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Date:   Thu, 13 Jul 2023 09:31:44 +0800
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From:   Chao Yu <chao@kernel.org>
Subject: Re: [f2fs-dev] [PATCH v2] f2fs: do not issue small discard commands
 during checkpoint
To:     Jaegeuk Kim <jaegeuk@kernel.org>
Cc:     Daejun Park <daejun7.park@samsung.com>,
        linux-f2fs-devel@lists.sourceforge.net,
        linux-kernel@vger.kernel.org
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On 2023/7/12 23:55, Jaegeuk Kim wrote:
> On 07/12, Chao Yu wrote:
>> On 2023/7/12 0:37, Jaegeuk Kim wrote:
>>> On 07/06, Chao Yu wrote:
>>>> On 2023/7/6 1:30, Jaegeuk Kim wrote:
>>>>> On 07/04, Chao Yu wrote:
>>>>>> On 2023/7/4 18:53, Jaegeuk Kim wrote:
>>>>>>> On 07/03, Chao Yu wrote:
>>>>>>>> On 2023/6/15 0:10, Jaegeuk Kim wrote:
>>>>>>>>> If there're huge # of small discards, this will increase checkpoint latency
>>>>>>>>> insanely. Let's issue small discards only by trim.
>>>>>>>>>
>>>>>>>>> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
>>>>>>>>> ---
>>>>>>>>>
>>>>>>>>>       Change log from v1:
>>>>>>>>>        - move the skip logic to avoid dangling objects
>>>>>>>>>
>>>>>>>>>       fs/f2fs/segment.c | 2 +-
>>>>>>>>>       1 file changed, 1 insertion(+), 1 deletion(-)
>>>>>>>>>
>>>>>>>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>>>>>>>> index 8c7af8b4fc47..0457d620011f 100644
>>>>>>>>> --- a/fs/f2fs/segment.c
>>>>>>>>> +++ b/fs/f2fs/segment.c
>>>>>>>>> @@ -2193,7 +2193,7 @@ void f2fs_clear_prefree_segments(struct f2fs_sb_info *sbi,
>>>>>>>>>       			len = next_pos - cur_pos;
>>>>>>>>>       			if (f2fs_sb_has_blkzoned(sbi) ||
>>>>>>>>> -			    (force && len < cpc->trim_minlen))
>>>>>>>>> +					!force || len < cpc->trim_minlen)
>>>>>>>>>       				goto skip;
>>>>>>>>
>>>>>>>> Sorry for late reply.
>>>>>>>>
>>>>>>>> We have a configuration for such case, what do you think of setting
>>>>>>>> max_small_discards to zero? otherwise, w/ above change, max_small_discards
>>>>>>>> logic may be broken?
>>>>>>>>
>>>>>>>> What:           /sys/fs/f2fs/<disk>/max_small_discards
>>>>>>>> Date:           November 2013
>>>>>>>> Contact:        "Jaegeuk Kim" <jaegeuk.kim@samsung.com>
>>>>>>>> Description:    Controls the issue rate of discard commands that consist of small
>>>>>>>>                     blocks less than 2MB. The candidates to be discarded are cached until
>>>>>>>>                     checkpoint is triggered, and issued during the checkpoint.
>>>>>>>>                     By default, it is disabled with 0.
>>>>>>>>
>>>>>>>> Or, if we prefer to disable small_discards by default, what about below change:
>>>>>>>
>>>>>>> I think small_discards is fine, but need to avoid long checkpoint latency only.
>>>>>>
>>>>>> I didn't get you, do you mean we can still issue small discard by
>>>>>> fstrim, so small_discards functionality is fine?
>>>>>
>>>>> You got the point.
>>>>
>>>> Well, actually, what I mean is max_small_discards sysfs entry's functionality
>>>> is broken. Now, the entry can not be used to control number of small discards
>>>> committed by checkpoint.
>>>
>>> Could you descrbie this problem first?
>>
>> Oh, alright, actually, I've described this problem literally, but maybe it's not
>> clear, let me give some examples as below:
>>
>> echo 0 > /sys/fs/f2fs/vdb/max_small_discards
>> xfs_io -f /mnt/f2fs/file -c "pwrite 0 2m" -c "fsync"
>> xfs_io /mnt/f2fs/file -c "fpunch 0 4k"
>> sync
>> cat /proc/fs/f2fs/vdb/discard_plist_info |head -2
>>
>> echo 100 > /sys/fs/f2fs/vdb/max_small_discards
>> rm /mnt/f2fs/file
>> xfs_io -f /mnt/f2fs/file -c "pwrite 0 2m" -c "fsync"
>> xfs_io /mnt/f2fs/file -c "fpunch 0 4k"
>> sync
>> cat /proc/fs/f2fs/vdb/discard_plist_info |head -2
>>
>> Before the patch:
>>
>> Discard pend list(Show diacrd_cmd count on each entry, .:not exist):
>>    0         .       .       .       .       .       .       .       .
>>
>> Discard pend list(Show diacrd_cmd count on each entry, .:not exist):
>>    0         3       1       .       .       .       .       .       .
>>
>> After the patch:
>> Discard pend list(Show diacrd_cmd count on each entry, .:not exist):
>>    0         .       .       .       .       .       .       .       .
>>
>> Discard pend list(Show diacrd_cmd count on each entry, .:not exist):
>>    0         .       .       .       .       .       .       .       .
>>
>> So, now max_small_discards can not be used to control small discard number
>> cached by checkpoint.

