linux-kernel - Re: [PATCH v5 01/14] slab: add opt-in caching layer of percpu sheaves

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Message-ID: <02568985-b6c0-45cd-924b-f86f7975c119@suse.cz>
Date: Mon, 15 Sep 2025 09:21:16 +0200
From: Vlastimil Babka <vbabka@...e.cz>
To: Hillf Danton <hdanton@...a.com>
Cc: linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 01/14] slab: add opt-in caching layer of percpu sheaves

On 9/15/25 02:11, Hillf Danton wrote:
> On Sun, 14 Sep 2025 22:24:19 +0200 Vlastimil Babka wrote:
>>On 9/14/25 04:22, Hillf Danton wrote:
>>> On Wed, 23 Jul 2025 15:34:34 +0200 Vlastimil Babka wrote:
>>> Migration is not disabled.
>>
>> Can you elaborate how it's not? There's a comment above the function saying
>> "Called from CPU work handler with migration disabled." and we have relied
>> on this before sheaves. queue_work_on() says it will run on the specific
>> cpu. AFAIK the workqueue workers are bound which is effectively disabled
>> migration (we hold the cpu hotplug lock).
>>
> CPU affinity and migration_disable() are two different things regardless of
> queue_work_on(), no?

Well as long as it means the execution of pcs_flush_all() won't change the
cpu it's running on in the middle of the operation, it's the same.

> I think you are right in accident rather than by design in this case.

I don't see a mistake in the design, there are many queue_work_on() users
relying on this property. It's only the comment wording where "make sure
migration is disabled" makes you expect there to be a migrate_disable()
somewhere? But why if it's not necessary?