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

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Date:	Sun, 6 Sep 2015 17:05:46 -0700
From:	Davidlohr Bueso <dave@...olabs.net>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Chinner <david@...morbit.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Waiman Long <Waiman.Long@...com>,
	Ingo Molnar <mingo@...nel.org>
Subject: Re: [4.2, Regression] Queued spinlocks cause major XFS performance
 regression

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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