Re: [PATCH v5 08/12] sched: move cfs task on a CPU with higher capacity

From: Preeti U Murthy
Date: Wed Sep 03 2014 - 08:26:40 EST

On 09/03/2014 05:14 PM, Vincent Guittot wrote:
> On 3 September 2014 11:11, Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx> wrote:
>> On 09/01/2014 02:15 PM, Vincent Guittot wrote:
>>> On 30 August 2014 19:50, Preeti U Murthy <preeti@xxxxxxxxxxxxxxxxxx> 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.
>
> yes you're right for the test if (rq->nr_running >= 2).
>
> It's not so straight forward for nr_busy_cpus which reflects how many
> CPUs have not stopped their tick. The scheduler assumes that the
> latter are busy with cfs tasks
>
>>
>> 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
>
> I have just replied to your comments on patch 3. Sorry for the delay
>
>> 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.
> This patch adds one additional condition that is based on
> rq->cfs.h_nr_running so it should not trigger any futile load balance.
> Then, I have also take advantage of this patch to clean up
> nohz_kick_needed as proposed by Peter but the conditions are the same
> than previously (except the one with rq->cfs.h_nr_running)
>
> I can prepare another patchset that will solve the concerns that you
> raised for nohz_kick_needed and in patch 3 but i would prefer not
> include them in this patchset which is large enough and which subject
> is a bit different.
> Does it seem ok for you ?

Sure Vincent, thanks! I have in fact sent out a mail raising my concern
over rq->nr_running. If others agree on the issue to be existing, maybe
we can work on this next patchset that can clean this up in all places
necessary and not just in nohz_kick_needed().

Regards
Preeti U Murthy
>
> Regards,
> Vincent
>>
>> Regards
>> Preeti U Murthy
>>
>

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/