Let me explain more:

Previously, we have two mechanisms to cache & submit small discards:

a) set max small discard number in /sys/fs/f2fs/vdb/max_small_discards, and checkpoint
will cache small discard candidates w/ configured maximum number.

b) call FITRIM ioctl, also, checkpoint in f2fs_trim_fs() will cache small discard
candidates w/ configured discard granularity, but w/o limitation of number. FSTRIM
interface is asynchronized, so it won't submit discard directly.

Finally, discard thread will submit them in background periodically.

So what I mean is the mechanism a) is broken, since no matter how we configure the
sysfs entry /sys/fs/f2fs/vdb/max_small_discards, checkpoint will not cache small
discard candidates any more.

So, it needs to fix max_small_discards sysfs functionality? or just drop the
functionality?

> 
> Since we do not submit small discards anymore during checkpoint. Why not relying
> on the discard thread to issue them?

Sorry, I'm not sure I get your point, do you mean max_small_discards functionality
is obsoleted, so it recommended to use fstrim to cache & submit small discards?

Let me know, if I'm missing something or misunderstanding the point.

Thanks,

> 
>>
>> Thanks,
>>
>>>
>>>>
>>>> I think there is another way to achieve "avoid long checkpoint latency caused
>>>> by committing huge # of small discards", the way is we can set max_small_discards
>>>> to small value or zero, w/ such configuration, it will take checkpoint much less
>>>> time or no time to committing small discard due to below control logic:
>>>>
>>>> f2fs_flush_sit_entries()
>>>> {
>>>> ...
>>>> 			if (!(cpc->reason & CP_DISCARD)) {
>>>> 				cpc->trim_start = segno;
>>>> 				add_discard_addrs(sbi, cpc, false);
>>>> 			}
>>>> ...
>>>> }
>>>>
>>>> add_discard_addrs()
>>>> {
>>>> ...
>>>> 	while (force || SM_I(sbi)->dcc_info->nr_discards <=
>>>> 				SM_I(sbi)->dcc_info->max_discards) {
>>>>
>>>> It will break the loop once nr_discards is larger than max_discards, if
>>>> max_discards is set to zero, checkpoint won't take time to handle small discards.
>>>>
>>>> ...
>>>> 		if (!de) {
>>>> 			de = f2fs_kmem_cache_alloc(discard_entry_slab,
>>>> 						GFP_F2FS_ZERO, true, NULL);
>>>> 			de->start_blkaddr = START_BLOCK(sbi, cpc->trim_start);
>>>> 			list_add_tail(&de->list, head);
>>>> 		}
>>>> ...
>>>> 	}
>>>> ...
