[PATCH v5 08/12] sched: move cfs task on a CPU with higher capacity
Preeti U Murthy
preeti at linux.vnet.ibm.com
Wed Sep 3 02:11:31 PDT 2014
On 09/01/2014 02:15 PM, Vincent Guittot wrote:
> On 30 August 2014 19:50, Preeti U Murthy <preeti at linux.vnet.ibm.com> wrote:
>> Hi Vincent,
>>> index 18db43e..60ae1ce 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -6049,6 +6049,14 @@ static bool update_sd_pick_busiest(struct lb_env *env,
>>> return true;
>>> }
>>>
>>> + /*
>>> + * The group capacity is reduced probably because of activity from other
>>> + * sched class or interrupts which use part of the available capacity
>>> + */
>>> + if ((sg->sgc->capacity_orig * 100) > (sgs->group_capacity *
>>> + env->sd->imbalance_pct))
>>
>> Wouldn't the check on avg_load let us know if we are packing more tasks
>> in this group than its capacity ? Isn't that the metric we are more
>> interested in?
>
> With this patch, we don't want to pack but we prefer to spread the
> task on another CPU than the one which handles the interruption if
> latter uses a significant part of the CPU capacity.
>
>>
>>> + return true;
>>> +
>>> return false;
>>> }
>>>
>>> @@ -6534,13 +6542,23 @@ static int need_active_balance(struct lb_env *env)
>>> struct sched_domain *sd = env->sd;
>>>
>>> if (env->idle == CPU_NEWLY_IDLE) {
>>> + int src_cpu = env->src_cpu;
>>>
>>> /*
>>> * ASYM_PACKING needs to force migrate tasks from busy but
>>> * higher numbered CPUs in order to pack all tasks in the
>>> * lowest numbered CPUs.
>>> */
>>> - if ((sd->flags & SD_ASYM_PACKING) && env->src_cpu > env->dst_cpu)
>>> + if ((sd->flags & SD_ASYM_PACKING) && src_cpu > env->dst_cpu)
>>> + return 1;
>>> +
>>> + /*
>>> + * If the CPUs share their cache and the src_cpu's capacity is
>>> + * reduced because of other sched_class or IRQs, we trig an
>>> + * active balance to move the task
>>> + */
>>> + if ((capacity_orig_of(src_cpu) * 100) > (capacity_of(src_cpu) *
>>> + sd->imbalance_pct))
>>> return 1;
>>> }
>>>
>>> @@ -6643,6 +6661,8 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>>>
>>> schedstat_add(sd, lb_imbalance[idle], env.imbalance);
>>>
>>> + env.src_cpu = busiest->cpu;
>>> +
>>> ld_moved = 0;
>>> if (busiest->nr_running > 1) {
>>> /*
>>> @@ -6652,7 +6672,6 @@ static int load_balance(int this_cpu, struct rq *this_rq,
>>> * correctly treated as an imbalance.
>>> */
>>> env.flags |= LBF_ALL_PINNED;
>>> - env.src_cpu = busiest->cpu;
>>> env.src_rq = busiest;
>>> env.loop_max = min(sysctl_sched_nr_migrate, busiest->nr_running);
>>>
>>> @@ -7359,10 +7378,12 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
>>>
>>> /*
>>> * Current heuristic for kicking the idle load balancer in the presence
>>> - * of an idle cpu is the system.
>>> + * of an idle cpu in the system.
>>> * - This rq has more than one task.
>>> - * - At any scheduler domain level, this cpu's scheduler group has multiple
>>> - * busy cpu's exceeding the group's capacity.
>>> + * - This rq has at least one CFS task and the capacity of the CPU is
>>> + * significantly reduced because of RT tasks or IRQs.
>>> + * - At parent of LLC scheduler domain level, this cpu's scheduler group has
>>> + * multiple busy cpu.
>>> * - For SD_ASYM_PACKING, if the lower numbered cpu's in the scheduler
>>> * domain span are idle.
>>> */
>>> @@ -7372,9 +7393,10 @@ static inline int nohz_kick_needed(struct rq *rq)
>>> struct sched_domain *sd;
>>> struct sched_group_capacity *sgc;
>>> int nr_busy, cpu = rq->cpu;
>>> + bool kick = false;
>>>
>>> if (unlikely(rq->idle_balance))
>>> - return 0;
>>> + return false;
>>>
>>> /*
>>> * We may be recently in ticked or tickless idle mode. At the first
>>> @@ -7388,38 +7410,45 @@ static inline int nohz_kick_needed(struct rq *rq)
>>> * balancing.
>>> */
>>> if (likely(!atomic_read(&nohz.nr_cpus)))
>>> - return 0;
>>> + return false;
>>>
>>> if (time_before(now, nohz.next_balance))
>>> - return 0;
>>> + return false;
>>>
>>> if (rq->nr_running >= 2)
>>
>> Will this check ^^ not catch those cases which this patch is targeting?
>
> This patch is not about how many tasks are running but if the capacity
> of the CPU is reduced because of side activity like interruptions
> which are only visible in the capacity value (with IRQ_TIME_ACCOUNTING
> config) but not in nr_running.
> Even if the capacity is reduced because of RT tasks, nothing ensures
> that the RT task will be running when the tick fires
>
> Regards,
> Vincent
>>
>> Regards
>> Preeti U Murthy
>>
>>> - goto need_kick;
>>> + return true;
>>>
>>> rcu_read_lock();
>>> sd = rcu_dereference(per_cpu(sd_busy, cpu));
>>> -
>>> if (sd) {
>>> sgc = sd->groups->sgc;
>>> nr_busy = atomic_read(&sgc->nr_busy_cpus);
>>>
>>> - if (nr_busy > 1)
>>> - goto need_kick_unlock;
>>> + if (nr_busy > 1) {
>>> + kick = true;
>>> + goto unlock;
>>> + }
>>> +
>>> }
>>>
>>> - sd = rcu_dereference(per_cpu(sd_asym, cpu));
>>> + sd = rcu_dereference(rq->sd);
>>> + if (sd) {
>>> + if ((rq->cfs.h_nr_running >= 1) &&
>>> + ((rq->cpu_capacity * sd->imbalance_pct) <
>>> + (rq->cpu_capacity_orig * 100))) {
Ok I understand your explanation above. But I was wondering if you would
need to add this check around rq->cfs.h_nr_running >= 1 in the above two
cases as well.
I have actually raised this concern over whether we should be using
rq->nr_running or cfs_rq->nr_running while we do load balancing in reply
to your patch3. While all our load measurements are about the cfs_rq
load, we use rq->nr_running, which may include tasks from other sched
classes as well. We divide them to get average load per task during load
balancing which is wrong, isn't it? Similarly during nohz_kick_needed(),
we trigger load balancing based on rq->nr_running again.
In this patch too, even if you know that the cpu is being dominated by
tasks that do not belong to cfs class, you would be triggering a futile
load balance if there are no fair tasks to move.
Regards
Preeti U Murthy
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