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

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Date: Fri, 2 Feb 2024 15:55:15 +0100
From: Juri Lelli <juri.lelli@...hat.com>
To: Waiman Long <longman@...hat.com>
Cc: Tejun Heo <tj@...nel.org>, 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 01/02/24 09:28, Waiman Long wrote:
> On 2/1/24 05:18, Juri Lelli wrote:
> > On 31/01/24 10:31, Waiman Long wrote:

..

> > My patch only uses the wq->unbound_attrs->cpumask to change the
> > associated rescuer cpumask, but I don't think your series modifies the
> > former?
> 
> I don't think so. The calling sequence of apply_wqattrs_prepare() and
> apply_wqattrs_commit() will copy unbound_cpumask into ctx->attrs which is
> copied into unbound_attrs. So unbound_attrs->cpumask should reflect the new
> global unbound cpumask. This code is there all along.

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.
:)

In the end we commit that last (overwritten) cpumask

apply_wqattrs_commit ->
  copy_workqueue_attrs(ctx->wq->unbound_attrs, ctx->attrs);

Now, my patch was wrong, as you pointed out, as it wasn't taking into
consideration the ordering guarantee. I thought maybe your changes (plus
and additional change to the above?) might fix the problem correctly.

Best,
Juri

1 - https://lore.kernel.org/lkml/20240116161929.232885-4-juri.lelli@redhat.com/