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SubjectRe: sched: Improve load balancing in the presence of idle CPUs
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> > ---
> > 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.



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