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

[Jason Low <jason.low2@hp.com>]

On Wed, 2015-04-08 at 16:42 +0530, Srikar Dronamraju wrote:
> * Jason Low <jason.low2@hp.com> [2015-04-07 17:07:46]:
> 
> > On Tue, 2015-04-07 at 16:28 -0700, Jason Low wrote:
> > 
> > > Okay, so perhaps we can also try continuing nohz load balancing if we
> > > find that there are overloaded CPUs in the system.
> > 
> > Something like the following.
> > 
> > ---
> > 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;
> 
> Why are we updating nohz.next_balance here?

This was just to make sure that since we're continuing the balancing on
another CPU that the nohz next_balance is guaranteed to be "now".

> > +	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 ||
> 
> Would it make sense to add need_resched here like
> http://mid.gmane.org/1427442750-8112-1-git-send-email-wanpeng.li@linux.intel.com

Yeah, we could have incorporated adding the need_resched there too for
testing purposes.

Though that probably wouldn't make too much of a difference in
performance with this patch, since this also modified the need_resched()
check in the loop + nohz.next_balance. So I think it would still be fine
to test this without the added need_resched().