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Subject[PATCH v2] sched/fair: reduce long-tail newly idle balance cost
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A long-tail load balance cost is observed on the newly idle path,
this is caused by a race window between the first nr_running check
of the busiest runqueue and its nr_running recheck in detach_tasks.

Before the busiest runqueue is locked, the tasks on the busiest
runqueue could be pulled by other CPUs and nr_running of the busiest
runqueu becomes 1 or even 0 if the running task becomes idle, this
causes detach_tasks breaks with LBF_ALL_PINNED flag set, and triggers
load_balance redo at the same sched_domain level.

In order to find the new busiest sched_group and CPU, load balance will
recompute and update the various load statistics, which eventually leads
to the long-tail load balance cost.

This patch clears LBF_ALL_PINNED flag for this race condition, and hence
reduces the long-tail cost of newly idle balance.

Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Mel Gorman <mgorman@techsingularity.net>
Cc: Andi Kleen <ak@linux.intel.com>
Cc: Tim Chen <tim.c.chen@linux.intel.com>
Cc: Srinivas Pandruvada <srinivas.pandruvada@linux.intel.com>
Cc: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Aubrey Li <aubrey.li@linux.intel.com>
---
kernel/sched/fair.c | 9 +++++++++
1 file changed, 9 insertions(+)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 04a3ce2..5c67804 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7675,6 +7675,15 @@ static int detach_tasks(struct lb_env *env)

lockdep_assert_held(&env->src_rq->lock);

+ /*
+ * Source run queue has been emptied by another CPU, clear
+ * LBF_ALL_PINNED flag as we will not test any task.
+ */
+ if (env->src_rq->nr_running <= 1) {
+ env->flags &= ~LBF_ALL_PINNED;
+ return 0;
+ }
+
if (env->imbalance <= 0)
return 0;

--
2.7.4
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