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Message-ID: <54588E89.1080108@gmail.com>
Date: Tue, 04 Nov 2014 16:30:01 +0800
From: Wanpeng Li <kernellwp@...il.com>
To: Vincent Guittot <vincent.guittot@...aro.org>, peterz@...radead.org,
mingo@...nel.org, linux-kernel@...r.kernel.org,
preeti@...ux.vnet.ibm.com, Morten.Rasmussen@....com,
kamalesh@...ux.vnet.ibm.com, linux@....linux.org.uk,
linux-arm-kernel@...ts.infradead.org
CC: riel@...hat.com, efault@....de, nicolas.pitre@...aro.org,
linaro-kernel@...ts.linaro.org
Subject: Re: [PATCH v8 03/10] sched: move cfs task on a CPU with higher capacity
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?
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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