Re: [PATCH] sched/fair: Fix cpu_util runnable_avg arithmetic

[Vincent Guittot]

On Fri, 5 Jun 2026 at 09:01, Hongyan Xia <hongyan.xia@xxxxxxxxxxxxx> wrote:
>
> From: Hongyan Xia <hongyan.xia@xxxxxxxxxxxxx>
>
> If we take runnable_avg in max(runnable_avg, util_avg) in cpu_util(), we
> should then add or subtract task runnable_avg, but the arithmetic below
> is still with task util_avg. This mixes runnable_avg with util_avg which
> is incorrect.
>
> Fix by always doing arithmetic with runnable_avg and only take
> max(runnable_avg, util_avg) at the last step.
>
> Fixes: 7d0583cf9ec7 ("sched/fair, cpufreq: Introduce 'runnable boosting'")
> Signed-off-by: Hongyan Xia <hongyan.xia@xxxxxxxxxxxxx>
> ---
>  kernel/sched/fair.c | 22 ++++++++++++++--------
>  1 file changed, 14 insertions(+), 8 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index 69361c63353a..050bce06efea 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -8231,25 +8231,31 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
>  static unsigned long
>  cpu_util(int cpu, struct task_struct *p, int dst_cpu, int boost)
>  {
> +       bool add_task = p && task_cpu(p) != cpu && dst_cpu == cpu;
> +       bool sub_task = p && task_cpu(p) == cpu && dst_cpu != cpu;
>         struct cfs_rq *cfs_rq = &cpu_rq(cpu)->cfs;
>         unsigned long util = READ_ONCE(cfs_rq->avg.util_avg);
>         unsigned long runnable;
>
> -       if (boost) {
> -               runnable = READ_ONCE(cfs_rq->avg.runnable_avg);
> -               util = max(util, runnable);
> -       }
> -
>         /*
>          * If @dst_cpu is -1 or @p migrates from @cpu to @dst_cpu remove its
>          * contribution. If @p migrates from another CPU to @cpu add its
>          * contribution. In all the other cases @cpu is not impacted by the
>          * migration so its util_avg is already correct.
>          */
> -       if (p && task_cpu(p) == cpu && dst_cpu != cpu)
> -               lsub_positive(&util, task_util(p));
> -       else if (p && task_cpu(p) != cpu && dst_cpu == cpu)
> +       if (add_task)
>                 util += task_util(p);
> +       else if (sub_task)
> +               lsub_positive(&util, task_util(p));
> +
> +       if (boost) {
> +               runnable = READ_ONCE(cfs_rq->avg.runnable_avg);
> +               if (add_task)
> +                       runnable += task_runnable(p);

Where did you get the task_runnable() function? I can't find it in
tip/sched/core so either create it or call
READ_ONCE(p->se.avg.runnable_avg) directly


> +               else if (sub_task)
> +                       lsub_positive(&runnable, task_runnable(p));
> +               util = max(util, runnable);
> +       }
>
>         if (sched_feat(UTIL_EST)) {
>                 unsigned long util_est;
> --
> 2.47.3
>