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Message-ID: <330c96f7-fbad-dd17-6368-f1378b3b5375@kernel.org>
Date:   Wed, 12 Jul 2023 09:33:45 +0800
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Subject: Re: [f2fs-dev] [PATCH v2] f2fs: do not issue small discard commands
 during checkpoint
Content-Language: en-US
To:     Jaegeuk Kim <jaegeuk@kernel.org>
Cc:     Daejun Park <daejun7.park@samsung.com>,
        linux-f2fs-devel@lists.sourceforge.net,
        linux-kernel@vger.kernel.org
References: <20230613203947.2745943-1-jaegeuk@kernel.org>
 <ZInmkgjDnAUD5Nk0@google.com>
 <50d5fa8c-4fe9-8a03-be78-0b5383e55b62@kernel.org>
 <ZKP6EJ5dZ4f4wScp@google.com>
 <65143701-4c19-ab66-1500-abd1162639cd@kernel.org>
 <ZKWovWZDiHjMavtB@google.com>
 <cadfb8d7-f5d0-a3ec-cafb-a0c06ad7d290@kernel.org>
 <ZK2FT9CUjxXvQ2K5@google.com>
From:   Chao Yu <chao@kernel.org>
In-Reply-To: <ZK2FT9CUjxXvQ2K5@google.com>
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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.

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,