Re: [PATCH 5/6] btrfs: simplify btrfs_select_ref_head and cleanup some local variables

[Lu Fengqi <lufq.fnst@cn.fujitsu.com>]

On Thu, Oct 11, 2018 at 09:40:52AM +0300, Nikolay Borisov wrote:
>
>
>On 11.10.2018 08:40, Lu Fengqi wrote:
>> If the return value of find_ref_head() is NULL, the only possibility is
>> that delayed_refs' head ref rbtree is empty. Hence, the second
>> find_ref_head() is pointless.
>> > Besides, the local variables loop and start are unnecessary, just remove
>> them.
>
>So the objective of that function is to get a reference to the first
>delayed head which is not processed. This is done by essentially keeping
>track of the last range that was processed in
>delayed_refs->run_delayed_start
>> 
>> Signed-off-by: Lu Fengqi <lufq.fnst@cn.fujitsu.com>
>> ---
>>  fs/btrfs/delayed-ref.c | 17 +++--------------
>>  1 file changed, 3 insertions(+), 14 deletions(-)
>> 
>> diff --git a/fs/btrfs/delayed-ref.c b/fs/btrfs/delayed-ref.c
>> index 885581852bea..2726d2fb4bbe 100644
>> --- a/fs/btrfs/delayed-ref.c
>> +++ b/fs/btrfs/delayed-ref.c
>> @@ -354,20 +354,11 @@ struct btrfs_delayed_ref_head *
>>  btrfs_select_ref_head(struct btrfs_delayed_ref_root *delayed_refs)
>>  {
>>  	struct btrfs_delayed_ref_head *head;
>> -	u64 start;
>> -	bool loop = false;
>>  
>>  again:
>> -	start = delayed_refs->run_delayed_start;
>> -	head = find_ref_head(delayed_refs, start, 1);
>> -	if (!head && !loop) {
>> +	head = find_ref_head(delayed_refs, delayed_refs->run_delayed_start, 1);
>> +	if (!head) {
>>  		delayed_refs->run_delayed_start = 0;
>> -		start = 0;
>> -		loop = true;
>> -		head = find_ref_head(delayed_refs, start, 1);
>> -		if (!head)
>> -			return NULL;
>> -	} else if (!head && loop) {
>
>I believe this will have a negative impact since it actually will
>prevent finding a head which was added BEFORE the last processed head.
>So when a ref head is selected in btrfs_obtain_ref_head then the
>delayed_refs->lock is dropped and the given head is locked and
>delayed_refs->run_delayed_start points to the end of the selected range
>that the head represents. At this point it's possible that another
>thread modifies a different range which is before the one we have
>selected so graphically it will be something like:
>
>
>---[HEAD2]----->[HEAD1]------
>0                            N
>
>Where HEAD1 is the head returned from first invocation of
>btrfs_obtain_ref_head. Once  btrfs_obtain_ref_head is called the 2nd
>time it will not find HEAD2 so will just reset run_delayed_start to 0
>and return. So it will be up to another run of the delayed refs to
>actually find head2. Essentially you made btrfs_obtain_ref_head less

Not exactly. In fact, find_ref_head hides such a logic. When
return_bigger is set, if there is no larger entry to return, the first
entry will be returned. Please see the comment I add in the PATCH 6.

Hence, the 2nd invocation of btrfs_obtain_ref_head still will return
HEAD2. There is no functional change here.

However, your question makes me consider whether such hidden logic
should be extracted from find_ref_head to btrfs_select_ref_head.

>greedy. Have you characterized what kind of performance impact this have?

I noticed that there is a macro called SCRAMBLE_DELAYED_REFS in the
extent-tree.c. I am a bit curious whether it has been forgotten by
everyone, I have not found any test results about its performance impact.

-- 
Thanks,
Lu

>
>
>
>
>>  		return NULL;
>>  	}
>>  
>> @@ -376,11 +367,9 @@ btrfs_select_ref_head(struct btrfs_delayed_ref_root *delayed_refs)
>>  
>>  		node = rb_next(&head->href_node);
>>  		if (!node) {
>> -			if (loop)
>> +			if (delayed_refs->run_delayed_start == 0)
>>  				return NULL;
>>  			delayed_refs->run_delayed_start = 0;
>> -			start = 0;
>> -			loop = true;
>>  			goto again;
>>  		}
>>  		head = rb_entry(node, struct btrfs_delayed_ref_head,
>> 
>
>