linux-kernel - Re: [PATCH 1/7] locking/pvqspinlock: Only kick CPU at unlock time

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Date:	Mon, 13 Jul 2015 14:02:02 +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>
Subject: Re: [PATCH 1/7] locking/pvqspinlock: Only kick CPU at unlock time

On Sat, Jul 11, 2015 at 04:36:52PM -0400, Waiman Long wrote:
> @@ -181,9 +187,9 @@ static void pv_wait_node(struct mcs_spinlock *node)
>  			pv_wait(&pn->state, vcpu_halted);
>  
>  		/*
> -		 * Reset the vCPU state to avoid unncessary CPU kicking
> +		 * Reset the state except when vcpu_hashed is set.
>  		 */
> -		WRITE_ONCE(pn->state, vcpu_running);
> +		cmpxchg(&pn->state, vcpu_halted, vcpu_running);

Why? Suppose we did get advanced into the hashed state, and then get a
(spurious) wakeup, this means we'll observe our ->locked == 1 condition
and fall out of pv_wait_node().

We'll then enter pv_wait_head(), which with your modification:

> @@ -229,19 +244,42 @@ static void pv_wait_head(struct qspinlock *lock, struct mcs_spinlock *node)
>  {
>  	struct pv_node *pn = (struct pv_node *)node;
>  	struct __qspinlock *l = (void *)lock;
> -	struct qspinlock **lp = NULL;
> +	struct qspinlock **lp;
>  	int loop;
>  
> +	/*
> +	 * Initialize lp to a non-NULL value if it has already been in the
> +	 * pv_hashed state so that pv_hash() won't be called again.
> +	 */
> +	lp = (READ_ONCE(pn->state) == vcpu_hashed) ? (struct qspinlock **)1
> +						   : NULL;
>  	for (;;) {
> +		WRITE_ONCE(pn->state, vcpu_running);

Will instantly and unconditionally write vcpu_running.


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