FROMLIST: sched/fair: Set rq->rd->overload when misfit

Idle balance is a great opportunity to pull a misfit task. However,
there are scenarios where misfit tasks are present but idle balance is
prevented by the overload flag.

A good example of this is a workload of n identical tasks. Let's suppose
we have a 2+2 Arm big.LITTLE system. We then spawn 4 fairly
CPU-intensive tasks - for the sake of simplicity let's say they are just
CPU hogs, even when running on big CPUs.

They are identical tasks, so on an SMP system they should all end at
(roughly) the same time. However, in our case the LITTLE CPUs are less
performing than the big CPUs, so tasks running on the LITTLEs will have
a longer completion time.

This means that the big CPUs will complete their work earlier, at which
point they should pull the tasks from the LITTLEs. What we want to
happen is summarized as follows:

a,b,c,d are our CPU-hogging tasks
_ signifies idling

LITTLE_0 | a a a a _ _
LITTLE_1 | b b b b _ _
---------|-------------
  big_0  | c c c c a a
  big_1  | d d d d b b
		  ^
		  ^
    Tasks end on the big CPUs, idle balance happens
    and the misfit tasks are pulled straight away

This however won't happen, because currently the overload flag is only
set when there is any CPU that has more than one runnable task - which
may very well not be the case here if our CPU-hogging workload is all
there is to run.

As such, this commit sets the overload flag in update_sg_lb_stats when
a group is flagged as having a misfit task.

cc: Ingo Molnar <mingo@redhat.com>
cc: Peter Zijlstra <peterz@infradead.org>

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Change-Id: I86d5aab67bd2c91a0b193f21844a4634a492f331

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index ab8bfd2..990dd03 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8370,7 +8370,7 @@ static bool update_nohz_stats(struct rq *rq, bool force)
  * @load_idx: Load index of sched_domain of this_cpu for load calc.
  * @local_group: Does group contain this_cpu.
  * @sgs: variable to hold the statistics for this group.
- * @overload: Indicate more than one runnable task for any CPU.
+ * @overload: Indicate pullable load (e.g. >1 runnable task).
  * @overutilized: Indicate overutilization for any CPU.
  */
 static inline void update_sg_lb_stats(struct lb_env *env,
@@ -8418,8 +8418,10 @@ static inline void update_sg_lb_stats(struct lb_env *env,
                        sgs->idle_cpus++;
 
                if (env->sd->flags & SD_ASYM_CPUCAPACITY &&
-                   sgs->group_misfit_task_load < rq->misfit_task_load)
+                   sgs->group_misfit_task_load < rq->misfit_task_load) {
                        sgs->group_misfit_task_load = rq->misfit_task_load;
+                       *overload = 1;
+               }
        }
 
        /* Adjust by relative CPU capacity of the group */

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 90e9386..ea33476 100644 (file)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -702,7 +702,11 @@ struct root_domain {
        cpumask_var_t           span;
        cpumask_var_t           online;
 
-       /* Indicate more than one runnable task for any CPU */
+       /*
+        * Indicate pullable load on at least one CPU, e.g:
+        * - More than one runnable task
+        * - Running task is misfit
+        */
        int                     overload;
 
        /* Indicate one or more cpus over-utilized (tipping point) */