Messages in this thread | | | From | Vincent Guittot <> | Date | Wed, 8 Jan 2020 09:05:20 +0100 | Subject | Re: [PATCH] sched/fair: Load balance aggressively for SCHED_IDLE CPUs |
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On Tue, 7 Jan 2020 at 13:42, Peter Zijlstra <peterz@infradead.org> wrote: > > On Tue, Dec 24, 2019 at 10:43:30AM +0530, Viresh Kumar wrote: > > The fair scheduler performs periodic load balance on every CPU to check > > if it can pull some tasks from other busy CPUs. The duration of this > > periodic load balance is set to sd->balance_interval for the idle CPUs > > and is calculated by multiplying the sd->balance_interval with the > > sd->busy_factor (set to 32 by default) for the busy CPUs. The > > multiplication is done for busy CPUs to avoid doing load balance too > > often and rather spend more time executing actual task. While that is > > the right thing to do for the CPUs busy with SCHED_OTHER or SCHED_BATCH > > tasks, it may not be the optimal thing for CPUs running only SCHED_IDLE > > tasks. > > > > With the recent enhancements in the fair scheduler around SCHED_IDLE > > CPUs, we now prefer to enqueue a newly-woken task to a SCHED_IDLE > > CPU instead of other busy or idle CPUs. The same reasoning should be > > applied to the load balancer as well to make it migrate tasks more > > aggressively to a SCHED_IDLE CPU, as that will reduce the scheduling > > latency of the migrated (SCHED_OTHER) tasks. > > > > This patch makes minimal changes to the fair scheduler to do the next > > load balance soon after the last non SCHED_IDLE task is dequeued from a > > runqueue, i.e. making the CPU SCHED_IDLE. Also the sd->busy_factor is > > ignored while calculating the balance_interval for such CPUs. This is > > done to avoid delaying the periodic load balance by few hundred > > milliseconds for SCHED_IDLE CPUs. > > > > This is tested on ARM64 Hikey620 platform (octa-core) with the help of > > rt-app and it is verified, using kernel traces, that the newly > > SCHED_IDLE CPU does load balancing shortly after it becomes SCHED_IDLE > > and pulls tasks from other busy CPUs. > > Nothing seems really objectionable here; I have a few comments below. > > Vincent?
The change makes sense to me. This should fix the last remaining long scheduling latency of SCHED_OTHER tasks in presence of SCHED_IDLE tasks
With the change proposed by Peter below you can add my Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
> > > > @@ -5324,6 +5336,7 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags) > > struct sched_entity *se = &p->se; > > int task_sleep = flags & DEQUEUE_SLEEP; > > int idle_h_nr_running = task_has_idle_policy(p); > > + bool was_sched_idle = sched_idle_rq(rq); > > > > for_each_sched_entity(se) { > > cfs_rq = cfs_rq_of(se); > > @@ -5370,6 +5383,10 @@ static void dequeue_task_fair(struct rq *rq, struct task_struct *p, int flags) > > if (!se) > > sub_nr_running(rq, 1); > > > > + /* balance early to pull high priority tasks */ > > + if (unlikely(!was_sched_idle && sched_idle_rq(rq))) > > + rq->next_balance = jiffies; > > + > > util_est_dequeue(&rq->cfs, p, task_sleep); > > hrtick_update(rq); > > } > > This can effectively set ->next_balance in the past, but given we only > tickle the balancer on every jiffy edge, that is of no concern. It just > made me stumble when reading this. > > Not sure it even deserves a comment or not.. > > > @@ -9531,6 +9539,7 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle) > > { > > int continue_balancing = 1; > > int cpu = rq->cpu; > > + int busy = idle != CPU_IDLE && !sched_idle_cpu(cpu); > > unsigned long interval; > > struct sched_domain *sd; > > /* Earliest time when we have to do rebalance again */ > > @@ -9567,7 +9576,7 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle) > > break; > > } > > > > - interval = get_sd_balance_interval(sd, idle != CPU_IDLE); > > + interval = get_sd_balance_interval(sd, busy); > > > > need_serialize = sd->flags & SD_SERIALIZE; > > if (need_serialize) { > > @@ -9582,10 +9591,16 @@ static void rebalance_domains(struct rq *rq, enum cpu_idle_type idle) > > * env->dst_cpu, so we can't know our idle > > * state even if we migrated tasks. Update it. > > */ > > - idle = idle_cpu(cpu) ? CPU_IDLE : CPU_NOT_IDLE; > > + if (idle_cpu(cpu)) { > > + idle = CPU_IDLE; > > + busy = 0; > > + } else { > > + idle = CPU_NOT_IDLE; > > + busy = !sched_idle_cpu(cpu); > > + } > > This is inconsistent vs the earlier code. That is, why not write it > like: > > idle = idle_cpu(cpu) ? CPU_IDLE : CPU_NOT_IDLE; > busy = idle != CPU_IDLE && !sched_idle_cpu(cpu);
This looks easier to read
> > > } > > sd->last_balance = jiffies; > > - interval = get_sd_balance_interval(sd, idle != CPU_IDLE); > > + interval = get_sd_balance_interval(sd, busy); > > } > > if (need_serialize) > > spin_unlock(&balancing);
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