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

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Date: Wed, 31 Jan 2024 10:31:09 -0500
From: Waiman Long <longman@...hat.com>
To: Juri Lelli <juri.lelli@...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 1/31/24 08:01, Juri Lelli wrote:
> Hi Waiman,
>
> Thanks for working on this!
>
> On 30/01/24 13:33, Waiman Long wrote:
>> Ordered workqueues does not currently follow changes made to the
>> global unbound cpumask because per-pool workqueue changes may break
>> the ordering guarantee. IOW, a work function in an ordered workqueue
>> may run on a cpuset isolated CPU.
>>
>> This series enables ordered workqueues to follow changes made to the
>> global unbound cpumask by temporaily saving the work items in an
>> internal queue until the old pwq has been properly flushed and to be
>> freed. At that point, those work items, if present, are queued back to
>> the new pwq to be executed.
> I took it for a quick first spin (on top of wq/for-6.9) and this is what
> I'm seeing.
>
> Let's take edac-poller ordered wq, as the behavior seems to be the same
> for the rest.
>
> Initially we have (using wq_dump.py)
>
> wq_unbound_cpumask=0xffffffff 000000ff
> ...
> pool[80] ref= 44 nice=  0 idle/workers=  2/  2 cpus=0xffffffff 000000ff pod_cpus=0xffffffff 000000ff
> ...
> edac-poller                      ordered    80 80 80 80 80 80 80 80 ...
> ...
> edac-poller                      0xffffffff 000000ff    345 0xffffffff 000000ff
>
> after I
>
> # echo 3 >/sys/devices/virtual/workqueue/cpumask
>
> I get
>
> wq_unbound_cpumask=00000003
> ...
> pool[86] ref= 44 nice=  0 idle/workers=  2/  2 cpus=00000003 pod_cpus=00000003
> ...
> edac-poller                      ordered    86 86 86 86 86 86 86 86 86 86 ...
> ...
> edac-poller                      0xffffffff 000000ff    345 0xffffffff 000000ff
>
> So, IIUC, the pool and wq -> pool settings are updated correctly, but
> the wq.unbound_cpus (and its associated rescure affinity) are left
> untouched. Is this expected or are we maybe still missing an additional
> step?

Isn't this what the 4th patch of your RFC workqueue patch series does?

https://lore.kernel.org/lkml/20240116161929.232885-5-juri.lelli@redhat.com/

The focus of this series is to make sure that we can update the pool 
cpumask of ordered workqueue to follow changes in global unbound 
workqueue cpumask. So I haven't touched anything related to rescuer at all.

I will include your 4th patch in the next version of this series.

Cheers,
Longman