Re: [PATCH] sched/nohz: Fix NOHZ imbalance by adding options for ILB CPU

[Valentin Schneider <vschneid@redhat.com>]

On 19/08/25 02:57, Adam Li wrote:
> A qualified CPU to run NOHZ idle load balancing (ILB) has to be:
> 1) housekeeping CPU in housekeeping_cpumask(HK_TYPE_KERNEL_NOISE)
> 2) and in nohz.idle_cpus_mask
> 3) and idle
> 4) and not current CPU
>
> If most CPUs are in nohz_full CPU list there is few housekeeping CPU left.
> In the worst case if all CPUs are in nohz_full only the boot CPU is used
> for housekeeping. And the housekeeping CPU is usually busier so it will
> be unlikely added to nohz.idle_cpus_mask.
>
> Therefore if there is few housekeeping CPUs, find_new_ilb() may likely
> failed to find any CPU to do NOHZ idle load balancing. Some NOHZ CPUs may
> stay idle while other CPUs are busy.
>
> This patch adds fallback options when looking for ILB CPU if there is
> no CPU meeting above requirements. Then it searches in bellow order:
> 1) Try looking for the first idle housekeeping CPU
> 2) Try looking for the first idle CPU in nohz.idle_cpus_mask if no SMT.
> 3) Select the first housekeeping CPU even if it is busy.
>
> With this patch the NOHZ idle balancing happens more frequently.
>

I'm not understanding why, in the scenarios outlined above, more NOHZ idle
balancing is a good thing.

Considering only housekeeping CPUs, they're all covered by wakeup, periodic
and idle balancing (on top of NOHZ idle balancing when relevant). So if
find_new_ilb() never finds a NOHZ-idle CPU, then that means your HK CPUs
are either always busy or never stopping the tick when going idle, IOW they
always have some work to do within a jiffy boundary.

Am I missing something?