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

[Waiman Long <longman@redhat.com>]

On 10/9/23 07:43, Xia Fukun wrote:
> 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?

It does look like the value of nr_deadline_tasks can be subjected to 
data race leading to incorrect value. Changing it to atomic_t should 
avoid that at the expense of a bit higher overhead.

Cheers,
Longman