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Date:   Sat, 26 Dec 2020 06:52:39 -0800
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Hillf Danton <hdanton@...a.com>
Cc:     Lai Jiangshan <jiangshanlai@...il.com>,
        linux-kernel@...r.kernel.org,
        Valentin Schneider <valentin.schneider@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Qian Cai <cai@...hat.com>,
        Vincent Donnefort <vincent.donnefort@....com>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH -tip V2 00/10] workqueue: break affinity initiatively

On Sat, Dec 26, 2020 at 06:34:21PM +0800, Hillf Danton wrote:
> On Wed, 23 Dec 2020 11:49:51 -0800 "Paul E. McKenney" wrote:
> >On Sat, Dec 19, 2020 at 01:09:09AM +0800, Lai Jiangshan wrote:
> >> From: Lai Jiangshan <laijs@...ux.alibaba.com>
> >> 
> >> 06249738a41a ("workqueue: Manually break affinity on hotplug")
> >> said that scheduler will not force break affinity for us.
> >> 
> >> But workqueue highly depends on the old behavior. Many parts of the codes
> >> relies on it, 06249738a41a ("workqueue: Manually break affinity on hotplug")
> >> is not enough to change it, and the commit has flaws in itself too.
> >> 
> >> It doesn't handle for worker detachment.
> >> It doesn't handle for worker attachement, mainly worker creation
> >>   which is handled by Valentin Schneider's patch [1].
> >> It doesn't handle for unbound workers which might be possible
> >> per-cpu-kthread.
> >> 
> >> We need to thoroughly update the way workqueue handles affinity
> >> in cpu hot[un]plug, what is this patchset intends to do and
> >> replace the Valentin Schneider's patch [1].  The equivalent patch
> >> is patch 10.
> >> 
> >> Patch 1 fixes a flaw reported by Hillf Danton <hdanton@...a.com>.
> >> I have to include this fix because later patches depends on it.
> >> 
> >> The patchset is based on tip/master rather than workqueue tree,
> >> because the patchset is a complement for 06249738a41a ("workqueue:
> >> Manually break affinity on hotplug") which is only in tip/master by now.
> >> 
> >> And TJ acked to route the series through tip.
> >> 
> >> Changed from V1:
> >> 	Add TJ's acked-by for the whole patchset
> >> 
> >> 	Add more words to the comments and the changelog, mainly derived
> >> 	from discussion with Peter.
> >> 
> >> 	Update the comments as TJ suggested.
> >> 	
> >> 	Update a line of code as Valentin suggested.
> >> 
> >> 	Add Valentin's ack for patch 10 because "Seems alright to me." and
> >> 	add Valentin's comments to the changelog which is integral.
> >> 
> >> [1]: https://lore.kernel.org/r/ff62e3ee994efb3620177bf7b19fab16f4866845.camel@redhat.com
> >> [V1 patcheset]: https://lore.kernel.org/lkml/20201214155457.3430-1-jiangshanlai@gmail.com/
> >> 
> >> Cc: Hillf Danton <hdanton@...a.com>
> >> Cc: Valentin Schneider <valentin.schneider@....com>
> >> Cc: Qian Cai <cai@...hat.com>
> >> Cc: Peter Zijlstra <peterz@...radead.org>
> >> Cc: Vincent Donnefort <vincent.donnefort@....com>
> >> Cc: Tejun Heo <tj@...nel.org>
> >
> >And rcutorture hits this, so thank you for the fix!
> 
> Can you please specify a bit what you encountered in rcutorture
> before this patchset? You know we cant have a correct estimation
> of the fix diameter without your help.

It triggers the following in sched_cpu_dying() in kernel/sched/core.c,
exactly the same as for Lai Jiangshan:

	BUG_ON(rq->nr_running != 1 || rq_has_pinned_tasks(rq))

Which is in fact the "this" in my earlier "rcutorture hits this".  ;-)

							Thanx, Paul

> >Tested-by: Paul E. McKenney <paulmck@...nel.org>
> >
> >> Lai Jiangshan (10):
> >>   workqueue: restore unbound_workers' cpumask correctly
> >>   workqueue: use cpu_possible_mask instead of cpu_active_mask to break
> >>     affinity
> >>   workqueue: Manually break affinity on pool detachment
> >>   workqueue: don't set the worker's cpumask when kthread_bind_mask()
> >>   workqueue: introduce wq_online_cpumask
> >>   workqueue: use wq_online_cpumask in restore_unbound_workers_cpumask()
> >>   workqueue: Manually break affinity on hotplug for unbound pool
> >>   workqueue: reorganize workqueue_online_cpu()
> >>   workqueue: reorganize workqueue_offline_cpu() unbind_workers()
> >>   workqueue: Fix affinity of kworkers when attaching into pool
> >> 
> >>  kernel/workqueue.c | 214 ++++++++++++++++++++++++++++-----------------
> >>  1 file changed, 132 insertions(+), 82 deletions(-)
> >> 
> >> -- 
> >> 2.19.1.6.gb485710b

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