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Date:   Thu, 23 Feb 2017 10:19:50 +0800
From:   Chao Yu <yuchao0@...wei.com>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
CC:     <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
        <linux-f2fs-devel@...ts.sourceforge.net>
Subject: Re: [f2fs-dev] [PATCH 3/6] f2fs: remove batched discard in
 f2fs_trim_fs

On 2017/2/23 5:55, Jaegeuk Kim wrote:
> On 02/22, Chao Yu wrote:
>> On 2017/1/13 6:44, Jaegeuk Kim wrote:
>>> We don't need to do multiple checkpoints, since we don't actually wait for
>>> completion of discard commands during checkpoint.
>>> Instead, we still need to avoid very big discard commands, since that large
>>> discard can interfere block allocation.
>>
>> I hope we can keep this functionality and related sysfs interface, the main
>> concern is that: for 16T size fragmented partition, at most there will be
>> ~sizeof(u32)/2 4K blocks which could be invalid and needed to be discarded.
>> After removal of in-batch discard, we have to handle billions of discard message
>> with gc_mutex & cp_rwsem held, which will freeze f2fs for very long time.
> 
> The baseline of this patch is that we have a kernel thread to issue discard
> commands asynchronously. So, I don't expect such the huge latency anymore.

Yes, I know async kernel thread was already introduced, and we can expect such
implementation can help to reduce huge handling latency. But still in checkpoint
we need to check region of invalid block and send the pending discard entries to
kernel thread, so what I concern is that in scenario of there is huge number of
discard messages, we will still face long latency with cp lock held.

