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

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aYJILJ8aplatbd2z@slm.duckdns.org>
Date: Tue, 3 Feb 2026 09:10:36 -1000
From: Tejun Heo <tj@...nel.org>
To: Chuck Lever <cel@...nel.org>
Cc: jiangshanlai@...il.com, linux-kernel@...r.kernel.org,
	Chuck Lever <chuck.lever@...cle.com>
Subject: Re: [RFC PATCH] workqueue: Automatic affinity scope fallback for
 single-pod topologies

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. 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.

Thanks.

-- 
tejun