Messages in this thread | | | Date | Wed, 15 Jul 2015 12:01:41 +0200 | From | Peter Zijlstra <> | Subject | Re: [PATCH v2 6/6] locking/pvqspinlock: Queue node adaptive spinning |
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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?
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