linux-kernel - Re: [RFC PATCH 0/3] workqueue: Enable unbound cpumask update on ordered workqueues

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Date: Fri, 2 Feb 2024 14:03:32 -0500
From: Waiman Long <longman@...hat.com>
To: Tejun Heo <tj@...nel.org>, Juri Lelli <juri.lelli@...hat.com>
Cc: Lai Jiangshan <jiangshanlai@...il.com>, linux-kernel@...r.kernel.org,
 Cestmir Kalina <ckalina@...hat.com>, Alex Gladkov <agladkov@...hat.com>
Subject: Re: [RFC PATCH 0/3] workqueue: Enable unbound cpumask update on
 ordered workqueues

On 2/2/24 12:07, Tejun Heo wrote:
> 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.

Thank for the explanation, we will use the new 
unbound_effective_cpumask() helper. It does look like there is a major 
restructuring of the workqueue code in 6.9. I will adapt my patch series 
to be based on the for-6.9 branch.

Cheers,
Longman