Re: [4.2, Regression] Queued spinlocks cause major XFS performance regression

[Davidlohr Bueso]

On Fri, 04 Sep 2015, Peter Zijlstra wrote:

> -static inline bool virt_queued_spin_lock(struct qspinlock *lock)
> +static inline bool virt_spin_lock(struct qspinlock *lock)

Given that we fall back to the cmpxchg loop even when PARAVIRT is not in the
picture, I believe this function is horribly misnamed.

> {
> 	if (!static_cpu_has(X86_FEATURE_HYPERVISOR))
> 		return false;
>
> -	while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0)
> -		cpu_relax();
> +	/*
> +	 * On hypervisors without PARAVIRT_SPINLOCKS support we fall
> +	 * back to a Test-and-Set spinlock, because fair locks have
> +	 * horrible lock 'holder' preemption issues.
> +	 */
> +

This comment is also misleading... but if you tuck the whole function
under some PARAVIRT option, it obviously makes sense to just leave as is.
And let native actually _use_ qspinlocks.

> +	do {
> +		while (atomic_read(&lock->val) != 0)
> +			cpu_relax();
> +	} while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0);

CCAS to the rescue again.

Thanks,
Davidlohr
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