Re: [PATCH RFC ticketlock] v2 Auto-queued ticketlock

[Steven Rostedt]

On Tue, 2013-06-11 at 10:02 -0700, Paul E. McKenney wrote:

> +#ifdef CONFIG_TICKET_LOCK_QUEUED
> +
> +#define __TKT_SPIN_INC 2
> +bool tkt_spin_pass(arch_spinlock_t *ap, struct __raw_tickets inc);
> +
> +#else /* #ifdef CONFIG_TICKET_LOCK_QUEUED */
> +
> +#define __TKT_SPIN_INC 1
> +static inline bool tkt_spin_pass(arch_spinlock_t *ap, struct __raw_tickets inc)
> +{
> +	return false;
> +}
> +
> +#endif /* #else #ifdef CONFIG_TICKET_LOCK_QUEUED */
> +
>  /*
>   * Ticket locks are conceptually two parts, one indicating the current head of
>   * the queue, and the other indicating the current tail. The lock is acquired
> @@ -49,17 +64,15 @@
>   */
>  static __always_inline void __ticket_spin_lock(arch_spinlock_t *lock)
>  {
> -	register struct __raw_tickets inc = { .tail = 1 };
> +	register struct __raw_tickets inc = { .tail = __TKT_SPIN_INC };
>  
>  	inc = xadd(&lock->tickets, inc);
> -
>  	for (;;) {
> -		if (inc.head == inc.tail)
> +		if (inc.head == inc.tail || tkt_spin_pass(lock, inc))
>  			break;
> -		cpu_relax();

Overheating the CPU are we ;-)

Keeping the cpu_relax() doesn't hurt, even when TICKET_LOCK_QUEUE is
enabled. As the only latency to worry about is when tkt_spin_pass()
returns true, where it breaks out of the loop anyway.

But if you really don't want the double call to cpu_relax(), we can
probably remove the cpu_relax from tkt_spin_pass() and keep this one, or
in the above tkt_spin_pass() where TICK_LOCK_QUEUED is not set, we can
do:

static inline bool tkt_spin_pass(arch_spinlock_t *ap, struct
__raw_tickets inc)
{
	cpu_relax();
	return false;
}

Honesty, I would say remove it from tkt_spin_pass() when returning
false.

-- Steve


>  		inc.head = ACCESS_ONCE(lock->tickets.head);
>  	}
> -	barrier();		/* make sure nothing creeps before the lock is taken */
> +	barrier(); /* Make sure nothing creeps in before the lock is taken. */
>  }
>  


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