,6 +18,7 @@
>  #include <linux/timer.h>
>  #include <linux/freezer.h>
>  #include <linux/sched/signal.h>
> +#include <linux/delay.h>

Why?

>  
>  #include "f2fs.h"
>  #include "segment.h"
> @@ -936,6 +937,7 @@ static void __init_discard_policy(struct f2fs_sb_info *sbi,
>  	/* common policy */
>  	dpolicy->type = discard_type;
>  	dpolicy->sync = true;
> +	dpolicy->ordered = false;
>  	dpolicy->granularity = granularity;
>  
>  	dpolicy->max_requests = DEF_MAX_DISCARD_REQUEST;
> @@ -947,6 +949,7 @@ static void __init_discard_policy(struct f2fs_sb_info *sbi,
>  		dpolicy->max_interval = DEF_MAX_DISCARD_ISSUE_TIME;
>  		dpolicy->io_aware = true;
>  		dpolicy->sync = false;
> +		dpolicy->ordered = true;
>  		if (utilization(sbi) > DEF_DISCARD_URGENT_UTIL) {
>  			dpolicy->granularity = 1;
>  			dpolicy->max_interval = DEF_MIN_DISCARD_ISSUE_TIME;
> @@ -1202,6 +1205,69 @@ static int __queue_discard_cmd(struct f2fs_sb_info *sbi,
>  	return 0;
>  }
>  
> +static unsigned int __issue_discard_cmd_orderly(struct f2fs_sb_info *sbi,
> +					struct discard_policy *dpolicy)
> +{
> +	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;
> +	unsigned int pos = dcc->next_pos;
> +	unsigned int issued = 0, iter = 0;
> +	bool io_interrupted;
> +
> +	mutex_lock(&dcc->cmd_lock);
> +	dc = (struct discard_cmd *)f2fs_lookup_rb_tree_ret(&dcc->root,
> +					NULL, pos,
> +					(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) {
> +		struct rb_node *node;
> +
> +		if (dc->state != D_PREP)
> +			goto next;
> +retry:
> +		io_interrupted = false;
> +
> +		if (dpolicy->io_aware && !is_idle(sbi)) {
> +			io_interrupted = true;
> +			goto skip;

Please don't try, if user is doing something.

> +		}
> +
> +		__submit_discard_cmd(sbi, dpolicy, dc);
> +		issued++;
> +		dcc->next_pos = dc->lstart + dc->len;
> +skip:
> +		if (++iter >= dpolicy->max_requests)
> +			break;
> +
> +		if (io_interrupted)
> +			goto retry;
> +next:
> +		node = rb_next(&dc->rb_node);
> +		dc = rb_entry_safe(node, struct discard_cmd, rb_node);
> +	}
> +
> +	blk_finish_plug(&plug);
> +
> +	if (!dc)
> +		dcc->next_pos = 0;
> +
> +	mutex_unlock(&dcc->cmd_lock);
> +
> +	if (!issued && io_interrupted)
> +		issued = -1;
> +
> +	return issued;
> +}
> +
>  static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>  					struct discard_policy *dpolicy)
>  {
> @@ -1215,6 +1281,10 @@ static int __issue_discard_cmd(struct f2fs_sb_info *sbi,
>  	for (i = MAX_PLIST_NUM - 1; i >= 0; i--) {
>  		if (i + 1 < dpolicy->granularity)
>  			break;
> +
> +		if (i < DEFAULT_DISCARD_GRANULARITY && dpolicy->ordered)
> +			return __issue_discard_cmd_orderly(sbi, dpolicy);

So, at this moment, we usually expect there'd be a bunch of small candidates
only, and thus, it'd be better to issue small chunks in LBA order?

> +
>  		pend_list = &dcc->pend_list[i];
>  
>  		mutex_lock(&dcc->cmd_lock);
> @@ -1786,6 +1856,7 @@ static int create_discard_cmd_control(struct f2fs_sb_info *sbi)
>  	dcc->nr_discards = 0;
>  	dcc->max_discards = MAIN_SEGS(sbi) << sbi->log_blocks_per_seg;
>  	dcc->undiscard_blks = 0;
> +	dcc->next_pos = 0;
>  	dcc->root = RB_ROOT;
>  	dcc->rbtree_check = false;
>  
> -- 
> 2.18.0.rc1