linux-kernel - Re: [PATCH 3/6] sched/cpuset: Keep track of SCHED

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Date:   Mon, 9 Oct 2023 19:43:10 +0800
From:   Xia Fukun <xiafukun@...wei.com>
To:     Juri Lelli <juri.lelli@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Qais Yousef <qyousef@...alina.io>,
        Waiman Long <longman@...hat.com>, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Hao Luo <haoluo@...gle.com>
CC:     Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        <linux-kernel@...r.kernel.org>, <luca.abeni@...tannapisa.it>,
        <claudio@...dence.eu.com>, <tommaso.cucinotta@...tannapisa.it>,
        <bristot@...hat.com>, <mathieu.poirier@...aro.org>,
        <cgroups@...r.kernel.org>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Wei Wang <wvw@...gle.com>, Rick Yiu <rickyiu@...gle.com>,
        Quentin Perret <qperret@...gle.com>,
        Heiko Carstens <hca@...ux.ibm.com>,
        Vasily Gorbik <gor@...ux.ibm.com>,
        Alexander Gordeev <agordeev@...ux.ibm.com>,
        Sudeep Holla <sudeep.holla@....com>
Subject: Re: [PATCH 3/6] sched/cpuset: Keep track of SCHED_DEADLINE task in
 cpusets

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@...il.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.