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Subject[PATCH v7 2/7] sched: move cfs task on a CPU with higher capacity
Date
If the CPU is used for handling lot of IRQs, trig a load balance to check if
it's worth moving its tasks on another CPU that has more capacity.

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 unconditionnaly 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@linaro.org>
---
kernel/sched/fair.c | 70 ++++++++++++++++++++++++++++++++++++++---------------
1 file changed, 50 insertions(+), 20 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index c3674da..9075dee 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5896,6 +5896,18 @@ 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.
*
@@ -6567,6 +6579,14 @@ static int need_active_balance(struct lb_env *env)
*/
if ((sd->flags & SD_ASYM_PACKING) && env->src_cpu > env->dst_cpu)
return 1;
+
+ /*
+ * The src_cpu's capacity is reduced because of other
+ * sched_class or IRQs, we trig an active balance to move the
+ * task
+ */
+ if (check_cpu_capacity(env->src_rq, sd))
+ return 1;
}

return unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2);
@@ -6668,6 +6688,9 @@ 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) {
/*
@@ -6677,8 +6700,6 @@ 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:
@@ -7378,10 +7399,12 @@ 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.
*/
@@ -7391,9 +7414,10 @@ 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
@@ -7407,38 +7431,44 @@ 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;
-
- rcu_read_unlock();
- return 0;
+ kick = true;

-need_kick_unlock:
+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) { }
--
1.9.1


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