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

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

On Fri, 2015-04-10 at 14:07 +0530, Srikar Dronamraju wrote:
> > > >
> > > >  #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".
> >
> 
> Since we are in nohz_idle_balance(), nohz.next_balance is guaranteed be
> less than now. We do check for time_before(now, nohz.next_balance) in
> nohz_kick_needed(). So in effect we are incrementing the nohz.next_balance.

Hi Srikar,

If now is equal to nohz.next_balance, we may attempt
nohz_balancer_kick().

After it does nohz.next_balance++ in nohz_balancer_kick(), now can be 1
less than the new nohz.next_balance value by the time
nohz_idle_balance() is attempted(). Without updating nohz.next_balance,
the time_before(now, nohz.next_balance) check in nohz_kick_needed() may
cause it to return false.