sched/fair: Spread out tasks evenly when not overloaded

When there is only one CPU per group, using the idle CPUs to evenly spread
tasks doesn't make sense and nr_running is a better metrics.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Ben Segall <bsegall@google.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mel Gorman <mgorman@suse.de>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Morten.Rasmussen@arm.com
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: hdanton@sina.com
Cc: parth@linux.ibm.com
Cc: pauld@redhat.com
Cc: quentin.perret@arm.com
Cc: riel@surriel.com
Cc: srikar@linux.vnet.ibm.com
Cc: valentin.schneider@arm.com
Link: https://lkml.kernel.org/r/1571405198-27570-8-git-send-email-vincent.guittot@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e6a3db08481c7be7ce440cbdc54cace0ffe289c6..f489f603f317e12412c44ae9a76b5f554b654034 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8591,18 +8591,34 @@ static struct sched_group *find_busiest_group(struct lb_env *env)
            busiest->sum_nr_running > local->sum_nr_running + 1)
                goto force_balance;
 
-       if (busiest->group_type != group_overloaded &&
-            (env->idle == CPU_NOT_IDLE ||
-             local->idle_cpus <= (busiest->idle_cpus + 1)))
-               /*
-                * If the busiest group is not overloaded
-                * and there is no imbalance between this and busiest group
-                * wrt. idle CPUs, it is balanced. The imbalance
-                * becomes significant if the diff is greater than 1 otherwise
-                * we might end up just moving the imbalance to another
-                * group.
-                */
-               goto out_balanced;
+       if (busiest->group_type != group_overloaded) {
+               if (env->idle == CPU_NOT_IDLE)
+                       /*
+                        * If the busiest group is not overloaded (and as a
+                        * result the local one too) but this CPU is already
+                        * busy, let another idle CPU try to pull task.
+                        */
+                       goto out_balanced;
+
+               if (busiest->group_weight > 1 &&
+                   local->idle_cpus <= (busiest->idle_cpus + 1))
+                       /*
+                        * If the busiest group is not overloaded
+                        * and there is no imbalance between this and busiest
+                        * group wrt idle CPUs, it is balanced. The imbalance
+                        * becomes significant if the diff is greater than 1
+                        * otherwise we might end up to just move the imbalance
+                        * on another group. Of course this applies only if
+                        * there is more than 1 CPU per group.
+                        */
+                       goto out_balanced;
+
+               if (busiest->sum_h_nr_running == 1)
+                       /*
+                        * busiest doesn't have any tasks waiting to run
+                        */
+                       goto out_balanced;
+       }
 
 force_balance:
        /* Looks like there is an imbalance. Compute it */