Re: [PATCH] sched: remove redundant cpu_cgrp_subsys->fork()

[Vincent Guittot]

On Wed, 22 Jun 2022 at 11:48, Chengming Zhou
<zhouchengming@xxxxxxxxxxxxx> wrote:
>
> We use cpu_cgrp_subsys->fork() to set task group for the new fair task
> in cgroup_post_fork().
>
> Since commit b1e8206582f9 ("sched: Fix yet more sched_fork() races")
> has already set task group for the new fair task in sched_cgroup_fork(),
> so cpu_cgrp_subsys->fork() can be removed.
>
>   cgroup_can_fork()     --> pin parent's sched_task_group
>   sched_cgroup_fork()
>     __set_task_cpu      --> set task group
>   cgroup_post_fork()
>     ss->fork() := cpu_cgroup_fork()     --> set again
>
> After this change, task_change_group_fair() only need to care about
> task cgroup migration, make the code much simplier.
>
> This patch move the task se depth setting to set_task_rq(), which
> is called when task moves across CPUs/groups, is a better place than
> attach_entity_cfs_rq(). The latter is also used when change task's
> sched_class, in which case the group/depth doesn't change actually.
>

The commit message needs some minor fixes but apart from this

Reviewed-by: Vincent Guittot <vincent.guittot@xxxxxxxxxx>

> Signed-off-by: Chengming Zhou <zhouchengming@xxxxxxxxxxxxx>
> ---
>  kernel/sched/core.c  | 27 ++++-----------------------
>  kernel/sched/fair.c  | 31 +------------------------------
>  kernel/sched/sched.h |  6 ++----
>  3 files changed, 7 insertions(+), 57 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c16f8cc5de08..f44bc5b889ab 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -481,8 +481,7 @@ sched_core_dequeue(struct rq *rq, struct task_struct *p, int flags) { }
>   *                             p->se.load, p->rt_priority,
>   *                             p->dl.dl_{runtime, deadline, period, flags, bw, density}
>   *  - sched_setnuma():         p->numa_preferred_nid
> - *  - sched_move_task()/
> - *    cpu_cgroup_fork():       p->sched_task_group
> + *  - sched_move_task():       p->sched_task_group
>   *  - uclamp_update_active()   p->uclamp*
>   *
>   * p->state <- TASK_*:
> @@ -10098,7 +10097,7 @@ void sched_release_group(struct task_group *tg)
>         spin_unlock_irqrestore(&task_group_lock, flags);
>  }
>
> -static void sched_change_group(struct task_struct *tsk, int type)
> +static void sched_change_group(struct task_struct *tsk)
>  {
>         struct task_group *tg;
>
> @@ -10114,7 +10113,7 @@ static void sched_change_group(struct task_struct *tsk, int type)
>
>  #ifdef CONFIG_FAIR_GROUP_SCHED
>         if (tsk->sched_class->task_change_group)
> -               tsk->sched_class->task_change_group(tsk, type);
> +               tsk->sched_class->task_change_group(tsk);
>         else
>  #endif
>                 set_task_rq(tsk, task_cpu(tsk));
> @@ -10145,7 +10144,7 @@ void sched_move_task(struct task_struct *tsk)
>         if (running)
>                 put_prev_task(rq, tsk);
>
> -       sched_change_group(tsk, TASK_MOVE_GROUP);
> +       sched_change_group(tsk);
>
>         if (queued)
>                 enqueue_task(rq, tsk, queue_flags);
> @@ -10223,23 +10222,6 @@ static void cpu_cgroup_css_free(struct cgroup_subsys_state *css)
>         sched_unregister_group(tg);
>  }
>
> -/*
> - * This is called before wake_up_new_task(), therefore we really only
> - * have to set its group bits, all the other stuff does not apply.
> - */
> -static void cpu_cgroup_fork(struct task_struct *task)
