linux-kernel - Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

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Date:	Mon, 14 Sep 2015 16:00:51 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Waiman Long <Waiman.Long@....com>
Cc:	Ingo Molnar <mingo@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, Scott J Norton <scott.norton@...com>,
	Douglas Hatch <doug.hatch@...com>,
	Davidlohr Bueso <dave@...olabs.net>
Subject: Re: [PATCH v6 5/6] locking/pvqspinlock: Allow 1 lock stealing attempt

On Fri, Sep 11, 2015 at 02:37:37PM -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.

> @@ -415,8 +458,12 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
>  
>  	for (;; waitcnt++) {
>  		for (loop = SPIN_THRESHOLD; loop; loop--) {
> -			if (!READ_ONCE(l->locked))
> -				return;
> +			/*
> +			 * Try to acquire the lock when it is free.
> +			 */
> +			if (!READ_ONCE(l->locked) &&
> +			   (cmpxchg(&l->locked, 0, _Q_LOCKED_VAL) == 0))
> +				goto gotlock;
>  			cpu_relax();
>  		}
>  

This isn't _once_, this is once per 'wakeup'. And note that interrupts
unrelated to the kick can equally wake the vCPU up.

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