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

[Qais Yousef <qyousef@layalina.io>]

On 03/15/23 12:18, 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);

nit:

I *think* task_cs() assumes rcu_read_lock() is held, right?

Would it make sense to WARN_ON(!rcu_read_lock_held()) to at least
annotate the deps?

Or maybe task_cs() should do that..

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

nit: ditto

> +
> +	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--;
> +		}
>  	}
>  
>  	/*
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 5902cbb5e751..d586a8440348 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -7683,6 +7683,16 @@ static int __sched_setscheduler(struct task_struct *p,
>  		goto unlock;
>  	}
>  
> +	/*
> +	 * In case a task is setscheduled to SCHED_DEADLINE, or if a task is
> +	 * moved to a different sched policy, we need to keep track of that on
> +	 * its cpuset (for correct bandwidth tracking).
> +	 */
> +	if (dl_policy(policy) && !dl_task(p))
> +		inc_dl_tasks_cs(p);
> +	else if (dl_task(p) && !dl_policy(policy))
> +		dec_dl_tasks_cs(p);
> +

Would it be better to use switched_to_dl()/switched_from_dl() instead to
inc/dec_dl_tasks_cs()?


Thanks!

--
Qais Yousef

>  	p->sched_reset_on_fork = reset_on_fork;
>  	oldprio = p->prio;
>  
> -- 
> 2.39.2
>