Re: [f2fs-dev] [PATCH v2] f2fs: do not issue small discard commands during checkpoint

[Chao Yu <chao@kernel.org>]

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@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?
> 
> 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?

- 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@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,


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