Subject: [PATCH v8 03/10] sched: move cfs task on a CPU with higher capacity

From: [email-masked] (Wanpeng Li)
Date: Tue, 04 Nov 2014 18:42:15 +0800

Hi Vincent,
On 14/11/4 ??5:41, Vincent Guittot wrote:
> On 4 November 2014 09:30, Wanpeng Li <[email-masked]> wrote:
>> On 14/10/31 ??4:47, Vincent Guittot wrote:
>>> When a CPU is used to handle a lot of IRQs or some RT tasks, the remaining
>>> capacity for CFS tasks can be significantly reduced. Once we detect such
>>> situation by comparing cpu_capacity_orig and cpu_capacity, we trig an idle
>>> load balance to check if it's worth moving its tasks on an idle CPU.
>>>
>>> Once the idle load_balance has selected the busiest CPU, it will look for
>>> an
>>> active load balance for only two cases :
>>> - there is only 1 task on the busiest CPU.
>>> - we haven't been able to move a task of the busiest rq.
>>>
>>> A CPU with a reduced capacity is included in the 1st case, and it's worth
>>> to
>>> actively migrate its task if the idle CPU has got full capacity. This test
>>> has
>>> been added in need_active_balance.
>>>
>>> As a sidenote, this will note generate more spurious ilb because we
>>> already
>>> trig an ilb if there is more than 1 busy cpu. If this cpu is the only one
>>> that
>>> has a task, we will trig the ilb once for migrating the task.
>>>
>>> The nohz_kick_needed function has been cleaned up a bit while adding the
>>> new
>>> test
>>>
>>> env.src_cpu and env.src_rq must be set unconditionnally because they are
>>> used
>>> in need_active_balance which is called even if busiest->nr_running equals
>>> 1
>>>
>>> Signed-off-by: Vincent Guittot <[email-masked]>
>>> ---
>>>    kernel/sched/fair.c | 86
>>> ++++++++++++++++++++++++++++++++++++++++-------------
>>>    1 file changed, 65 insertions(+), 21 deletions(-)
>>>
>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>> index bd214d2..54468f3 100644
>>> --- a/kernel/sched/fair.c
>>> +++ b/kernel/sched/fair.c
>>> @@ -5889,6 +5889,18 @@ fix_small_capacity(struct sched_domain *sd, struct
>>> sched_group *group)
>>>    }
>>>      /*
>>> + * Check whether the capacity of the rq has been noticeably reduced by
>>> side
>>> + * activity. The imbalance_pct is used for the threshold.
>>> + * Return true is the capacity is reduced
>>> + */
>>> +static inline int
>>> +check_cpu_capacity(struct rq *rq, struct sched_domain *sd)
>>> +{
>>> +       return ((rq->cpu_capacity * sd->imbalance_pct) <
>>> +                               (rq->cpu_capacity_orig * 100));
>>> +}
>>> +
>>> +/*
>>>     * Group imbalance indicates (and tries to solve) the problem where
>>> balancing
>>>     * groups is inadequate due to tsk_cpus_allowed() constraints.
>>>     *
>>> @@ -6562,6 +6574,28 @@ static int need_active_balance(struct lb_env *env)
>>>                          return 1;
>>>          }
>>>    +     /*
>>> +        * The dst_cpu is idle and the src_cpu CPU has only 1 CFS task.
>>
>> Why specify one task instead of not less than one?
> if cfs.h_nr_running == 0 (which should not occurs at that point), we
> don't need to do more test to check if it's worth moving the task
> because there is no task to move.

Sorry for my confusing statement. I mean cfs.h_nr_running >= 1.

Regards,
Wanpeng Li

>
> Regards,
> Vincent
>> Regards,
>> Wanpeng Li
>>
>>
>>> +        * It's worth migrating the task if the src_cpu's capacity is
>>> reduced
>>> +        * because of other sched_class or IRQs whereas capacity stays
>>> +        * available on dst_cpu.
>>> +        */
>>> +       if ((env->idle != CPU_NOT_IDLE) &&
>>> +                       (env->src_rq->cfs.h_nr_running == 1)) {
>>> +               unsigned long src_eff_capacity, dst_eff_capacity;
>>> +
>>> +               dst_eff_capacity = 100;
>>> +               dst_eff_capacity *= capacity_of(env->dst_cpu);
>>> +               dst_eff_capacity *= capacity_orig_of(env->src_cpu);
>>> +
>>> +               src_eff_capacity = sd->imbalance_pct;
>>> +               src_eff_capacity *= capacity_of(env->src_cpu);
>>> +               src_eff_capacity *= capacity_orig_of(env->dst_cpu);
>>> +
>>> +               if (src_eff_capacity < dst_eff_capacity)
>>> +                       return 1;
>>> +       }
>>> +
>>>          return unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2);
>>>    }
>>>    @@ -6661,6 +6695,9 @@ static int load_balance(int this_cpu, struct rq
>>> *this_rq,
>>>          schedstat_add(sd, lb_imbalance[idle], env.imbalance);
>>>    +     env.src_cpu = busiest->cpu;
>>> +       env.src_rq = busiest;
>>> +
>>>          ld_moved = 0;
>>>          if (busiest->nr_running > 1) {
>>>                  /*
>>> @@ -6670,8 +6707,6 @@ 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);
>>>      more_balance:
>>> @@ -7371,22 +7406,25 @@ 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.
>>>     */
>>> -static inline int nohz_kick_needed(struct rq *rq)
>>> +static inline bool nohz_kick_needed(struct rq *rq)
>>>    {
>>>          unsigned long now = jiffies;
>>>          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
>>> @@ -7400,38 +7438,44 @@ 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)
>>> -               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) &&
>>> +                               check_cpu_capacity(rq, sd)) {
>>> +                       kick = true;
>>> +                       goto unlock;
>>> +               }
>>> +       }
>>>    +     sd = rcu_dereference(per_cpu(sd_asym, cpu));
>>>          if (sd && (cpumask_first_and(nohz.idle_cpus_mask,
>>>                                    sched_domain_span(sd)) < cpu))
>>> -               goto need_kick_unlock;
>>> +               kick = true;
>>>    +unlock:
>>>          rcu_read_unlock();
>>> -       return 0;
>>> -
>>> -need_kick_unlock:
>>> -       rcu_read_unlock();
>>> -need_kick:
>>> -       return 1;
>>> +       return kick;
>>>    }
>>>    #else
>>>    static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type
>>> idle) { }
>>