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>
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index b15e2bb4cd424244c00302995f9f515141bafaae..b7fbfebbc024cc503d1d94046044f1fcdf0136e9 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -8052,8 +8052,10 @@ 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;
+                       *sg_status |= SG_OVERLOAD;
+               }
        }
 
        /* Adjust by relative CPU capacity of the group */

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 73d4432f66a220635b56ecc9ab5b4861f92e635f..dcb1be6b3f5a3816bfcdc7e8d90f424665462b86 100644 (file)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -726,7 +726,11 @@ 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) */