Re: [PATCH v2 6/6] locking/pvqspinlock: Queue node adaptive spinning

Peter Zijlstra
2015-07-15T10:02:08Z

On Tue, Jul 14, 2015 at 10:13:37PM -0400, Waiman Long wrote:
> +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;
> + bool wait_early, can_wait_early;
> int loop;
>
> for (;;) {
> - for (loop = SPIN_THRESHOLD; loop; loop--) {
> + /*
> + * Spin less if the previous vCPU was in the halted state
> + * and it is not the queue head.
> + */
> + can_wait_early = (pn->waithist > PV_WAITHIST_THRESHOLD);
> + wait_early = can_wait_early && !READ_ONCE(prev->locked) &&
> + (READ_ONCE(pp->state) == vcpu_halted);
> + loop = wait_early ? QNODE_SPIN_THRESHOLD_SHORT
> + : QNODE_SPIN_THRESHOLD;
> + for (; loop; loop--, cpu_relax()) {
> + bool halted;
> +
> if (READ_ONCE(node->locked))
> return;
> - cpu_relax();
> +
> + if (!can_wait_early || (loop & QNODE_SPIN_CHECK_MASK))
> + continue;
> +
> + /*
> + * Look for state transition at previous node.
> + *
> + * running => halted:
> + * call pv_wait() now if kick-ahead is enabled
> + * or reduce spin threshold to
> + * QNODE_SPIN_THRESHOLD_SHORT or less.
> + * halted => running:
> + * reset spin threshold to QNODE_SPIN_THRESHOLD
> + */
> + halted = (READ_ONCE(pp->state) == vcpu_halted) &&
> + !READ_ONCE(prev->locked);
> + if (wait_early == halted)
> + continue;
> + wait_early = halted;
> +
> + if (!wait_early)
> + loop = QNODE_SPIN_THRESHOLD;
> + else if (pv_kick_ahead)
> + break;
> + else if (loop > QNODE_SPIN_THRESHOLD_SHORT)
> + loop = QNODE_SPIN_THRESHOLD_SHORT;
> }
> + if (wait_early)
> + pvstat_inc(pvstat_wait_early);
> +
> + /*
> + * A pv_wait while !wait_early has higher weight than when
> + * wait_early is true.
> + */
> + if (pn->waithist < PV_WAITHIST_MAX)
> + pn->waithist += wait_early ? 1 : PV_WAIT_INC;

So when you looked at this patch, you didn't go like, OMFG!?

So what was wrong with something like:

static inline int pv_spin_threshold(struct pv_node *prev)
{
if (READ_ONCE(prev->locked)) /* it can run, wait for it */
return SPIN_THRESHOLD;

if (READ_ONCE(prev->state) == vcpu_halted) /* its not running, do not wait */
return 1;

return SPIN_THRESHOLD;
}

static void pv_wait_head(...)
{
for (;;) {
for (loop = pv_spin_threshold(pp); loop; loop--) {
if (READ_ONCE(node->locked))
return;
cpu_relax();
}

if (!lp) {
...
}
pv_wait(&l->locked, _Q_SLOW_VAL);
}
}

What part of: "keep it simple" and "gradual complexity" have you still
not grasped?