Re: [PATCH RFC ticketlock] Auto-queued ticketlock

[Davidlohr Bueso]

On Tue, 2013-06-11 at 14:41 -0400, Waiman Long wrote:
> On 06/11/2013 12:36 PM, Paul E. McKenney wrote:
> >
> >> I am a bit concern about the size of the head queue table itself.
> >> RHEL6, for example, had defined CONFIG_NR_CPUS to be 4096 which mean
> >> a table size of 256. Maybe it is better to dynamically allocate the
> >> table at init time depending on the actual number of CPUs in the
> >> system.
> > But if your kernel is built for 4096 CPUs, the 32*256=8192 bytes of memory
> > is way down in the noise.  Systems that care about that small an amount
> > of memory probably have a small enough number of CPUs that they can just
> > turn off queueing at build time using CONFIG_TICKET_LOCK_QUEUED=n, right?
> 
> My concern is more about the latency on the table scan than the actual 
> memory that was used.
> 
> >
> >>> +/*
> >>> + * Return a pointer to the queue header associated with the specified lock,
> >>> + * or return NULL if there is no queue for the lock or if the lock's queue
> >>> + * is in transition.
> >>> + */
> >>> +static struct tkt_q_head *tkt_q_find_head(arch_spinlock_t *asp)
> >>> +{
> >>> +	int i;
> >>> +	int start;
> >>> +
> >>> +	start = i = tkt_q_hash(asp);
> >>> +	do
> >>> +		if (tkt_q_heads[i].ref == asp)
> >>> +			return&tkt_q_heads[i];
> >>> +	while ((i = tkt_q_next_slot(i)) != start);
> >>> +	return NULL;
> >>> +}
> >> With a table size of 256 and you have to scan the whole table to
> >> find the right head queue. This can be a significant overhead. I
> >> will suggest setting a limiting of how many entries it scans before
> >> it aborts rather than checking the whole table.
> > But it will scan 256 entries only if there are 256 other locks in queued
> > mode, which is -very- unlikely, even given 4096 CPUs.  That said, if you
> > show me that this results in a real latency problem on a real system,
> > I would be happy to provide a way to limit the search.
> 
> Looking at the code more carefully, the chance of actually scanning 256 
> entries is very small. However, I now have some concern on the way you 
> set up the initial queue.
> 
> +/*
> + * Start handling a period of high contention by finding a queue to associate
> + * with this lock.  Returns true if successful (in which case we hold the
> + * lock) and false otherwise (in which case we do -not- hold the lock).
> + */
> +bool tkt_q_start_contend(arch_spinlock_t *asp, struct __raw_tickets inc)
> +{
> +	int i;
> +	int start;
> +
> +	/* Hash the lock address to find a starting point. */
> +	start = i = tkt_q_hash(asp);
> +
> +	/*
> +	 * Each pass through the following loop attempts to associate
> +	 * the lock with the corresponding queue.
> +	 */
> +	do {
> +		/*
> +		 * Use 0x1 to mark the queue in use, but also avoiding
> +		 * any spinners trying to use it before we get it all
> +		 * initialized.
> +		 */
> +		if (cmpxchg(&tkt_q_heads[i].ref,
> +			    NULL,
> +			    (arch_spinlock_t *)0x1) == NULL) {
> +
> +			/* Succeeded, now go initialize it. */
> +			return tkt_q_init_contend(i, asp, inc);
> +		}
> +
> +		/* If someone beat us to it, go spin on their queue. */
> +		if (ACCESS_ONCE(asp->tickets.head)&  0x1)
> +			return tkt_q_do_spin(asp, inc);
> +	} while ((i = tkt_q_next_slot(i)) != start);
> +
> +	/* All the queues are in use, revert to spinning on the ticket lock. */
> +	return false;
> +}
> +
> 
> Unconditional cmpxchg() can be a source of high contention by itself. 
> Considering that 16 threads may be doing cmpxchg() more or less 
> simultaneously on the same cache line, it can cause a lot of contention. 
> It will be better if you check to see if tkt_q_heads[i] is NULL first 
> before doing cmpxchg.

Good point, we already noticed good benefits in mutexes and rwsems when
using test and cmpxchg techniques.

Thanks,
davidlohr

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