Re: [PATCH 2/3] sched/fair: Make cfs_rq::idle the sum of ancestor tg::idle

[Kayra Cizmeci <kayracizmeci@gmail.com>]

:-(.

Look here: https://lore.kernel.org/oe-kbuild-all/202611121431.06YgiErz-lkp@intel.com/

On Thu, Nov 05, 2026 at 05:55:27PM +0100, Peter Zijlstra wrote:
> Rather than keeping cfs_rq::idle as a copy of tg::idle, make it the sum of
> all ancestor tg::idle. This makes it trivial to tell if the current cgroup
> is marked idle somewhere along its ancestral path.
> 
> This makes se_id_idle() behave differently. Notably is will now be true
> when there is a idle cgroup anywhere in the ancestor path. This
> addresses an issue where preemption of cgroup::idle was overlooked when
> flattening the runqueue.
> 
> Fixes: 85570f10a4c6 ("sched/eevdf: Move to a single runqueue")
> Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
> ---
>  kernel/sched/core.c  |    2 -
>  kernel/sched/debug.c |    1 
>  kernel/sched/fair.c  |   73 ++++++++++++++++++++++++++++++++-------------------
>  kernel/sched/sched.h |    4 +-
>  4 files changed, 50 insertions(+), 30 deletions(-)
> 
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8970,7 +8970,7 @@ void __init sched_init(void)
>  		rq->nr_running = 0;
>  		rq->calc_load_active = 0;
>  		rq->calc_load_update = jiffies + LOAD_FREQ;
> -		init_cfs_rq(&rq->cfs);
> +		init_cfs_rq(&rq->cfs, NULL);
>  		init_rt_rq(&rq->rt);
>  		init_dl_rq(&rq->dl);
>  #ifdef CONFIG_FAIR_GROUP_SCHED
> --- a/kernel/sched/debug.c
> +++ b/kernel/sched/debug.c
> @@ -1090,6 +1090,7 @@ void print_cfs_rq(struct seq_file *m, in
>  	SEQ_printf(m, "  .%-30s: %d\n", "h_nr_runnable", cfs_rq->h_nr_runnable);
>  	SEQ_printf(m, "  .%-30s: %d\n", "h_nr_queued", cfs_rq->h_nr_queued);
>  	SEQ_printf(m, "  .%-30s: %d\n", "h_nr_idle", cfs_rq->h_nr_idle);
> +	SEQ_printf(m, "  .%-30s: %d\n", "idle", cfs_rq->idle);

idle is only in the CONFIG_FAIR_GROUP_SCHED :-(.
So as the robot says, this breaks compiling in !CONFIG_FAIR_GROUP_SCHED.

AFAICT tho, moving this down below in CONFIG_FAIR_GROUP_SCHED next to
h_load and other guys will solve the issue no?


Cheers,

Kayra