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Date:   Mon, 10 Apr 2023 21:52:52 +0800
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To:     Jaegeuk Kim <jaegeuk@kernel.org>
Cc:     linux-f2fs-devel@lists.sourceforge.net,
        linux-kernel@vger.kernel.org
References: <20230324071028.336982-1-chao@kernel.org>
 <ZCyZGgf4RSEjyHTF@google.com>
 <a4e49177-3959-eb2b-996c-5d07b7390495@kernel.org>
 <ZC2aA+i5+HpdJ6M2@google.com>
From:   Chao Yu <chao@kernel.org>
Subject: Re: [PATCH] f2fs: fix to trigger a checkpoint in the end of
 foreground garbage collection
In-Reply-To: <ZC2aA+i5+HpdJ6M2@google.com>
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On 2023/4/5 23:55, Jaegeuk Kim wrote:
> On 04/05, Chao Yu wrote:
>> On 2023/4/5 5:39, Jaegeuk Kim wrote:
>>> Can we do like this?
>>>
>>>   From 9a58f0e59364241aa31b555cfe793d278e39b0dc Mon Sep 17 00:00:00 2001
>>> From: Jaegeuk Kim <jaegeuk@kernel.org>
>>> Date: Tue, 4 Apr 2023 14:36:00 -0700
>>> Subject: [PATCH] f2fs: do checkpoint when there's not enough free sections
>>>
>>> We didn't do checkpoint in FG_GC case, which may cause losing to reclaim prefree
>>> sctions in time.
>>>
>>> Fixes: 6f8d4455060d ("f2fs: avoid fi->i_gc_rwsem[WRITE] lock in f2fs_gc")
>>> Signed-off-by: Chao Yu <chao@kernel.org>
>>> Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>
>>> ---
>>>    fs/f2fs/gc.c | 24 +++++++++++-------------
>>>    1 file changed, 11 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/fs/f2fs/gc.c b/fs/f2fs/gc.c
>>> index 56c53dbe05c9..f1d0dd9c5a6c 100644
>>> --- a/fs/f2fs/gc.c
>>> +++ b/fs/f2fs/gc.c
>>> @@ -1806,6 +1806,7 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
>>>    	};
>>>    	unsigned int skipped_round = 0, round = 0;
>>>    	unsigned int upper_secs;
>>> +	bool stop_gc = false;
>>>    	trace_f2fs_gc_begin(sbi->sb, gc_type, gc_control->no_bg_gc,
>>>    				gc_control->nr_free_secs,
>>> @@ -1876,19 +1877,15 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
>>>    				(gc_type == FG_GC) ? sec_freed : 0, 0)) {
>>>    		if (gc_type == FG_GC && sec_freed < gc_control->nr_free_secs)
>>>    			goto go_gc_more;
>>> -		goto stop;
>>> -	}
>>> -
>>> -	/* FG_GC stops GC by skip_count */
>>> -	if (gc_type == FG_GC) {
>>> +		stop_gc = true;
>>
>> I guess below condition is for emergency recycle of prefree segments during
>> foreground GC, in order to avoid exhausting free sections due to to many
>> metadata allocation during CP.
>>
>> 	if (free_sections(sbi) <= upper_secs + NR_GC_CHECKPOINT_SECS &&
>> 				prefree_segments(sbi)) {
>>
>> But for common case, free_sections() is close to reserved_segments(), and
>> upper_secs + NR_GC_CHECKPOINT_SECS value may be far smaller than free_sections(),
>> so checkpoint may not be trggered as expected, IIUC.
>>
>> So it's fine to just trigger CP in the end of foreground garbage collection?
> 
> My major concern is to avoid unnecessary checkpointing given multiple FG_GC
> requests were pending in parallel. And, I don't want to add so many combination
> which gives so many corner cases, and feel f2fs_gc() needs to call checkpoint
> automatically in the worst case scenario only.

Alright.

> 
> By the way, do we just need to call checkpoint here including FG_GC as well?

I didn't get it, do you mean?

- f2fs_balance_fs()
  - f2fs_gc() creates prefree segments but not call checkpoint to reclaim

- f2fs_balance_fs()
  - f2fs_gc()
   - detect prefree segments created by last f2fs_balance_fs, then call
f2fs_write_checkpoint to reclaim

Or could you please provide a draft patch? :-P

Thanks,

> 
> 1832
> 1833         if (gc_type == BG_GC && has_not_enough_free_secs(sbi, 0, 0)) {
> 1834                 /*
> 1835                  * For example, if there are many prefree_segments below given
> 1836                  * threshold, we can make them free by checkpoint. Then, we
> 1837                  * secure free segments which doesn't need fggc any more.
> 1838                  */
> 1839                 if (prefree_segments(sbi)) {
> 1840                         ret = f2fs_write_checkpoint(sbi, &cpc);
> 1841                         if (ret)
> 1842                                 goto stop;
> 1843                 }
> 1844                 if (has_not_enough_free_secs(sbi, 0, 0))
> 1845                         gc_type = FG_GC;
> 1846         }
> 
>>
>> One other concern is for those path as below:
>> - disable_checkpoint
>> - ioc_gc
>> - ioc_gc_range
>> - ioc_resize
>> ...
> 
> I think the upper caller should decide to call checkpoint, if they want to
> reclaim the prefree likewise f2fs_disable_checkpoint.
> 
>>
>> We've passed gc_type as FG_GC, but the demand here is to migrate block in time,
>> rather than dirtying blocks, and callers don't expect checkpoint in f2fs_gc(),
>> instead the callers will do the checkpoit as it needs.
>>
>> That means it's better to decouple FG_GC and write_checkpoint behavior, so I
>> added another parameter .reclaim_space to just let f2fs_balance_fs() to trigger
>> checkpoit in the end of f2fs_gc().
> 
>>
>> Thanks,
>>
>>> +	} else if (gc_type == FG_GC) {
>>> +		/* FG_GC stops GC by skip_count */
>>>    		if (sbi->skipped_gc_rwsem)
>>>    			skipped_round++;
>>>    		round++;
>>>    		if (skipped_round > MAX_SKIP_GC_COUNT &&
>>> -				skipped_round * 2 >= round) {
>>> -			ret = f2fs_write_checkpoint(sbi, &cpc);
>>> -			goto stop;
>>> -		}
>>> +				skipped_round * 2 >= round)
>>> +			stop_gc = true;
>>>    	}
>>>    	__get_secs_required(sbi, NULL, &upper_secs, NULL);
>>> @@ -1901,12 +1898,13 @@ int f2fs_gc(struct f2fs_sb_info *sbi, struct f2fs_gc_control *gc_control)
>>>    				prefree_segments(sbi)) {
>>>    		ret = f2fs_write_checkpoint(sbi, &cpc);
>>>    		if (ret)
>>> -			goto stop;
>>> +			stop_gc = true;
>>>    	}
>>>    go_gc_more:
>>> -	segno = NULL_SEGNO;
>>> -	goto gc_more;
>>> -
>>> +	if (!stop_gc) {
>>> +		segno = NULL_SEGNO;
>>> +		goto gc_more;
>>> +	}
>>>    stop:
>>>    	SIT_I(sbi)->last_victim[ALLOC_NEXT] = 0;
>>>    	SIT_I(sbi)->last_victim[FLUSH_DEVICE] = gc_control->victim_segno;