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Date:   Tue, 25 Jul 2023 21:18:34 +0800
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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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From:   Chao Yu <chao@kernel.org>
Subject: Re: [f2fs-dev] [PATCH v2] f2fs: do not issue small discard commands
 during checkpoint
In-Reply-To: <ZLrpMAaOkmrKQFv+@google.com>
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On 2023/7/22 4:23, Jaegeuk Kim wrote:
> On 07/13, Chao Yu wrote:
>> 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.
> 
> Ok, it seems what I encountered before was adding this small discard even
> after issuing it by checkpoint. Thoughts?

Do you mean: in f2fs_clear_prefree_segments(), small discard may overlap
segment granularity discard?

e.g.
- f2fs_clear_prefree_segments
  - f2fs_issue_discard(0, 512)  --- segment granularity discard
  - f2fs_issue_discard(0, 1)  --- small discard
  - f2fs_issue_discard(5, 1)  --- small discard

Thanks,

> 
> node=0], cp [data=0, node=0, meta=0], app [read=0 (direct=0, buffered=0), mapped=0], compr(buffered=0, mapped=0)], fs [data=0, (gc_data=0, cdata=0), node=0, meta=0]
>   f2fs_discard-25-752     [003] .....  9744.173085: f2fs_iostat_latency: dev = (254,51), iotype [peak lat.(ms)/avg lat.(ms)/count], rd_data [0/0/0], rd_node [0/0/0], rd_meta [0/0/0], wr_sync_data [0/0/0], wr_sync_node [0/0/0], wr_sync_meta [0/0/0], wr_async_data [0/0/0], wr_async_node [0/0/0], wr_async_meta [0/0/0]
>   f2fs_discard-25-752     [003] .....  9744.173089: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c0c6, blklen = 0x1
>   f2fs_discard-25-752     [003] .....  9744.173111: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c0ca, blklen = 0x1
>   f2fs_discard-25-752     [003] .....  9744.173126: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c116, blklen = 0x1
>   f2fs_discard-25-752     [003] .....  9744.173140: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c121, blklen = 0x1
>   f2fs_discard-25-752     [003] .....  9744.173154: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c15b, blklen = 0x1
>   f2fs_discard-25-752     [003] .....  9744.173169: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c16e, blklen = 0x1
>   f2fs_discard-25-752     [003] .....  9744.173183: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c181, blklen = 0x1
>   f2fs_discard-25-752     [004] .....  9744.175272: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c0a9, blklen = 0x1
>   f2fs_discard-25-752     [004] .....  9744.175345: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c0c6, blklen = 0x1
>   f2fs_discard-25-752     [004] .....  9744.175348: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c0ca, blklen = 0x1
>   f2fs_discard-25-752     [004] .....  9744.175352: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c116, blklen = 0x1
>   f2fs_discard-25-752     [004] .....  9744.175354: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c121, blklen = 0x1
>   f2fs_discard-25-752     [004] .....  9744.175360: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c15b, blklen = 0x1
>   f2fs_discard-25-752     [004] .....  9744.175362: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c16e, blklen = 0x1
>   f2fs_discard-25-752     [004] .....  9744.175367: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c181, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.228775: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c197, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.228950: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c1ad, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.228965: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c1b1, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.228994: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c1b3, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.229006: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c27a, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.229017: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c2a3, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.229028: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c2ab, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.229041: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c304, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.230811: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c197, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.230926: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c1ad, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.230929: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c1b1, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.230932: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c1b3, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.230940: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c27a, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.230945: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c2a3, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.230947: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c2ab, blklen = 0x1
>   f2fs_discard-25-752     [002] .....  9744.230952: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c304, blklen = 0x1
> 
>>
>> 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,