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

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Date: Thu, 1 Feb 2024 11:18:10 +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 31/01/24 10:31, Waiman Long wrote:
> 
> 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.

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?

Thanks,
Juri