>>>>
>>>> Thanks,
>>>>
>>>>>
>>>>>>
>>>>>> Thanks,
>>>>>>
>>>>>>>
>>>>>>>>
>>>>>>>>     From eb89d9b56e817e3046d7fa17165b12416f09d456 Mon Sep 17 00:00:00 2001
>>>>>>>> From: Chao Yu <chao@kernel.org>
>>>>>>>> Date: Mon, 3 Jul 2023 09:06:53 +0800
>>>>>>>> Subject: [PATCH] Revert "f2fs: enable small discard by default"
>>>>>>>>
>>>>>>>> This reverts commit d618ebaf0aa83d175658aea5291e0c459d471d39 in order
>>>>>>>> to disable small discard by default, so that if there're huge number of
>>>>>>>> small discards, it will decrease checkpoint's latency obviously.
>>>>>>>>
>>>>>>>> Also, this patch reverts 9ac00e7cef10 ("f2fs: do not issue small discard
>>>>>>>> commands during checkpoint"), due to it breaks small discard feature which
>>>>>>>> may be configured via sysfs entry max_small_discards.
>>>>>>>>
>>>>>>>> Fixes: 9ac00e7cef10 ("f2fs: do not issue small discard commands during checkpoint")
>>>>>>>> Signed-off-by: Chao Yu <chao@kernel.org>
>>>>>>>> ---
>>>>>>>>      fs/f2fs/segment.c | 4 ++--
>>>>>>>>      1 file changed, 2 insertions(+), 2 deletions(-)
>>>>>>>>
>>>>>>>> diff --git a/fs/f2fs/segment.c b/fs/f2fs/segment.c
>>>>>>>> index 14c822e5c9c9..0a313368f18b 100644
>>>>>>>> --- a/fs/f2fs/segment.c
>>>>>>>> +++ b/fs/f2fs/segment.c
>>>>>>>> @@ -2193,7 +2193,7 @@ void f2fs_clear_prefree_segments(struct f2fs_sb_info *sbi,
>>>>>>>>      			len = next_pos - cur_pos;
>>>>>>>>
>>>>>>>>      			if (f2fs_sb_has_blkzoned(sbi) ||
>>>>>>>> -					!force || len < cpc->trim_minlen)
>>>>>>>> +			    (force && len < cpc->trim_minlen))
>>>>>>>>      				goto skip;
>>>>>>>>
>>>>>>>>      			f2fs_issue_discard(sbi, entry->start_blkaddr + cur_pos,
>>>>>>>> @@ -2269,7 +2269,7 @@ static int create_discard_cmd_control(struct f2fs_sb_info *sbi)
>>>>>>>>      	atomic_set(&dcc->queued_discard, 0);
>>>>>>>>      	atomic_set(&dcc->discard_cmd_cnt, 0);
>>>>>>>>      	dcc->nr_discards = 0;
>>>>>>>> -	dcc->max_discards = MAIN_SEGS(sbi) << sbi->log_blocks_per_seg;
>>>>>>>> +	dcc->max_discards = 0;
>>>>>>>>      	dcc->max_discard_request = DEF_MAX_DISCARD_REQUEST;
>>>>>>>>      	dcc->min_discard_issue_time = DEF_MIN_DISCARD_ISSUE_TIME;
>>>>>>>>      	dcc->mid_discard_issue_time = DEF_MID_DISCARD_ISSUE_TIME;
>>>>>>>> -- 
>>>>>>>> 2.40.1
>>>>>>>>
>>>>>>>>
>>>>>>>>
>>>>>>>>>       			f2fs_issue_discard(sbi, entry->start_blkaddr + cur_pos,