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

[Tim Chen <tim.c.chen@linux.intel.com>]

On Tue, 2015-04-07 at 10:42 -0700, Jason Low wrote:
> On Fri, 2015-04-03 at 15:35 -0700, Tim Chen wrote:
> > I think we can get rid of the done_balancing boolean 
> > and make it a bit easier to read if we change the above code to
> > 
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index bcfe320..08317dc 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -7557,8 +7557,13 @@ 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()) {
> > +                       /* preparing to bail, kicking other cpu to continue */
> > +                       clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
> > +                       if (nohz_kick_needed(this_rq))
> > +                               nohz_balance_kick();
> > +                       return;
> > +               }
> 
> Hi Tim,
> 
> We would also need the nohz_kick_needed/nohz_balance_kick if we
> initially find that the current CPU is not idle (at the beginning of
> nohz_idle_balance). In the above case, we would need to add the code to
> 2 locations.
> 
> Would it be better to still keep the done_balancing to avoid having
> duplicate code?
> 

How about consolidating the code for passing the
nohz balancing and call it at both places.  
Something like below.  Make the code more readable.

Tim

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 40667cb..16f6904 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7531,6 +7531,15 @@ out:
 }
 
 #ifdef CONFIG_NO_HZ_COMMON
+static inline int nohz_kick_needed(struct rq *rq);
+
+static void inline pass_nohz_balance(struct rq *this_rq, int this_cpu)
+{
+       clear_bit(NOHZ_BALANCE_KICK, nohz_flags(this_cpu));
+       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.
@@ -7542,8 +7551,10 @@ 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;
+       }
 
        for_each_cpu(balance_cpu, nohz.idle_cpus_mask) {
                if (balance_cpu == this_cpu || !idle_cpu(balance_cpu))
@@ -7554,8 +7565,10 @@ 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);
 
@@ -7575,7 +7588,6 @@ 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));
 }