linux-kernel - Re: [PATCH tip/locking/core v9 6/6] locking/pvqspinlock: Queue node adaptive spinning

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Date:	Fri, 6 Nov 2015 16:01:49 +0100
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 tip/locking/core v9 6/6] locking/pvqspinlock: Queue node
 adaptive spinning

On Fri, Oct 30, 2015 at 07:26:37PM -0400, Waiman Long wrote:
> +++ b/kernel/locking/qspinlock_paravirt.h
> @@ -23,6 +23,19 @@
>  #define _Q_SLOW_VAL	(3U << _Q_LOCKED_OFFSET)
>  
>  /*
> + * Queue Node Adaptive Spinning
> + *
> + * A queue node vCPU will stop spinning if the vCPU in the previous node is
> + * not running. The one lock stealing attempt allowed at slowpath entry
> + * mitigates the slight slowdown for non-overcommitted guest with this
> + * aggressive wait-early mechanism.
> + *
> + * The status of the previous node will be checked at fixed interval
> + * controlled by PV_PREV_CHECK_MASK.
> + */
> +#define PV_PREV_CHECK_MASK	0xff
> +
> +/*
>   * Queue node uses: vcpu_running & vcpu_halted.
>   * Queue head uses: vcpu_running & vcpu_hashed.
>   */
> @@ -202,6 +215,20 @@ static struct pv_node *pv_unhash(struct qspinlock *lock)
>  }
>  
>  /*
> + * Return true if when it is time to check the previous node which is not
> + * in a running state.
> + */
> +static inline bool
> +pv_wait_early(struct pv_node *prev, int loop)
> +{
> +
> +	if ((loop & PV_PREV_CHECK_MASK) != 0)
> +		return false;
> +
> +	return READ_ONCE(prev->state) != vcpu_running;
> +}

So it appears to me the sole purpose of PV_PREV_CHECK_MASK it to avoid
touching the prev->state cacheline too hard. Yet that is not mentioned
anywhere above.


> +static void pv_wait_node(struct mcs_spinlock *node, struct mcs_spinlock *prev)
>  {
>  	struct pv_node *pn = (struct pv_node *)node;
> +	struct pv_node *pp = (struct pv_node *)prev;
>  	int waitcnt = 0;
>  	int loop;
> +	bool wait_early;
>  
>  	/* waitcnt processing will be compiled out if !QUEUED_LOCK_STAT */
>  	for (;; waitcnt++) {
> -		for (loop = SPIN_THRESHOLD; loop; loop--) {
> +		for (wait_early = false, loop = SPIN_THRESHOLD; loop; loop--) {
>  			if (READ_ONCE(node->locked))
>  				return;
> +			if (pv_wait_early(pp, loop)) {
> +				wait_early = true;
> +				break;
> +			}
>  			cpu_relax();
>  		}
>  

So if prev points to another node, it will never see vcpu_running. Was
that fully intended?

FYI, I think I've now seen all patches ;-)
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