UPSTREAM: sched/fair: Avoid pulling tasks from non-overloaded higher capacity groups

For asymmetric CPU capacity systems it is counter-productive for
throughput if low capacity CPUs are pulling tasks from non-overloaded
CPUs with higher capacity. The assumption is that higher CPU capacity is
preferred over running alone in a group with lower CPU capacity.

This patch rejects higher CPU capacity groups with one or less task per
CPU as potential busiest group which could otherwise lead to a series of
failing load-balancing attempts leading to a force-migration.

Change-Id: I428875bb6267c780026ef75e2882300738d016e7
Signed-off-by: Morten Rasmussen <morten.rasmussen@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: dietmar.eggemann@arm.com
Cc: freedom.tan@mediatek.com
Cc: keita.kobayashi.ym@renesas.com
Cc: mgalbraith@suse.de
Cc: sgurrappadi@nvidia.com
Cc: vincent.guittot@linaro.org
Cc: yuyang.du@intel.com
Link: http://lkml.kernel.org/r/1476452472-24740-5-git-send-email-morten.rasmussen@arm.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>
(cherry picked from commit 9e0994c0a1c1f82c705f1f66388e1bcffcee8bb9)
Signed-off-by: Chris Redpath <chris.redpath@arm.com>

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index d18e2c7b67e59868a64d3ab15af49c394b15d524..c00a8ae48a46201c4aae8401ad66e4bb37d459bc 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7569,14 +7569,20 @@ static bool update_sd_pick_busiest(struct lb_env *env,
        if (sgs->avg_load <= busiest->avg_load)
                return false;
 
+       if (!(env->sd->flags & SD_ASYM_CPUCAPACITY))
+               goto asym_packing;
+
        /*
-        * Candiate sg has no more than one task per cpu and has higher
-        * per-cpu capacity. No reason to pull tasks to less capable cpus.
+        * Candidate sg has no more than one task per CPU and
+        * has higher per-CPU capacity. Migrating tasks to less
+        * capable CPUs may harm throughput. Maximize throughput,
+        * power/energy consequences are not considered.
         */
        if (sgs->sum_nr_running <= sgs->group_weight &&
            group_smaller_cpu_capacity(sds->local, sg))
                return false;
 
+asym_packing:
        /* This is the busiest node in its class. */
        if (!(env->sd->flags & SD_ASYM_PACKING))
                return true;