Re: [PATCH 3/6] sched/cpuset: Keep track of SCHED_DEADLINE task in cpusets

Xia Fukun
2023-10-09T11:43:29Z

On 2023/3/29 20:55, Juri Lelli wrote:

> To fix the problem keep track of the number of DEADLINE tasks belonging
> to each cpuset and then use this information (followup patch) to only
> perform the above iteration if DEADLINE tasks are actually present in
> the cpuset for which a corresponding root domain is being rebuilt.
>
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index 935e8121b21e..ff27b2d2bf0b 100644

> @@ -6673,6 +6674,9 @@ void cgroup_exit(struct task_struct *tsk)
> list_add_tail(&tsk->cg_list, &cset->dying_tasks);
> cset->nr_tasks--;
>
> + if (dl_task(tsk))
> + dec_dl_tasks_cs(tsk);
> +
> WARN_ON_ONCE(cgroup_task_frozen(tsk));
> if (unlikely(!(tsk->flags & PF_KTHREAD) &&
> test_bit(CGRP_FREEZE, &task_dfl_cgroup(tsk)->flags)))

The cgroup_exit() function decrements the value of the nr_deadline_tasks by one.

> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index fbc10b494292..eb0854ef9757 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -193,6 +193,12 @@ struct cpuset {
> + /*
> + * number of SCHED_DEADLINE tasks attached to this cpuset, so that we
> + * know when to rebuild associated root domain bandwidth information.
> + */
> + int nr_deadline_tasks;
> +

> +void inc_dl_tasks_cs(struct task_struct *p)
> +{
> + struct cpuset *cs = task_cs(p);
> +
> + cs->nr_deadline_tasks++;
> +}
> +
> +void dec_dl_tasks_cs(struct task_struct *p)
> +{
> + struct cpuset *cs = task_cs(p);
> +
> + cs->nr_deadline_tasks--;
> +}
> +

> @@ -2477,6 +2497,11 @@ static int cpuset_can_attach(struct cgroup_taskset *tset)
> ret = security_task_setscheduler(task);
> if (ret)
> goto out_unlock;
> +
> + if (dl_task(task)) {
> + cs->nr_deadline_tasks++;
> + cpuset_attach_old_cs->nr_deadline_tasks--;
> + }
> }

The cpuset_can_attach() function increments the value of the nr_deadline_tasks by one.

> /*
> diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
> index 4cc7e1ca066d..8f92f0f87383 100644
> --- a/kernel/sched/deadline.c
> +++ b/kernel/sched/deadline.c
> @@ -16,6 +16,8 @@
> * Fabio Checconi <fchecconi@gmail.com>
> */
>
> +#include <linux/cpuset.h>
> +
> /*
> * Default limits for DL period; on the top end we guard against small util
> * tasks still getting ridiculously long effective runtimes, on the bottom end we
> @@ -2595,6 +2597,12 @@ static void switched_from_dl(struct rq *rq, struct task_struct *p)
> if (task_on_rq_queued(p) && p->dl.dl_runtime)
> task_non_contending(p);
>
> + /*
> + * In case a task is setscheduled out from SCHED_DEADLINE we need to
> + * keep track of that on its cpuset (for correct bandwidth tracking).
> + */
> + dec_dl_tasks_cs(p);
> +
> if (!task_on_rq_queued(p)) {
> /*
> * Inactive timer is armed. However, p is leaving DEADLINE and
> @@ -2635,6 +2643,12 @@ static void switched_to_dl(struct rq *rq, struct task_struct *p)
> if (hrtimer_try_to_cancel(&p->dl.inactive_timer) == 1)
> put_task_struct(p);
>
> + /*
> + * In case a task is setscheduled to SCHED_DEADLINE we need to keep
> + * track of that on its cpuset (for correct bandwidth tracking).
> + */
> + inc_dl_tasks_cs(p);
> +
> /* If p is not queued we will update its parameters at next wakeup. */
> if (!task_on_rq_queued(p)) {
> add_rq_bw(&p->dl, &rq->dl);

And both switched_from_dl() and switched_to_dl() can change the value of
nr_deadline_tasks.

I suspect that changing the values of the nr_deadline_tasks in these
4 paths will cause data race problems.

And this patch([PATCH 6/6] cgroup/cpuset: Iterate only if DEADLINE tasks are present)
has the following judgment:

diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
index f8ebec66da51..05c0a1255218 100644
--- a/kernel/cgroup/cpuset.c
+++ b/kernel/cgroup/cpuset.c
@@ -1092,6 +1092,9 @@ static void dl_update_tasks_root_domain(struct cpuset *cs)
struct css_task_iter it;
struct task_struct *task;

+ if (cs->nr_deadline_tasks == 0)
+ return;
+
css_task_iter_start(&cs->css, 0, &it);

while ((task = css_task_iter_next(&it)))
--

The uncertainty of nr_deadline_tasks can lead to logical problems.

May I ask what experts think of the Data Race problem?

I would like to inquire if there is a problem and if so, is it
necessary to use atomic operations to avoid it?

Thank you all.