Thanks,

> 
> Thanks,
> 
>>
>> So I suggest we just set trim_sections to 512 segments (covers one GB) by
>> default. How do you think?
>>
>> Thanks,
>>
>>>
>>> Signed-off-by: Jaegeuk Kim <jaegeuk@...nel.org>
>>> ---
>>>  Documentation/ABI/testing/sysfs-fs-f2fs |  6 ------
>>>  fs/f2fs/f2fs.h                          |  9 +-------
>>>  fs/f2fs/segment.c                       | 38 +++++++++++----------------------
>>>  fs/f2fs/super.c                         |  2 --
>>>  4 files changed, 14 insertions(+), 41 deletions(-)
>>>
>>> diff --git a/Documentation/ABI/testing/sysfs-fs-f2fs b/Documentation/ABI/testing/sysfs-fs-f2fs
>>> index a809f6005f14..df6b3f6e164a 100644
>>> --- a/Documentation/ABI/testing/sysfs-fs-f2fs
>>> +++ b/Documentation/ABI/testing/sysfs-fs-f2fs
>>> @@ -75,12 +75,6 @@ Contact:	"Jaegeuk Kim" <jaegeuk.kim@...sung.com>
>>>  Description:
>>>  		 Controls the memory footprint used by f2fs.
>>>  
>>> -What:		/sys/fs/f2fs/<disk>/trim_sections
>>> -Date:		February 2015
>>> -Contact:	"Jaegeuk Kim" <jaegeuk@...nel.org>
>>> -Description:
>>> -		 Controls the trimming rate in batch mode.
>>> -
>>>  What:		/sys/fs/f2fs/<disk>/cp_interval
>>>  Date:		October 2015
>>>  Contact:	"Jaegeuk Kim" <jaegeuk@...nel.org>
>>> diff --git a/fs/f2fs/f2fs.h b/fs/f2fs/f2fs.h
>>> index 548e75d18ec1..a2850bf2a487 100644
>>> --- a/fs/f2fs/f2fs.h
>>> +++ b/fs/f2fs/f2fs.h
>>> @@ -128,11 +128,7 @@ enum {
>>>  	CP_DISCARD,
>>>  };
>>>  
>>> -#define DEF_BATCHED_TRIM_SECTIONS	2
>>> -#define BATCHED_TRIM_SEGMENTS(sbi)	\
>>> -		(SM_I(sbi)->trim_sections * (sbi)->segs_per_sec)
>>> -#define BATCHED_TRIM_BLOCKS(sbi)	\
>>> -		(BATCHED_TRIM_SEGMENTS(sbi) << (sbi)->log_blocks_per_seg)
>>> +#define MAX_DISCARD_BLOCKS(sbi) (1 << (sbi)->log_blocks_per_seg)
>>>  #define DEF_CP_INTERVAL			60	/* 60 secs */
>>>  #define DEF_IDLE_INTERVAL		5	/* 5 secs */
>>>  
>>> @@ -638,9 +634,6 @@ struct f2fs_sm_info {
>>>  	int nr_discards;			/* # of discards in the list */
>>>  	int max_discards;			/* max. discards to be issued */
>>>  
>>> -	/* for batched trimming */
>>> -	unsigned int trim_sections;		/* # of sections to trim */
>>> -
>>>  	struct list_head sit_entry_set;	/* sit entry set list */
>>>  
>>>  	unsigned int ipu_policy;	/* in-place-update policy */
>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>> index 73282108fa33..e6f3c6db7616 100644
>>> --- a/fs/f2fs/segment.c
>>> +++ b/fs/f2fs/segment.c
>>> @@ -823,7 +823,8 @@ static void __add_discard_entry(struct f2fs_sb_info *sbi,
>>>  	if (!list_empty(head)) {
>>>  		last = list_last_entry(head, struct discard_entry, list);
>>>  		if (START_BLOCK(sbi, cpc->trim_start) + start ==
>>> -						last->blkaddr + last->len) {
>>> +				last->blkaddr + last->len &&
>>> +				last->len <= MAX_DISCARD_BLOCKS(sbi)) {
>>>  			last->len += end - start;
>>>  			goto done;
>>>  		}
>>> @@ -1513,36 +1514,25 @@ int f2fs_trim_fs(struct f2fs_sb_info *sbi, struct fstrim_range *range)
>>>  			"Found FS corruption, run fsck to fix.");
>>>  		goto out;
>>>  	}
>>> +	if (sbi->discard_blks == 0)
>>> +		goto out;
>>>  
>>>  	/* start/end segment number in main_area */
>>>  	start_segno = (start <= MAIN_BLKADDR(sbi)) ? 0 : GET_SEGNO(sbi, start);
>>>  	end_segno = (end >= MAX_BLKADDR(sbi)) ? MAIN_SEGS(sbi) - 1 :
>>>  						GET_SEGNO(sbi, end);
>>> +	/*
>>> +	 * do checkpoint to issue discard commands safely since we now can
>>> +	 * use async discard.
>>> +	 */
>>>  	cpc.reason = CP_DISCARD;
>>>  	cpc.trim_minlen = max_t(__u64, 1, F2FS_BYTES_TO_BLK(range->minlen));
>>> +	cpc.trim_start = start_segno;
>>> +	cpc.trim_end = end_segno;
>>>  
>>> -	/* do checkpoint to issue discard commands safely */
>>> -	for (; start_segno <= end_segno; start_segno = cpc.trim_end + 1) {
>>> -		cpc.trim_start = start_segno;
>>> -
>>> -		if (sbi->discard_blks == 0)
>>> -			break;
>>> -		else if (sbi->discard_blks < BATCHED_TRIM_BLOCKS(sbi))
>>> -			cpc.trim_end = end_segno;
>>> -		else
>>> -			cpc.trim_end = min_t(unsigned int,
>>> -				rounddown(start_segno +
>>> -				BATCHED_TRIM_SEGMENTS(sbi),
>>> -				sbi->segs_per_sec) - 1, end_segno);
>>> -
>>> -		mutex_lock(&sbi->gc_mutex);
>>> -		err = write_checkpoint(sbi, &cpc);
>>> -		mutex_unlock(&sbi->gc_mutex);
>>> -		if (err)
>>> -			break;
>>> -
>>> -		schedule();
>>> -	}
>>> +	mutex_lock(&sbi->gc_mutex);
>>> +	err = write_checkpoint(sbi, &cpc);
>>> +	mutex_unlock(&sbi->gc_mutex);
>>>  out:
>>>  	range->len = F2FS_BLK_TO_BYTES(cpc.trimmed);
>>>  	return err;
>>> @@ -2708,8 +2698,6 @@ int build_segment_manager(struct f2fs_sb_info *sbi)
>>>  	sm_info->nr_discards = 0;
>>>  	sm_info->max_discards = 0;
>>>  
>>> -	sm_info->trim_sections = DEF_BATCHED_TRIM_SECTIONS;
>>> -
>>>  	INIT_LIST_HEAD(&sm_info->sit_entry_set);
>>>  
>>>  	if (test_opt(sbi, FLUSH_MERGE) && !f2fs_readonly(sbi->sb)) {
>>> diff --git a/fs/f2fs/super.c b/fs/f2fs/super.c
>>> index 461c29043aec..37d40d8aa9c4 100644
>>> --- a/fs/f2fs/super.c
>>> +++ b/fs/f2fs/super.c
>>> @@ -284,7 +284,6 @@ F2FS_RW_ATTR(GC_THREAD, f2fs_gc_kthread, gc_no_gc_sleep_time, no_gc_sleep_time);
>>>  F2FS_RW_ATTR(GC_THREAD, f2fs_gc_kthread, gc_idle, gc_idle);
>>>  F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, reclaim_segments, rec_prefree_segments);
>>>  F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, max_small_discards, max_discards);
>>> -F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, batched_trim_sections, trim_sections);
>>>  F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, ipu_policy, ipu_policy);
>>>  F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, min_ipu_util, min_ipu_util);
>>>  F2FS_RW_ATTR(SM_INFO, f2fs_sm_info, min_fsync_blocks, min_fsync_blocks);
>>> @@ -309,7 +308,6 @@ static struct attribute *f2fs_attrs[] = {
>>>  	ATTR_LIST(gc_idle),
>>>  	ATTR_LIST(reclaim_segments),
>>>  	ATTR_LIST(max_small_discards),
>>> -	ATTR_LIST(batched_trim_sections),
>>>  	ATTR_LIST(ipu_policy),
>>>  	ATTR_LIST(min_ipu_util),
>>>  	ATTR_LIST(min_fsync_blocks),
>>>
>>
>>
>> ------------------------------------------------------------------------------
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>> _______________________________________________
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>> Linux-f2fs-devel@...ts.sourceforge.net
>> https://lists.sourceforge.net/lists/listinfo/linux-f2fs-devel
> 
> .
> 

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