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Date: Wed, 3 Sep 2014 13:44:58 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Preeti U Murthy <preeti@...ux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Russell King - ARM Linux <linux@....linux.org.uk>,
LAK <linux-arm-kernel@...ts.infradead.org>,
Rik van Riel <riel@...hat.com>,
Morten Rasmussen <Morten.Rasmussen@....com>,
Mike Galbraith <efault@....de>,
Nicolas Pitre <nicolas.pitre@...aro.org>,
"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>,
Daniel Lezcano <daniel.lezcano@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>
Subject: Re: [PATCH v5 08/12] sched: move cfs task on a CPU with higher capacity
On 3 September 2014 11:11, Preeti U Murthy <preeti@...ux.vnet.ibm.com> wrote:
> On 09/01/2014 02:15 PM, Vincent Guittot wrote:
>> On 30 August 2014 19:50, Preeti U Murthy <preeti@...ux.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.
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 ?
Regards,
Vincent
>
> Regards
> Preeti U Murthy
>
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