[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] Re: [Xen-devel] [PATCH v2 5/7] xen: credit1: increase efficiency and scalability of load balancing.
On Fri, 2017-04-07 at 17:26 +0200, Dario Faggioli wrote: > On Fri, 2017-04-07 at 16:17 +0100, George Dunlap wrote: > > It seems like trying to sort out most of the refcounting > > inside fo those two functions (perhaps with runq_insert() which did > > reference counting, and __runq_insert() that didn't, or > > something like > > that) would be a better approach. > > > Right. I've tried already, but without success, and I had to stop an > resort to what's in this patch. > > As I said, I am ok with this approach, so I just went for it. I can > try > to think harder at whether it is really possible to do something like > you suggest above... lemme try. > So, how about something like the below? For almost all cases, all it's done inside runqueue remove and insert helpers. There still are two special cases, though, one where I must explicitly call inc_nr_runnable(), the other where I need dec_nr_runnable(). I've added comments trying to explain why that is the case. Let me know. Thanks and Regards, Dario --- diff --git a/xen/common/sched_credit.c b/xen/common/sched_credit.c index 59b87f7..a0ad167 100644 --- a/xen/common/sched_credit.c +++ b/xen/common/sched_credit.c @@ -172,6 +172,7 @@ struct csched_pcpu { struct timer ticker; unsigned int tick; unsigned int idle_bias; + unsigned int nr_runnable; }; /* @@ -262,9 +263,26 @@ static inline bool_t is_runq_idle(unsigned int cpu) } static inline void +inc_nr_runnable(unsigned int cpu) +{ + ASSERT(spin_is_locked(per_cpu(schedule_data, cpu).schedule_lock)); + CSCHED_PCPU(cpu)->nr_runnable++; + +} + +static inline void +dec_nr_runnable(unsigned int cpu) +{ + ASSERT(spin_is_locked(per_cpu(schedule_data, cpu).schedule_lock)); + CSCHED_PCPU(cpu)->nr_runnable--; + ASSERT(CSCHED_PCPU(cpu)->nr_runnable >= 0); +} + +static inline void __runq_insert(struct csched_vcpu *svc) { - const struct list_head * const runq = RUNQ(svc->vcpu->processor); + unsigned int cpu = svc->vcpu->processor; + const struct list_head * const runq = RUNQ(cpu); struct list_head *iter; BUG_ON( __vcpu_on_runq(svc) ); @@ -292,12 +310,25 @@ __runq_insert(struct csched_vcpu *svc) } static inline void +runq_insert(struct csched_vcpu *svc) +{ + __runq_insert(svc); + inc_nr_runnable(svc->vcpu->processor); +} + +static inline void __runq_remove(struct csched_vcpu *svc) { BUG_ON( !__vcpu_on_runq(svc) ); list_del_init(&svc->runq_elem); } +static inline void +runq_remove(struct csched_vcpu *svc) +{ + dec_nr_runnable(svc->vcpu->processor); + __runq_remove(svc); +} #define for_each_csched_balance_step(step) \ for ( (step) = 0; (step) <= CSCHED_BALANCE_HARD_AFFINITY; (step)++ ) @@ -601,6 +632,7 @@ init_pdata(struct csched_private *prv, struct csched_pcpu *spc, int cpu) /* Start off idling... */ BUG_ON(!is_idle_vcpu(curr_on_cpu(cpu))); cpumask_set_cpu(cpu, prv->idlers); + spc->nr_runnable = 0; } static void @@ -1052,7 +1084,7 @@ csched_vcpu_insert(const struct scheduler *ops, struct vcpu *vc) lock = vcpu_schedule_lock_irq(vc); if ( !__vcpu_on_runq(svc) && vcpu_runnable(vc) && !vc->is_running ) - __runq_insert(svc); + runq_insert(svc); vcpu_schedule_unlock_irq(lock, vc); @@ -1117,7 +1149,7 @@ csched_vcpu_sleep(const struct scheduler *ops, struct vcpu *vc) cpu_raise_softirq(cpu, SCHEDULE_SOFTIRQ); } else if ( __vcpu_on_runq(svc) ) - __runq_remove(svc); + runq_remove(svc); } static void @@ -1177,7 +1209,7 @@ csched_vcpu_wake(const struct scheduler *ops, struct vcpu *vc) } /* Put the VCPU on the runq and tickle CPUs */ - __runq_insert(svc); + runq_insert(svc); __runq_tickle(svc); } @@ -1679,8 +1711,14 @@ csched_runq_steal(int peer_cpu, int cpu, int pri, int balance_step) SCHED_VCPU_STAT_CRANK(speer, migrate_q); SCHED_STAT_CRANK(migrate_queued); WARN_ON(vc->is_urgent); - __runq_remove(speer); + runq_remove(speer); vc->processor = cpu; + /* + * speer will start executing directly on cpu, without having to + * go through runq_insert(). So we must update the runnable count + * for cpu here. + */ + inc_nr_runnable(cpu); return speer; } } @@ -1736,7 +1774,7 @@ csched_load_balance(struct csched_private *prv, int cpu, peer_node = node; do { - /* Find out what the !idle are in this node */ + /* Select the pCPUs in this node that have work we can steal. */ cpumask_andnot(&workers, online, prv->idlers); cpumask_and(&workers, &workers, &node_to_cpumask(peer_node)); __cpumask_clear_cpu(cpu, &workers); @@ -1746,6 +1784,40 @@ csched_load_balance(struct csched_private *prv, int cpu, goto next_node; do { + spinlock_t *lock; + + /* + * If there is only one runnable vCPU on peer_cpu, it means + * there's no one to be stolen in its runqueue, so skip it. + * + * Checking this without holding the lock is racy... But that's + * the whole point of this optimization! + * + * In more details: + * - if we race with dec_nr_runnable(), we may try to take the + * lock and call csched_runq_steal() for no reason. This is + * not a functional issue, and should be infrequent enough. + * And we can avoid that by re-checking nr_runnable after + * having grabbed the lock, if we want; + * - if we race with inc_nr_runnable(), we skip a pCPU that may + * have runnable vCPUs in its runqueue, but that's not a + * problem because: + * + if racing with csched_vcpu_insert() or csched_vcpu_wake(), + * __runq_tickle() will be called afterwords, so the vCPU + * won't get stuck in the runqueue for too long; + * + if racing with csched_runq_steal(), it may be that a + * vCPU that we could have picked up, stays in a runqueue + * until someone else tries to steal it again. But this is + * no worse than what can happen already (without this + * optimization), it the pCPU would schedule right after we + * have taken the lock, and hence block on it. + */ + if ( CSCHED_PCPU(peer_cpu)->nr_runnable <= 1 ) + { + TRACE_2D(TRC_CSCHED_STEAL_CHECK, peer_cpu, /* skipp'n */ 0); + goto next_cpu; + } + /* * Get ahold of the scheduler lock for this peer CPU. * @@ -1753,14 +1825,13 @@ csched_load_balance(struct csched_private *prv, int cpu, * could cause a deadlock if the peer CPU is also load * balancing and trying to lock this CPU. */ - spinlock_t *lock = pcpu_schedule_trylock(peer_cpu); + lock = pcpu_schedule_trylock(peer_cpu); SCHED_STAT_CRANK(steal_trylock); if ( !lock ) { SCHED_STAT_CRANK(steal_trylock_failed); TRACE_2D(TRC_CSCHED_STEAL_CHECK, peer_cpu, /* skip */ 0); - peer_cpu = cpumask_cycle(peer_cpu, &workers); - continue; + goto next_cpu; } TRACE_2D(TRC_CSCHED_STEAL_CHECK, peer_cpu, /* checked */ 1); @@ -1777,6 +1848,7 @@ csched_load_balance(struct csched_private *prv, int cpu, return speer; } + next_cpu: peer_cpu = cpumask_cycle(peer_cpu, &workers); } while( peer_cpu != cpumask_first(&workers) ); @@ -1907,7 +1979,11 @@ csched_schedule( if ( vcpu_runnable(current) ) __runq_insert(scurr); else + { BUG_ON( is_idle_vcpu(current) || list_empty(runq) ); + /* Current has blocked. Update the runnable counter for this cpu. */ + dec_nr_runnable(cpu); + } snext = __runq_elem(runq->next); ret.migrated = 0; @@ -2024,7 +2100,8 @@ csched_dump_pcpu(const struct scheduler *ops, int cpu) runq = &spc->runq; cpumask_scnprintf(cpustr, sizeof(cpustr), per_cpu(cpu_sibling_mask, cpu)); - printk("CPU[%02d] sort=%d, sibling=%s, ", cpu, spc->runq_sort_last, cpustr); + printk("CPU[%02d] nr_run=%d, sort=%d, sibling=%s, ", + cpu, spc->nr_runnable, spc->runq_sort_last, cpustr); cpumask_scnprintf(cpustr, sizeof(cpustr), per_cpu(cpu_core_mask, cpu)); printk("core=%s\n", cpustr); -- <<This happens because I choose it to happen!>> (Raistlin Majere) ----------------------------------------------------------------- Dario Faggioli, Ph.D, http://about.me/dario.faggioli Senior Software Engineer, Citrix Systems R&D Ltd., Cambridge (UK) Attachment:
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