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

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Message-ID: <20150914140456.GU18489@twins.programming.kicks-ass.net>
Date:	Mon, 14 Sep 2015 16:04:56 +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.


> @@ -416,7 +414,8 @@ queue:
>  	 * does not imply a full barrier.
>  	 *
>  	 */

If it really were once, like the Changelog says it is, then you could
have simply added:

	if (pv_try_steal_lock(...))
		goto release;

here, and not wrecked pv_wait_head() like you did. Note that if you do
it like this, you also do not need to play games with the hash, because
you'll never get into that situation.

> -	pv_wait_head(lock, node);
> +	if (pv_wait_head_and_lock(lock, node, tail))
> +		goto release;
>  	while ((val = smp_load_acquire(&lock->val.counter)) & _Q_LOCKED_PENDING_MASK)
>  		cpu_relax();
>  
--
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