Re: [PATCH 2/2] [PATCH] sched: Add smp_rmb() in task rq locking cycles

[Peter Zijlstra <peterz@infradead.org>]

On Tue, Apr 28, 2015 at 12:58:55PM -0400, Chris Metcalf wrote:
> On 04/28/2015 12:40 PM, Peter Zijlstra wrote:
> >On Tue, Apr 28, 2015 at 11:53:21AM -0400, Chris Metcalf wrote:
> >
> >>The reason we use two 32-bit fields on tilepro is that the only available
> >>atomic instruction is tns (test and set), which sets a 32-bit "1" value
> >>into the target memory and returns the old 32-bit value.
> >And you want a ticket lock as opposed to the test-and-set lock because
> >with 64 tiles starvation under contention is a real worry?
> 
> We see substantial unfairness under load with a plain spinlock,
> basically because nearer cores on the mesh network can exponentially
> crowd out further cores.  The ticket lock avoids that, though we
> have to be careful to do backoff when checking the lock to avoid
> DDoS in the mesh network.

Does your arch have 16bit atomic load/stores ? If so, would something
like the below not make sense?

typedef struct {
	union {
		struct {
			unsigned short head;
			unsigned short tail;
		};
		unsigned int tickets;
	};
	unsigned int lock;
} arch_spinlock_t;

static inline void ___tns_lock(unsigned int *lock)
{
	while (tns(lock))
		cpu_relax();
}

static inline void ___tns_unlock(unsigned int *lock)
{
	WRITE_ONCE(*lock, 0);
}

static inline void arch_spin_lock(arch_spinlock_t *lock)
{
	unsigned short head, tail;

	___tns_lock(&lock->lock); /* XXX does the TNS imply a ___sync?  */
	head = lock->head;
	lock->head++;
	___tns_unlock(&lock->lock);

	while (READ_ONCE(lock->tail) != head)
		cpu_relax();
}

static inline void arch_spin_unlock(arch_spinlock_t *lock)
{
	/*
	 * can do with regular load/store because the lock owner
	 * is the only one going to do stores to the tail
	 */
	unsigned short tail = READ_ONCE(lock->tail);
	smp_mb(); /* MB is stronger than RELEASE */
	WRITE_ONCE(lock->tail, tail + 1);
}

static inline void arch_spin_unlock_wait(arch_spinlock_t *lock)
{
	union {
		struct {
			unsigned short head;
			unsigned short tail;
		};
		unsigned int tickets;
	} x;

	for (;;) {
		x.tickets = READ_ONCE(lock->tickets);
		if (x.head == x.tail)
			break;
		cpu_relax();
	}
}