Re: [RFC PATCH v3 06/10] sched/fair: Prepare switched_from & switched_to for per-task throttling

[Peter Zijlstra]

On Thu, Jul 11, 2024 at 03:00:00PM +0200, Valentin Schneider wrote:
> Later commits will change CFS bandwidth control throttling from a
> per-cfs_rq basis to a per-task basis. This means special care needs to be
> taken around any transition a task can have into and out of a cfs_rq.
> 
> To ease reviewing, the transitions are patched with dummy-helpers that are
> implemented later on.
> 
> Add helpers to switched_from_fair() and switched_to_fair() to cover class
> changes. If switching from CFS, a task may need to be unthrottled. If
> switching to CFS, a task may need to be throttled.
> 
> Signed-off-by: Valentin Schneider <vschneid@xxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 12 ++++++++++++
>  1 file changed, 12 insertions(+)
> 
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 095357bd17f0e..acac0829c71f3 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -5694,6 +5694,10 @@ static inline int throttled_hierarchy(struct cfs_rq *cfs_rq)
>  	return cfs_bandwidth_used() && cfs_rq->throttle_count;
>  }
>  
> +static inline bool task_needs_throttling(struct task_struct *p) { return false; }
> +static inline void task_throttle_setup(struct task_struct *p) { }
> +static inline void task_throttle_cancel(struct task_struct *p) { }
> +
>  /*
>   * Ensure that neither of the group entities corresponding to src_cpu or
>   * dest_cpu are members of a throttled hierarchy when performing group
> @@ -6622,6 +6626,10 @@ static inline int throttled_lb_pair(struct task_group *tg,
>  	return 0;
>  }
>  
> +static inline bool task_needs_throttling(struct task_struct *p) { return false; }
> +static inline void task_throttle_setup(struct task_struct *p) { }
> +static inline void task_throttle_cancel(struct task_struct *p) { }
> +
>  #ifdef CONFIG_FAIR_GROUP_SCHED
>  void init_cfs_bandwidth(struct cfs_bandwidth *cfs_b, struct cfs_bandwidth *parent) {}
>  static void init_cfs_rq_runtime(struct cfs_rq *cfs_rq) {}
> @@ -12847,11 +12855,15 @@ static void attach_task_cfs_rq(struct task_struct *p)
>  static void switched_from_fair(struct rq *rq, struct task_struct *p)
>  {
>  	detach_task_cfs_rq(p);
> +	if (cfs_bandwidth_used())
> +		task_throttle_cancel(p);
>  }
>  
>  static void switched_to_fair(struct rq *rq, struct task_struct *p)
>  {
>  	attach_task_cfs_rq(p);
> +	if (cfs_bandwidth_used() && task_needs_throttling(p))
> +		task_throttle_setup(p);
>  
>  	set_task_max_allowed_capacity(p);

Other functions seem to have cfs_bandwidth_used() inside them, and not
bother the caller with this detail.