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

[Peter Zijlstra]

On Tue, Oct 13, 2026 at 06:54:39PM +0200, Vincent Guittot wrote:
> 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

My build scripts argued it should be. It breaks with the fancy new
for_each_sched_entity().