Re: [PATCH v2 5/6] locking/pvqspinlock: Opportunistically defer kicking to unlock time

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

On 07/15/2015 06:03 AM, Peter Zijlstra wrote:
> On Tue, Jul 14, 2015 at 10:13:36PM -0400, Waiman Long wrote:
>> +static void pv_kick_node(struct qspinlock *lock, struct mcs_spinlock *node)
>>   {
>>   	struct pv_node *pn = (struct pv_node *)node;
>>
>> +	if (xchg(&pn->state, vcpu_running) == vcpu_running)
>> +		return;
>> +
>>   	/*
>> +	 * Kicking the next node at lock time can actually be a bit faster
>> +	 * than doing it at unlock time because the critical section time
>> +	 * overlaps with the wakeup latency of the next node. However, if the
>> +	 * VM is too overcommmitted, it can happen that we need to kick the
>> +	 * CPU again at unlock time (double-kick). To avoid that and also to
>> +	 * fully utilize the kick-ahead functionality at unlock time,
>> +	 * the kicking will be deferred under either one of the following
>> +	 * 2 conditions:
>>   	 *
>> +	 * 1) The VM guest has too few vCPUs that kick-ahead is not even
>> +	 *    enabled. In this case, the chance of double-kick will be
>> +	 *    higher.
>> +	 * 2) The node after the next one is also in the halted state.
>>   	 *
>> +	 * In this case, the hashed flag is set to indicate that hashed
>> +	 * table has been filled and _Q_SLOW_VAL is set.
>>   	 */
>> -	if (xchg(&pn->state, vcpu_running) == vcpu_halted) {
>> -		pvstat_inc(pvstat_lock_kick);
>> -		pv_kick(pn->cpu);
>> +	if ((!pv_kick_ahead || pv_get_kick_node(pn, 1))&&
>> +	    (xchg(&pn->hashed, 1) == 0)) {
>> +		struct __qspinlock *l = (void *)lock;
>> +
>> +		/*
>> +		 * As this is the same vCPU that will check the _Q_SLOW_VAL
>> +		 * value and the hash table later on at unlock time, no atomic
>> +		 * instruction is needed.
>> +		 */
>> +		WRITE_ONCE(l->locked, _Q_SLOW_VAL);
>> +		(void)pv_hash(lock, pn);
>> +		return;
>>   	}
>> +
>> +	/*
>> +	 * Kicking the vCPU even if it is not really halted is safe.
>> +	 */
>> +	pvstat_inc(pvstat_lock_kick);
>> +	pv_kick(pn->cpu);
>>   }
>>
>>   /*
>> @@ -513,6 +545,13 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
>>   			cpu_relax();
>>   		}
>>
>> +		if (!lp&&  (xchg(&pn->hashed, 1) == 1))
>> +			/*
>> +			 * The hashed table&  _Q_SLOW_VAL had been filled
>> +			 * by the lock holder.
>> +			 */
>> +			lp = (struct qspinlock **)-1;
>> +
>>   		if (!lp) { /* ONCE */
>>   			lp = pv_hash(lock, pn);
>>   			/*
> *groan*, so you complained the previous version of this patch was too
> complex, but let me say I vastly preferred it to this one :/

I said it was complex as maintaining a tri-state variable needed more 
thought than 2 bi-state variables. I can revert it back to the tri-state 
variable as doing an unconditional kick in unlock simplifies the code at 
pv_wait_head().

Cheers,
Longman