From: Dietmar Eggemann dietmar.eggemann@arm.com
There is an error scenario where on a 1x1 HMP system (weight of the hmp_slow_cpu_mask is 1) the short-cut of restricting the allowed cpu mask of an rt tasks leads to triggering a kernel bug in the rt sched class set_cpus_allowed function set_cpus_allowed_rt().
In case the task is on the run-queue and the weight of the required cpu mask is 1 and this is different to the p->nr_cpus_allowed value, this back-end function interprets this in such a way that a task changed from being migratable to not migratable anymore and decrements the rt_nr_migratory counter. There is a BUG_ON(!rq->rt.rt_nr_migratory) check in this code path which triggers in this situation.
To circumvent this issue, set the number of allowed cpus for a task p to the weight of the hmp_slow_cpu_mask before calling do_set_cpus_allowed() in __setscheduler(). It will be set to this value in do_set_cpus_allowed() after the call to the sched class related backend function any way. By doing this, set_cpus_allowed_rt() returns without trying to update the rt_nr_migratory counter.
This patch has been tested with a test device driver requiring a threaded irq handler on a TC2 system with a reduced cpu mask (1 Cortex A15, 1 Cortex A7).
Signed-off-by: Dietmar Eggemann dietmar.eggemann@arm.com Signed-off-by: Jon Medhurst tixy@linaro.org --- kernel/sched/core.c | 5 ++++- 1 file changed, 4 insertions(+), 1 deletion(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index 134d815..277e355 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -3854,8 +3854,11 @@ __setscheduler(struct rq *rq, struct task_struct *p, int policy, int prio) p->sched_class = &rt_sched_class; #ifdef CONFIG_SCHED_HMP if (!cpumask_empty(&hmp_slow_cpu_mask)) - if (cpumask_equal(&p->cpus_allowed, cpu_all_mask)) + if (cpumask_equal(&p->cpus_allowed, cpu_all_mask)) { + p->nr_cpus_allowed = + cpumask_weight(&hmp_slow_cpu_mask); do_set_cpus_allowed(p, &hmp_slow_cpu_mask); + } #endif } else