Re: [PATCH v2] sched/task_group: Re-layout structure to reduce false sharing

From: Aaron Lu
Date: Mon Jun 26 2023 - 08:53:59 EST


On Mon, Jun 26, 2023 at 03:52:17PM +0800, Chen Yu wrote:
> Besides the cache line alignment, if the task is not a rt one,
> why do we have to touch that, I wonder if the following change can avoid that:
>
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index ec7b3e0a2b20..067f1310bad2 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1958,8 +1958,10 @@ static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
> #endif
>
> #ifdef CONFIG_RT_GROUP_SCHED
> - p->rt.rt_rq = tg->rt_rq[cpu];
> - p->rt.parent = tg->rt_se[cpu];
> + if (p->sched_class = &rt_sched_class) {
== :-)

> + p->rt.rt_rq = tg->rt_rq[cpu];
> + p->rt.parent = tg->rt_se[cpu];
> + }
> #endif
> }

If a task starts life as a SCHED_NORMAL one and then after some time
it's changed to a RT one, then during its next ttwu(), if it didn't
migrate, then set_task_rq() will not be called and p->rt.rt_rq will
keep as NULL which will cause problem when this task gets enqueued as
a rt one.

The follow diff seems to cure this issue:

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c7db597e8175..8c57148e668c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7801,6 +7801,20 @@ static int __sched_setscheduler(struct task_struct *p,
}
__setscheduler_uclamp(p, attr);

+#ifdef CONFIG_RT_GROUP_SCHED
+ /*
+ * Make sure when this task becomes a rt one,
+ * its rt fields have valid value.
+ */
+ if (rt_prio(newprio)) {
+ struct task_group *tg = task_group(p);
+ int cpu = cpu_of(rq);
+
+ p->rt.rt_rq = tg->rt_rq[cpu];
+ p->rt.parent = tg->rt_se[cpu];
+ }
+#endif
+
if (queued) {
/*
* We enqueue to tail when the priority of a task is

But I'm not sure if it's worth the trouble.

Thanks,
Aaron