Messages in this thread Patch in this message | | | From | Lucas De Marchi <> | Date | Mon, 26 Oct 2009 14:11:13 -0200 | Subject | Bug when changing cpus_allowed of RT tasks? |
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I think there's a problem when balancing RT tasks: both find_lowest_rq_rt and select_task_rq_rt check for p->rt.nr_cpus_allowed being greater than 1 to wake a task in another cpu:
static int select_task_rq_rt(struct task_struct *p, int sd_flag, int flags) { [...] if (unlikely(rt_task(rq->curr)) && (p->rt.nr_cpus_allowed > 1)) { int cpu = find_lowest_rq(p);
return (cpu == -1) ? task_cpu(p) : cpu; } [...] }
static int find_lowest_rq(struct task_struct *task) { struct sched_domain *sd; struct cpumask *lowest_mask = __get_cpu_var(local_cpu_mask); int this_cpu = smp_processor_id(); int cpu = task_cpu(task); cpumask_var_t domain_mask;
if (task->rt.nr_cpus_allowed == 1) return -1; /* No other targets possible */ [...] }
What happens if rt.nr_cpus_allowed continues to be 1, but the allowed cpu is not the one in which task is being woken up? Since set_cpus_allowed_rt just changed the mask and updated the value of nr_cpus_allowed_rt, as far as I can see, it remains on same CPU even if the new mask does not allow.
A possible fix below, and in case it is right: Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>
Lucas De Marchi
--
diff --git a/kernel/sched_rt.c b/kernel/sched_rt.c index a4d790c..531c946 100644 --- a/kernel/sched_rt.c +++ b/kernel/sched_rt.c @@ -962,8 +962,7 @@ static int select_task_rq_rt(struct task_struct *p, int sd_flag, int flags) * that is just being woken and probably will have * cold cache anyway. */ - if (unlikely(rt_task(rq->curr)) && - (p->rt.nr_cpus_allowed > 1)) { + if (unlikely(rt_task(rq->curr))) { int cpu = find_lowest_rq(p);
return (cpu == -1) ? task_cpu(p) : cpu; @@ -1177,7 +1176,8 @@ static int find_lowest_rq(struct task_struct *task) int cpu = task_cpu(task); cpumask_var_t domain_mask;
- if (task->rt.nr_cpus_allowed == 1) + if ((task->rt.nr_cpus_allowed == 1) && + likely(cpumask_test_cpu(cpu, &task->cpus_allowed))) return -1; /* No other targets possible */
if (!cpupri_find(&task_rq(task)->rd->cpupri, task, lowest_mask))
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