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Message-ID: <1428965360.2605.33.camel@j-VirtualBox>
Subject: Re: sched: Improve load balancing in the presence of idle CPUs
From: Jason Low <jason.low2@hp.com>
To: Preeti U Murthy <preeti@linux.vnet.ibm.com>
Cc: Morten Rasmussen <morten.rasmussen@arm.com>,
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Date: Mon, 13 Apr 2015 15:49:20 -0700
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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.