> -{
> -       struct rq_flags rf;
> -       struct rq *rq;
> -
> -       rq = task_rq_lock(task, &rf);
> -
> -       update_rq_clock(rq);
> -       sched_change_group(task, TASK_SET_GROUP);
> -
> -       task_rq_unlock(rq, task, &rf);
> -}
> -
>  static int cpu_cgroup_can_attach(struct cgroup_taskset *tset)
>  {
>         struct task_struct *task;
> @@ -11136,7 +11118,6 @@ struct cgroup_subsys cpu_cgrp_subsys = {
>         .css_released   = cpu_cgroup_css_released,
>         .css_free       = cpu_cgroup_css_free,
>         .css_extra_stat_show = cpu_extra_stat_show,
> -       .fork           = cpu_cgroup_fork,
>         .can_attach     = cpu_cgroup_can_attach,
>         .attach         = cpu_cgroup_attach,
>         .legacy_cftypes = cpu_legacy_files,
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 8bed75757e65..fdb9125fe34e 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -11325,14 +11325,6 @@ static void attach_entity_cfs_rq(struct sched_entity *se)
>  {
>         struct cfs_rq *cfs_rq = cfs_rq_of(se);
>
> -#ifdef CONFIG_FAIR_GROUP_SCHED
> -       /*
> -        * Since the real-depth could have been changed (only FAIR
> -        * class maintain depth value), reset depth properly.
> -        */
> -       se->depth = se->parent ? se->parent->depth + 1 : 0;
> -#endif
> -
>         /* Synchronize entity with its cfs_rq */
>         update_load_avg(cfs_rq, se, sched_feat(ATTACH_AGE_LOAD) ? 0 : SKIP_AGE_LOAD);
>         attach_entity_load_avg(cfs_rq, se);
> @@ -11431,15 +11423,7 @@ void init_cfs_rq(struct cfs_rq *cfs_rq)
>  }
>
>  #ifdef CONFIG_FAIR_GROUP_SCHED
> -static void task_set_group_fair(struct task_struct *p)
> -{
> -       struct sched_entity *se = &p->se;
> -
> -       set_task_rq(p, task_cpu(p));
> -       se->depth = se->parent ? se->parent->depth + 1 : 0;
> -}
> -
> -static void task_move_group_fair(struct task_struct *p)
> +static void task_change_group_fair(struct task_struct *p)
>  {
>         detach_task_cfs_rq(p);
>         set_task_rq(p, task_cpu(p));
> @@ -11451,19 +11435,6 @@ static void task_move_group_fair(struct task_struct *p)
>         attach_task_cfs_rq(p);
>  }
>
> -static void task_change_group_fair(struct task_struct *p, int type)
> -{
> -       switch (type) {
> -       case TASK_SET_GROUP:
> -               task_set_group_fair(p);
> -               break;
> -
> -       case TASK_MOVE_GROUP:
> -               task_move_group_fair(p);
> -               break;
> -       }
> -}
> -
>  void free_fair_sched_group(struct task_group *tg)
>  {
>         int i;
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 317480d535b0..8433e22da3ab 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1901,6 +1901,7 @@ static inline void set_task_rq(struct task_struct *p, unsigned int cpu)
>         set_task_rq_fair(&p->se, p->se.cfs_rq, tg->cfs_rq[cpu]);
>         p->se.cfs_rq = tg->cfs_rq[cpu];
>         p->se.parent = tg->se[cpu];
> +       p->se.depth = tg->se[cpu] ? tg->se[cpu]->depth + 1 : 0;
>  #endif
>
>  #ifdef CONFIG_RT_GROUP_SCHED
> @@ -2163,11 +2164,8 @@ struct sched_class {
>
>         void (*update_curr)(struct rq *rq);
>
> -#define TASK_SET_GROUP         0
> -#define TASK_MOVE_GROUP                1
> -
>  #ifdef CONFIG_FAIR_GROUP_SCHED
> -       void (*task_change_group)(struct task_struct *p, int type);
> +       void (*task_change_group)(struct task_struct *p);
>  #endif
>  };
>
> --
> 2.36.1
>