Re: [PATCH v2 3/4] sched/fair: Extend for_each_sched_entity() with a back-link

[Vincent Guittot]

On Tue, 29 Sept 2026 at 10:55, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
>
> Leave a trail of bread crumbs, such that we can find our way back down the
> hierarchy. No actual users yet, but split out because its a bit tricky.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

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

> ---
>  kernel/sched/fair.c  |   14 +++++++++++---
>  kernel/sched/sched.h |    1 +
>  2 files changed, 12 insertions(+), 3 deletions(-)
>
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -318,8 +318,13 @@ const struct sched_class fair_sched_clas
>  #ifdef CONFIG_FAIR_GROUP_SCHED
>
>  /* Walk up scheduling entities hierarchy */
> -#define for_each_sched_entity(se, cfs_rq)                              \
> -       for (; (se) && ((cfs_rq) = cfs_rq_of(se)); (se) = (se)->parent)
> +#define for_each_sched_entity(se, cfs_rq)                                      \
> +       for (struct sched_entity *_BL = NULL;                                   \
> +            (se) && ((cfs_rq) = cfs_rq_of(se), (cfs_rq)->backlink = _BL, true);\
> +            (se) = (se)->parent, _BL = (se))
> +
> +#define for_each_sched_entity_bl(se, cfs_rq) \
> +       for (; ((se) = (cfs_rq)->backlink); (cfs_rq) = group_cfs_rq(se))
>
>  static inline bool list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
>  {
> @@ -456,6 +461,9 @@ static int se_is_idle(struct sched_entit
>  #define for_each_sched_entity(se, cfs_rq) \
>         for (; (se) && ((cfs_rq) = cfs_rq_of(se)); (se) = NULL)
>
> +#define for_each_sched_entity_bl(se, cfs_rq) \
> +       for (; ((se) = NULL);)
> +
>  static inline bool list_add_leaf_cfs_rq(struct cfs_rq *cfs_rq)
>  {
>         return true;
> @@ -15233,7 +15241,7 @@ static inline void task_tick_core(struct
>  /*
>   * se_fi_update - Update the cfs_rq->zero_vruntime_fi in a CFS hierarchy if needed.
>   */
> -static void se_fi_update(const struct sched_entity *se, unsigned int fi_seq,
> +static void se_fi_update(struct sched_entity *se, unsigned int fi_seq,

Not sure this should be part of this patch

>                          bool forceidle)
>  {
>         struct cfs_rq *cfs_rq;
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -726,6 +726,7 @@ struct cfs_rq {
>         unsigned long           tg_runnable_avg_contrib;
>         long                    propagate;
>         long                    prop_runnable_sum;
> +       struct sched_entity     *backlink;
>
>         /*
>          *   h_load = weight * f(tg)
>
>