Re: [RFC PATCH] workqueue: Automatic affinity scope fallback for single-pod topologies

[Chuck Lever]

On 2/3/26 2:10 PM, Tejun Heo wrote:
> Hello,
> 
> On Tue, Feb 03, 2026 at 09:37:44AM -0500, Chuck Lever wrote:
>> On such systems WQ_AFFN_CACHE, WQ_AFFN_SMT, and WQ_AFFN_NUMA scopes all
>> collapse to a single pod.
> 
> WQ_AFFN_SMT should be on CPU core boundaries, right?
> 
>> Add wq_effective_affn_scope() to detect when a selected affinity scope
>> provides only one pod despite having multiple CPUs, and automatically
>> fall back to a finer-grained scope. This ensures reasonable lock
>> distribution without requiring manual configuration via the
>> workqueue.default_affinity_scope parameter or per-workqueue sysfs
>> tuning.
>>
>> The fallback is conservative: it triggers only when a scope degenerates
>> to exactly one pod, and respects explicitly configured (non-default)
>> scopes.
> 
> While I understand the problem, I don't think dropping down to core boundary
> for unbound workqueues by default makes sense. That may help with some use
> cases but cause problem with others.

I've never seen a case where it doesn't help. In order to craft an
alternative, I'll need to have some examples to avoid. Is it only the
SMT case that is concerning?


> Given that WQ_AFFN_CACHE is the same as
> WQ_AFFN_NUMA on these machines, maybe we can shard it automatically
> according to some heuristics or maybe we can introduce another affinity
> level between CACHE and SMT which is sharded on machines with too many CPUs
> in a single cache domain.


-- 
Chuck Lever