Re: [PATCH] SCHED: scatter nohz idle balance target cpus

[Vincent Guittot]

On Tue, 18 Mar 2025 at 03:27, Jianyong Wu <wujianyong@xxxxxxxx> wrote:
>
> Currently, cpu selection logic for nohz idle balance lacks history info
> that leads to cpu0 is always chosen if it's in nohz cpu mask. It's not
> fair fot the tasks reside in numa node0. It's worse in the machine with
> large cpu number, nohz idle balance may be very heavy.

Could you provide more details about why it's not fair for tasks that
reside on numa node 0 ? cpu0 is idle so ilb doesn't steal time to
other tasks.

Do you have figures or use cases to highlight this unfairness ?

>
> To address this issue, adding a member to "nohz" to indicate who is
> chosen last time and choose next for this round of nohz idle balance.
>
> Signed-off-by: Jianyong Wu <wujianyong@xxxxxxxx>
> ---
>  kernel/sched/fair.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index c798d2795243..ba6930c79e25 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7197,6 +7197,7 @@ static struct {
>         atomic_t nr_cpus;
>         int has_blocked;                /* Idle CPUS has blocked load */
>         int needs_update;               /* Newly idle CPUs need their next_balance collated */
> +       int last_cpu;                   /* Last cpu chosen to do nohz idle balance */
>         unsigned long next_balance;     /* in jiffy units */
>         unsigned long next_blocked;     /* Next update of blocked load in jiffies */
>  } nohz ____cacheline_aligned;
> @@ -12266,13 +12267,15 @@ static inline int find_new_ilb(void)
>
>         hk_mask = housekeeping_cpumask(HK_TYPE_KERNEL_NOISE);
>
> -       for_each_cpu_and(ilb_cpu, nohz.idle_cpus_mask, hk_mask) {
> +       for_each_cpu_wrap(ilb_cpu, nohz.idle_cpus_mask, nohz.last_cpu + 1) {
>
> -               if (ilb_cpu == smp_processor_id())
> +               if (ilb_cpu == smp_processor_id() || !cpumask_test_cpu(ilb_cpu, hk_mask))
>                         continue;
>
> -               if (idle_cpu(ilb_cpu))
> +               if (idle_cpu(ilb_cpu)) {
> +                       nohz.last_cpu = ilb_cpu;
>                         return ilb_cpu;
> +               }
>         }
>
>         return -1;
> --
> 2.43.0
>