Re: [PATCH v7 4/5] locking/pvqspinlock: Allow 1 lock stealing attempt

[Waiman Long <waiman.long@hpe.com>]

On 10/13/2015 03:39 PM, Peter Zijlstra wrote:
> On Tue, Sep 22, 2015 at 04:50:43PM -0400, Waiman Long wrote:
>> This patch allows one attempt for the lock waiter to steal the lock
>> when entering the PV slowpath.  This helps to reduce the performance
>> penalty caused by lock waiter preemption while not having much of
>> the downsides of a real unfair lock.
>>
> Changelog does not explain the implementation, which is subtle enough to
> warrant a few words.

Will add more information into the changelog.

>
>> @@ -417,7 +415,8 @@ queue:
>>   	 * does not imply a full barrier.
>>   	 *
>>   	 */
>> -	pv_wait_head(lock, node);
>> +	if (pv_wait_head_and_lock(lock, node, tail))
>> +		goto release;
> That's very much: pv_wait_head_or_lock(), maybe _or_steal() is even
> better.

I am not very good at naming function. Changing it to _or_steal() is 
fine for me.

>>   	while ((val = smp_load_acquire(&lock->val.counter))&  _Q_LOCKED_PENDING_MASK)
>>   		cpu_relax();
>>
>> @@ -454,7 +453,6 @@ queue:
>>   		cpu_relax();
>>
>>   	arch_mcs_spin_unlock_contended(&next->locked);
>> -	pv_kick_node(lock, next);
> Not sure about removing that, breaks symmetry.
>
>>   /*
>> + * Allow one unfair trylock when entering the PV slowpath to reduce the
>> + * performance impact of lock waiter preemption (either explicitly via
>> + * pv_wait or implicitly via PLE). This function will be called once when
>> + * a lock waiter enter the slowpath before being queued.
>> + *
>> + * A little bit of unfairness here can improve performance without many
>> + * of the downsides of a real unfair lock.
>> + */
>> +#define queued_spin_trylock(l)	pv_queued_spin_trylock_unfair(l)
>> +static inline bool pv_queued_spin_trylock_unfair(struct qspinlock *lock)
>> +{
>> +	struct __qspinlock *l = (void *)lock;
>> +
>> +	if (READ_ONCE(l->locked))
>> +		return 0;
>> +	/*
>> +	 * Wait a bit here to ensure that an actively spinning queue head vCPU
>> +	 * has a fair chance of getting the lock.
>> +	 */
>> +	cpu_relax();
>> +
>> +	return cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0;
>> +}
> This doesn't seem to make any sense.. Its also very much distinct from
> the rest of the patch and can easily be added in a separate patch with
> separate performance numbers to show it does (or does not) make a
> difference.

If you mean I don't need an extra cpu_relax() here, I can take that out. 
It was there to make the active queue head vCPU having a higher chance 
of getting the lock, but it is not essential.

Cheers,
Longman