Re: [PATCH 1/2] sched/fair: Prefer fully-idle SMT cores in asym-capacity idle selection

[K Prateek Nayak]

Hello Andrea,

On 4/20/2026 2:06 PM, Andrea Righi wrote:
>> With your changes, only two places actually care about test_idle_cores():
>>
>> - select_idle_capacity()
>> - select_idle_cpu()
>>
>> If we go into select_idle_capacity(), we don't do select_idle_cpu() so
>> the two paths are mutually exclusive.
>>
>> In nohz_balancer_kick(), if we find, sd_asym_cpucapacity, we simply
>> don't care about the sd_llc_shared->nr_busy_cpus during balancing so
>> that begs the question if we can simply track idle_cores at
>> sd_asym_cpucapacity for these systems?
> 
> Yeah, makes sense to me. I was planning to test something similar, so thanks for
> sharing this patch. :) I'll give it a try and report back.

Thank you for taking it for a spin!

[..snip..]

>> I still have one question: Can first SD_ASYM_CPUCAPACITY_FULL be set at
>> a SD_NUMA?
>>
>> We'll need to deal with overlapping domains then but seems like it could
>> be possible with weird cpusets :-(
>>
>> But in that case, do we even want to search CPUs outside the NUMA in
>> select_idle_capacity()? I don't think anything stops this currently but
>> I might be wrong.
> 
> My $0.02 on this.
> 
> In theory it could happen with unusual topologies or constrained cpusets,
> although it should be quite rare. That said, select_idle_capacity() already
> operates on the span of sd_asym_cpucapacity, so if that domain crosses NUMA
> boundaries, we're already scanning across NUMA today. This patch doesn't
> fundamentally alter this behavior.

Ack! I was just thinking loud from the topology standpoint since
sd->shared is not designed to handle the overlapping domains like
sg->sgc does but we can probably figure some way to make it work.

Using the ring topology example from topology.c:

  0 ----- 1
  |       |
  |       |
  |       |
  3 ----- 2

Consider NUMA-1 below gets the SD_ASYM_CPUCAPACITY_FULL flag:

  NUMA-2       0-3             0-3             0-3             0-3
   groups:     {0-1,3},{1-3}   {0-2},{0,2-3}   {1-3},{0-1,3}   {0,2-3},{0-2}

  NUMA-1       0-1,3           0-2             1-3             0,2-3
   groups:     {0},{1},{3}     {0},{1},{2}     {1},{2},{3}     {0},{2},{3}

  NUMA-0       0               1               2               3


The "sd->shared" assignments at NUMA-1 will put first, second, and the
last domain in the same "shared" range by today's logic since the first
CPU in their span is the same although their spans are slightly
different.

The third will be standalone since the first CPU of the domain span
will be different.

> If we think cross-NUMA scanning is undesirable, that's probably a more general
> issue in select_idle_capacity(), rather than something specific to this change
> and we can address this later.

Ack! That is a tangential problem but may require some looking at if
we decide to extend the sd->shared object to SD_NUMA domains. I guess
if anyone is running such setup, this bit will be the least of their
worries.

-- 
Thanks and Regards,
Prateek