Date:   Tue, 8 Aug 2023 08:50:18 +0800
From:   Chao Yu <chao@...nel.org>
To:     Jaegeuk Kim <jaegeuk@...nel.org>
Cc:     Daejun Park <daejun7.park@...sung.com>,
        linux-f2fs-devel@...ts.sourceforge.net,
        linux-kernel@...r.kernel.org
Subject: Re: [f2fs-dev] [PATCH v2] f2fs: do not issue small discard commands
 during checkpoint

On 2023/8/8 3:55, Jaegeuk Kim wrote:
> On 08/07, Chao Yu wrote:
>> On 2023/8/5 4:52, Jaegeuk Kim wrote:
>>> On 07/25, Chao Yu wrote:
>>>> 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@...nel.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@...sung.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?
>>>
>>> I didn't dig enough tho, don't think so. Somehow I got a loop as below which
>>> said the same LBA was issued and added back repeatedly, not seen this short log
>>> unfortunately.
>>>
>>>>
>>>> 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,
>>
>> [snip]
>>
>>>>>     f2fs_discard-25-752     [003] .....  9744.173111: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c0ca, blklen = 0x1
>>>>>     f2fs_discard-25-752     [004] .....  9744.175348: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c0ca, blklen = 0x1
>>
>> I don't see any loop via above log, am I missing something?
>>
>> The traces were printed in below call paths, and it printed the same LBAs as
>> expected?
> 
> I said the log was cut to show the loop enoughly. I'll try to reproduce this

Oh, I misunderstood it...

> later locally.

Alright.

Thanks,

> 
>>
>> - issue_discard_thread
>>   - __issue_discard_cmd
>>    - __submit_discard_cmd
>>     - trace_f2fs_issue_discard
>> 9744.173111: f2fs_issue_discard: dev = (254,51), blkstart = 0x18c0ca, blklen = 0x1
>>   - __wait_all_discard_cmd
>>    - __wait_discard_cmd_range
>>     - __remove_discard_cmd
>>      - trace_f2fs_remove_discard
>> 9744.175348: f2fs_remove_discard: dev = (254,51), blkstart = 0x18c0ca, blklen = 0x1
>>
>> Thanks,
>>
>>>>>
>>>>>>
>>>>>> 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@...nel.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@...nel.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,

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