by Jason Low

[permalink] [raw]

Subject: Re: sched: Improve load balancing in the presence of idle CPUs

> > ---
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index fdae26e..d636bf7 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -7620,6 +7620,16 @@ out:
> > }
> >
> > #ifdef CONFIG_NO_HZ_COMMON
> > +static inline bool nohz_kick_needed(struct rq *rq);
> > +
> > +static inline void pass_nohz_balance(struct rq *this_rq, int this_cpu)
> > +{
> > + clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
> > + nohz.next_balance = jiffies;
> > + if (nohz_kick_needed(this_rq))
> > + nohz_balancer_kick();
> > +}
> > +
> > /*
> > * In CONFIG_NO_HZ_COMMON case, the idle balance kickee will do the
> > * rebalancing for all the cpus for whom scheduler ticks are stopped.
> > @@ -7631,8 +7641,10 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
> > int balance_cpu;
> >
> > if (idle != CPU_IDLE ||
> > - !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
> > - goto end;
> > + !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu))) {
> > + pass_nohz_balance(this_rq, this_cpu);
> > + return;
> > + }

hmm, so taking a look at the patch again, it looks like we pass nohz
balance even when the NOHZ_BALANCE_KICK is not set on the current CPU.
We should separate the 2 conditions:

if (!test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
return;

if (idle != CPU_IDLE) {
/* another CPU continue balancing */
pass_nohz_balance(this_rq, this_cpu);
return;
}

In general, separating the check also optimizes nohz_idle_balance() to
avoid clearing the bit when it is not set.

2015-04-14 03:00:19

by Jason Low

[permalink] [raw]

Subject: Re: sched: Improve load balancing in the presence of idle CPUs

On Mon, 2015-04-13 at 15:49 -0700, Jason Low wrote:

> hmm, so taking a look at the patch again, it looks like we pass nohz
> balance even when the NOHZ_BALANCE_KICK is not set on the current CPU.
> We should separate the 2 conditions:
>
> if (!test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
> return;
>
> if (idle != CPU_IDLE) {
> /* another CPU continue balancing */
> pass_nohz_balance(this_rq, this_cpu);
> return;
> }

Here's the example patch with the above update.

---
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ffeaa41..9aa48f7 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7622,6 +7622,16 @@ out:
}

#ifdef CONFIG_NO_HZ_COMMON
+static inline bool nohz_kick_needed(struct rq *rq);
+
+static inline void pass_nohz_balance(struct rq *this_rq, int this_cpu)
+{
+ clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
+ nohz.next_balance = jiffies;
+ if (nohz_kick_needed(this_rq))
+ nohz_balancer_kick();
+}
+
/*
* In CONFIG_NO_HZ_COMMON case, the idle balance kickee will do the
* rebalancing for all the cpus for whom scheduler ticks are stopped.
@@ -7632,9 +7642,13 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
struct rq *rq;
int balance_cpu;

- if (idle != CPU_IDLE ||
- !test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
- goto end;
+ if (!test_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu)))
+ return;
+
+ if (idle != CPU_IDLE) {
+ pass_nohz_balance(this_rq, this_cpu);
+ return;
+ }

for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
@@ -7645,8 +7659,10 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
* work being done for other cpus. Next load
* balancing owner will pick it up.
*/
- if (need_resched())
- break;
+ if (need_resched()) {
+ pass_nohz_balance(this_rq, this_cpu);
+ return;
+ }

rq = cpu_rq(balance_cpu);

@@ -7666,7 +7682,6 @@ static void nohz_idle_balance(struct rq *this_rq, enum cpu_idle_type idle)
this_rq->next_balance = rq->next_balance;
}
nohz.next_balance = this_rq->next_balance;
-end:
clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
}

@@ -7689,7 +7704,7 @@ static inline bool nohz_kick_needed(struct rq *rq)
int nr_busy, cpu = rq->cpu;
bool kick = false;

- if (unlikely(rq->idle_balance))
+ if (unlikely(idle_cpu(cpu)))
return false;

/*
@@ -7709,7 +7724,7 @@ static inline bool nohz_kick_needed(struct rq *rq)
if (time_before(now, nohz.next_balance))
return false;

- if (rq->nr_running >= 2)
+ if (rq->nr_running >= 2 || rq->rd->overload)
return true;

rcu_read_lock();
@@ -7759,16 +7774,14 @@ static void run_rebalance_domains(struct softirq_action *h)
enum cpu_idle_type idle = this_rq->idle_balance ?
CPU_IDLE : CPU_NOT_IDLE;

+ rebalance_domains(this_rq, idle);
+
/*
* If this cpu has a pending nohz_balance_kick, then do the
* balancing on behalf of the other idle cpus whose ticks are
- * stopped. Do nohz_idle_balance *before* rebalance_domains to
- * give the idle cpus a chance to load balance. Else we may
- * load balance only within the local sched_domain hierarchy
- * and abort nohz_idle_balance altogether if we pull some load.
+ * stopped.
*/
nohz_idle_balance(this_rq, idle);
- rebalance_domains(this_rq, idle);
}

/*