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Sender: Tejun Heo <htejun@gmail.com>
Date: Fri, 2 Feb 2024 07:07:40 -1000
From: Tejun Heo <tj@kernel.org>
To: Juri Lelli <juri.lelli@redhat.com>
Cc: Waiman Long <longman@redhat.com>,
	Lai Jiangshan <jiangshanlai@gmail.com>,
	linux-kernel@vger.kernel.org, Cestmir Kalina <ckalina@redhat.com>,
	Alex Gladkov <agladkov@redhat.com>
Subject: Re: [RFC PATCH 0/3] workqueue: Enable unbound cpumask update on
 ordered workqueues
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Hello,

On Fri, Feb 02, 2024 at 03:55:15PM +0100, Juri Lelli wrote:
> Indeed. I believe this is what my 3/4 [1] was trying to cure, though. I
> still think that with current code the new_attr->cpumask gets first
> correctly initialized considering unbound_cpumask
> 
> apply_wqattrs_prepare ->
>   copy_workqueue_attrs(new_attrs, attrs);
>   wqattrs_actualize_cpumask(new_attrs, unbound_cpumask);
> 
> but then overwritten further below using cpu_possible_mask
> 
> apply_wqattrs_prepare ->
>   copy_workqueue_attrs(new_attrs, attrs);
>   cpumask_and(new_attrs->cpumask, new_attrs->cpumask, cpu_possible_mask);
> 
> operation that I honestly seem to still fail to grasp why we need to do.
> :)

So, imagine the following scenario on a system with four CPUs:

1. Initially both wq_unbound_cpumask and wq A's cpumask are 0xf.

2. wq_unbound_cpumask is set to 0x3. A's effective is 0x3.

3. A's cpumask is set to 0xe, A's effective is 0x3.

4. wq_unbound_cpumask is restore to 0xf. A's effective should become 0xe.

The reason why we're saving what user requested rather than effective is to
be able to do #4 so that the effective is always what's currently allowed
from what the user specified for the workqueue.

Now, if you want the current effective cpumask, that always coincides with
the workqueue's dfl_pwq's __pod_cpumask and if you look at the current
wq/for-6.9 branch, that's accessible through unbound_effective_cpumask()
helper.

Thanks.

-- 
tejun