[RFC PATCH v2 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>
---
 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 fc93d96..f1a4c24 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5153,13 +5153,16 @@ static bool numa_allow_migration(struct task_struct *p, int prev_cpu, int new_cp
 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->nr_running) >
-			rq->avg.runnable_avg_period);
+	return ((sum << rq->nr_running) > period);
 }
 
 static bool is_light_task(struct task_struct *p)
-- 
1.7.9.5