Re: [PATCH v7 5/5] locking/qspinlock: Introduce the shuffle reduction optimization into CNA

[Waiman Long <longman@redhat.com>]

On 12/10/19 1:56 PM, Alex Kogan wrote:
> ----- longman@redhat.com wrote:
>
>> On 11/25/19 4:07 PM, Alex Kogan wrote:
>>> @@ -234,12 +263,13 @@ __always_inline u32 cna_pre_scan(struct
>> qspinlock *lock,
>>>  	struct cna_node *cn = (struct cna_node *)node;
>>>  
>>>  	/*
>>> -	 * setting @pre_scan_result to 1 indicates that no post-scan
>>> +	 * setting @pre_scan_result to 1 or 2 indicates that no post-scan
>>>  	 * should be made in cna_pass_lock()
>>>  	 */
>>>  	cn->pre_scan_result =
>>> -		cn->intra_count == intra_node_handoff_threshold ?
>>> -			1 : cna_scan_main_queue(node, node);
>>> +		(node->locked <= 1 && probably(SHUFFLE_REDUCTION_PROB_ARG)) ?
>>> +			1 : cn->intra_count == intra_node_handoff_threshold ?
>>> +			2 : cna_scan_main_queue(node, node);
>>>  
>>>  	return 0;
>>>  }
>>> @@ -253,12 +283,15 @@ static inline void cna_pass_lock(struct
>> mcs_spinlock *node,
>>>  
>>>  	u32 scan = cn->pre_scan_result;
>>>  
>>> +	if (scan == 1)
>>> +		goto pass_lock;
>>> +
>>>  	/*
>>>  	 * check if a successor from the same numa node has not been found
>> in
>>>  	 * pre-scan, and if so, try to find it in post-scan starting from
>> the
>>>  	 * node where pre-scan stopped (stored in @pre_scan_result)
>>>  	 */
>>> -	if (scan > 1)
>>> +	if (scan > 2)
>>>  		scan = cna_scan_main_queue(node, decode_tail(scan));
>>>  
>>>  	if (!scan) { /* if found a successor from the same numa node */
>>> @@ -281,5 +314,6 @@ static inline void cna_pass_lock(struct
>> mcs_spinlock *node,
>>>  		tail_2nd->next = next;
>>>  	}
>>>  
>>> +pass_lock:
>>>  	arch_mcs_pass_lock(&next_holder->locked, val);
>>>  }
>> I think you might have mishandled the proper accounting of
>> intra_count.
>> How about something like:
>>
>> diff --git a/kernel/locking/qspinlock_cna.h
>> b/kernel/locking/qspinlock_cna.h
>> index f1eef6bece7b..03f8fdec2b80 100644
>> --- a/kernel/locking/qspinlock_cna.h
>> +++ b/kernel/locking/qspinlock_cna.h
>> @@ -268,7 +268,7 @@ __always_inline u32 cna_pre_scan(struct qspinlock
>> *lock,
>>          */
>>         cn->pre_scan_result =
>>                 (node->locked <= 1 &&
>> probably(SHUFFLE_REDUCTION_PROB_ARG)) ?
>> -                       1 : cn->intra_count ==
>> intra_node_handoff_threshold ?
>> +                       1 : cn->intra_count >=
>> intra_node_handoff_threshold ?
> We reset ‘intra_count' in cna_init_node(), which is called before we enter 
> the slow path, and set it at most once when we pass the internal (CNA) lock
> by taking the owner’s value + 1. Only after we get the internal lock, we
> call this cna_pre_scan() function, where we check the threshold. 
> IOW, having 'intra_count > intra_node_handoff_threshold' would mean a bug, 
> and having “>=“ would mask it. 
> Perhaps I can add WARN_ON(cn->intra_count > intra_node_handoff_threshold)
> here instead, although I'm not sure if that is a good idea performance-wise.

The code that I added below could have the possibility of making
intra_count > intra_node_handoff_threshold. I agree with your assessment
of the current code. This conditional check is fine if no further change
is made.


>>                         2 : cna_scan_main_queue(node, node);
>>  
>>         return 0;
>> @@ -283,9 +283,6 @@ static inline void cna_pass_lock(struct
>> mcs_spinlock
>> *node,
>>  
>>         u32 scan = cn->pre_scan_result;
>>  
>> -       if (scan == 1)
>> -               goto pass_lock;
>> -
> The thing is that we want to avoid as much of the shuffling-related overhead
> as we can when the spinlock is only lightly contended. That's why we have this
> early exit here that avoids the rest of the logic of triaging through possible
> 'scan' values.
That is a valid point. Maybe you can document that fact you are
optimizing for performance instead of better correctness.
>>         /*
>>          * check if a successor from the same numa node has not been
>> found in
>>          * pre-scan, and if so, try to find it in post-scan starting
>> from the
>> @@ -294,7 +291,13 @@ static inline void cna_pass_lock(struct
>> mcs_spinlock *node,
>>         if (scan > 2)
>>                 scan = cna_scan_main_queue(node, decode_tail(scan));
>>  
>> -       if (!scan) { /* if found a successor from the same numa node
>> */
>> +       if (scan <= 1) { /* if found a successor from the same numa
>> node */
>> +               /* inc @intra_count if the secondary queue is not
>> empty */
>> +               ((struct cna_node *)next_holder)->intra_count =
>> +                       cn->intra_count + (node->locked > 1);
>> +               if ((scan == 1)
>> +                       goto pass_lock;
>> +
> Hmm, I am not sure this makes the code any better/more readable,
> while this does add the overhead of going through 3 branches before
> jumping to 'pass_lock'.
>
This is just a suggestion for improving the correctness of the code. I
am fine if you opt for better performance.

Cheers,
Longman


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