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Message-ID: <5458AD87.7020709@gmail.com>
Date: Tue, 04 Nov 2014 18:42:15 +0800
From: Wanpeng Li <kernellwp@...il.com>
To: Vincent Guittot <vincent.guittot@...aro.org>
CC: Peter Zijlstra <peterz@...radead.org>,
Ingo Molnar <mingo@...nel.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Preeti U Murthy <preeti@...ux.vnet.ibm.com>,
Morten Rasmussen <Morten.Rasmussen@....com>,
Kamalesh Babulal <kamalesh@...ux.vnet.ibm.com>,
Russell King - ARM Linux <linux@....linux.org.uk>,
LAK <linux-arm-kernel@...ts.infradead.org>,
Rik van Riel <riel@...hat.com>,
Mike Galbraith <efault@....de>,
Nicolas Pitre <nicolas.pitre@...aro.org>,
"linaro-kernel@...ts.linaro.org" <linaro-kernel@...ts.linaro.org>
Subject: Re: [PATCH v8 03/10] sched: move cfs task on a CPU with higher capacity
Hi Vincent,
On 14/11/4 下午5:41, Vincent Guittot wrote:
> On 4 November 2014 09:30, Wanpeng Li <kernellwp@...il.com> wrote:
>> On 14/10/31 下午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 <vincent.guittot@...aro.org>
>>> ---
>>> 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) { }
>>
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