[RFC PATCH v3 4/6] sched: secure access to other CPU statistics

[Vincent Guittot]

If a CPU accesses the runnable_avg_sum and runnable_avg_period fields of its
buddy CPU while the latter updates it, it can get the new version of a field
and the old version of the other one. This can generate erroneous decisions.
We don't want to use a lock mechanism for ensuring the coherency because of
the overhead in this critical path. The previous attempt can't ensure
coherency of both fields for 100% of the platform and use case as it will
depend of the toolchain and the platform architecture.
The runnable_avg_period of a runqueue tends to the max value in less than
345ms after plugging a CPU, which implies that we could use the max value
instead of reading runnable_avg_period after 345ms. During the starting phase,
we must ensure a minimum of coherency between the fields. A simple rule is
runnable_avg_sum <= runnable_avg_period.

Signed-off-by: Vincent Guittot <vincent.guittot at linaro.org>
Reviewed-by: Morten Rasmussen <morten.rasmussen at arm.com>
---
 kernel/sched/fair.c |    7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 021c7b7..b636199 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -3364,13 +3364,16 @@ done:
 static bool is_buddy_busy(int cpu)
 {
 	struct rq *rq = cpu_rq(cpu);
+	u32 sum = rq->avg.runnable_avg_sum;
+	u32 period = rq->avg.runnable_avg_period;
+
+	sum = min(sum, period);
 
 	/*
 	 * A busy buddy is a CPU with a high load or a small load with a lot of
 	 * running tasks.
 	 */
-	return (rq->avg.runnable_avg_sum >
-			(rq->avg.runnable_avg_period / (rq->nr_running + 2)));
+	return (sum > (period / (rq->nr_running + 2)));
 }
 
 static bool is_light_task(struct task_struct *p)
-- 
1.7.9.5