Re: [RFC PATCH 2/3] sched/cpuset: Keep track of SCHED_DEADLINE tasks in cpusets

[Waiman Long <longman@redhat.com>]

On 3/15/23 08:18, Juri Lelli wrote:
> Qais reported that iterating over all tasks when rebuilding root domains
> for finding out which ones are DEADLINE and need their bandwidth
> correctly restored on such root domains can be a costly operation (10+
> ms delays on suspend-resume).
>
> 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.
>
> Reported-by: Qais Yousef <qyousef@layalina.io>
> Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
> ---
>   include/linux/cpuset.h |  4 ++++
>   kernel/cgroup/cgroup.c |  4 ++++
>   kernel/cgroup/cpuset.c | 25 +++++++++++++++++++++++++
>   kernel/sched/core.c    | 10 ++++++++++
>   4 files changed, 43 insertions(+)
>
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index 355f796c5f07..0348dba5680e 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -71,6 +71,8 @@ extern void cpuset_init_smp(void);
>   extern void cpuset_force_rebuild(void);
>   extern void cpuset_update_active_cpus(void);
>   extern void cpuset_wait_for_hotplug(void);
> +extern void inc_dl_tasks_cs(struct task_struct *task);
> +extern void dec_dl_tasks_cs(struct task_struct *task);
>   extern void cpuset_lock(void);
>   extern void cpuset_unlock(void);
>   extern void cpuset_cpus_allowed(struct task_struct *p, struct cpumask *mask);
> @@ -196,6 +198,8 @@ static inline void cpuset_update_active_cpus(void)
>   
>   static inline void cpuset_wait_for_hotplug(void) { }
>   
> +static inline void inc_dl_tasks_cs(struct task_struct *task) { }
> +static inline void dec_dl_tasks_cs(struct task_struct *task) { }
>   static inline void cpuset_lock(void) { }
>   static inline void cpuset_unlock(void) { }
>   
> diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
> index c099cf3fa02d..357925e1e4af 100644
> --- a/kernel/cgroup/cgroup.c
> +++ b/kernel/cgroup/cgroup.c
> @@ -57,6 +57,7 @@
>   #include <linux/file.h>
>   #include <linux/fs_parser.h>
>   #include <linux/sched/cputime.h>
> +#include <linux/sched/deadline.h>
>   #include <linux/psi.h>
>   #include <net/sock.h>
>   
> @@ -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)))
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index 8d82d66d432b..57bc60112618 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -193,6 +193,12 @@ struct cpuset {
>   	int use_parent_ecpus;
>   	int child_ecpus_count;
>   
> +	/*
> +	 * 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;
> +
>   	/* Invalid partition error code, not lock protected */
>   	enum prs_errcode prs_err;
>   
> @@ -245,6 +251,20 @@ static inline struct cpuset *parent_cs(struct cpuset *cs)
>   	return css_cs(cs->css.parent);
>   }
>   
> +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--;
> +}
> +
>   /* bits in struct cpuset flags field */
>   typedef enum {
>   	CS_ONLINE,
> @@ -2472,6 +2492,11 @@ 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--;
> +		}
>   	}

Any one of the tasks in the cpuset can cause the test to fail and abort 
the attachment. I would suggest that you keep a deadline task transfer 
count in the loop and then update cs and cpouset_attach_old_cs only 
after all the tasks have been iterated successfully.

Cheers,
Longman