Subject: Re: [PATCH v5 3/5] locking/qspinlock: Introduce CNA into the slow path of qspinlock

From: Alex Kogan <[email-masked]>
Date: Fri, 18 Oct 2019 17:37:51 -0400

> On Oct 18, 2019, at 12:03 PM, Waiman Long <[email-masked]> wrote:
> 
> On 10/16/19 12:29 AM, Alex Kogan wrote:
>> +static inline void cna_pass_lock(struct mcs_spinlock *node,
>> +				 struct mcs_spinlock *next)
>> +{
>> +	struct cna_node *cn = (struct cna_node *)node;
>> +	struct mcs_spinlock *next_holder = next, *tail_2nd;
>> +	u32 val = 1;
>> +
>> +	u32 scan = cn->pre_scan_result;
>> +
>> +	/*
>> +	 * 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 > 0)
>> +		scan = cna_scan_main_queue(node, decode_tail(scan));
>> +
>> +	if (!scan) { /* if found a successor from the same numa node */
>> +		next_holder = node->next;
>> +		/*
>> +		 * make sure @val gets 1 if current holder's @locked is 0 as
>> +		 * we have to store a non-zero value in successor's @locked
>> +		 * to pass the lock
>> +		 */
>> +		val = node->locked + (node->locked == 0);
> 
> node->locked can be 0 when the cpu enters into an empty MCS queue. We
> could unconditionally set node->locked to 1 for this case in qspinlock.c
> or with your above code.

Right, I was doing that in the first two versions of the series. It adds 
unnecessary store into @locked for non-CNA variants, and even if it does not
have any real performance implications, I think Peter did not like that (or, 
at least, the comment I had to explain why we needed that store).

> Perhaps, a comment about when node->locked will
> be 0.
Yeah, I was tinkering with this comment. Here is how it read in v3:
/*
 * We unlock a successor by passing a non-zero value,
 * so set @val to 1 iff @locked is 0, which will happen
 * if we acquired the MCS lock when its queue was empty
 */

I can change back to something like that if it is better.

> 
> It may be easier to understand if you just do
> 
>     val = node->locked ? node->locked : 1;
You’re right, that’s another possibility.
However, it adds yet another if-statement on the critical path, which I was
trying to avoid that.

Best regards,
— Alex

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