Re: [PATCH RFC ticketlock] Auto-queued ticketlock

[Steven Rostedt]

On Tue, 2013-06-11 at 13:32 -0700, Paul E. McKenney wrote:

> 		/*
> 		 * This lock has lots of spinners, but no queue.  Go create
> 		 * a queue to spin on.
> 		 *
> 		 * In the common case, only the single task that
> 		 * sees the head and tail tickets being different by
> 		 * exactly TKT_Q_SWITCH will come here set up the queue,
> 		 * which prevents a "thundering herd" of queue setups.
> 		 * Although it is still possible for an unfortunate series
> 		 * of lock handoffs and newly arrived tasks to result
> 		 * in more than one task performing a queue setup, this
> 		 * is unlikely.  Of course, this situation must still be
> 		 * handled correctly, which is the job of the cmpxchg()
> 		 * in tkt_q_start_contend().
> 		 */
> 		if (tkt_q_start_contend(ap, inc))
> 			return true;
> 
> Does that help?

Yes, very good.

> 
> > As TKT_Q_SWITCH doesn't have a type, I'm not sure how C will evaluate
> > this. I always screw type conversions up, and just add in the type casts
> > to be safe.
> > 
> > You could also give TKT_Q_SWITCH a type too.
> 
> This is an excellent point as well -- things might well get confused.
> My solution was to take your last suggestion and given TKT_Q_SWITCH the
> same type as inc.tail and inc.head, and also apply type-safety paranoia
> to TKT_Q_NQUEUES:
> 
> /*
>  * TKT_Q_SWITCH is twice the number of CPUs that must be spinning on a
>  * given ticket lock to motivate switching to spinning on a queue.
>  * The reason that it is twice the number is because the bottom bit of
>  * the ticket is reserved for the bit that indicates that a queue is
>  * associated with the lock.
>  */
> #define TKT_Q_SWITCH  ((__ticket_t)(CONFIG_TICKET_LOCK_QUEUED_SWITCH * 2))
> 
> /*
>  * TKT_Q_NQUEUES is the number of queues to maintain.  Large systems
>  * might have multiple highly contended locks, so provide more queues for
>  * systems with larger numbers of CPUs.
>  */
> #define TKT_Q_NQUEUES (2 * DIV_ROUND_UP(NR_CPUS + ((int)TKT_Q_SWITCH) - 1, \
> 					(int)TKT_Q_SWITCH))
> 
> Does that look OK?  (The limits on the value of TKT_Q_SWITCH should avoid
> signed integer overflow.)
> 

Looks fine.

-- Steve


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