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 03:28:15PM +0300, Nikolay Borisov wrote:
>
>
>On 11.10.2018 15:15, Lu Fengqi wrote:
>> 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.
>
>Right I agree with your. As it stands I will expect that if
>return_bigger is true to specifically return a bigger entry or if
>nothing is found to return null. IMO this behavior is higher level and

This is also exactly what I want. The patch is on the way.

>belongs to btrfs_delayed_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.
>
>I guess it was used during testing but nothing currently sets it. I.e it
>might make sense to enable it if BTRFS_DEBUG is set.
>

Make sense.

-- 
Thanks,
Lu