@@ -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)
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@linaro.org> --- kernel/sched/fair.c | 7 +++++-- 1 file changed, 5 insertions(+), 2 deletions(-)