linux-kernel - Re: [PATCH -tip V3 3/8] workqueue: introduce wq_online

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Date:   Tue, 5 Jan 2021 16:23:44 +0800
From:   Lai Jiangshan <jiangshanlai@...il.com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Qian Cai <cai@...hat.com>,
        Vincent Donnefort <vincent.donnefort@....com>,
        Dexuan Cui <decui@...rosoft.com>,
        Lai Jiangshan <laijs@...ux.alibaba.com>,
        Tejun Heo <tj@...nel.org>
Subject: Re: [PATCH -tip V3 3/8] workqueue: introduce wq_online_cpumask

On Tue, Jan 5, 2021 at 10:41 AM Lai Jiangshan <jiangshanlai@...il.com> wrote:
>
> On Mon, Jan 4, 2021 at 9:56 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Sat, Dec 26, 2020 at 10:51:11AM +0800, Lai Jiangshan wrote:
> > > From: Lai Jiangshan <laijs@...ux.alibaba.com>
> > >
> > > wq_online_cpumask is the cached result of cpu_online_mask with the
> > > going-down cpu cleared.
> >
> > You can't use cpu_active_mask ?
>
>
> When a cpu is going out:
> (cpu_active_mask is not protected by workqueue mutexs.)
>
> create_worker() for unbound pool  |  cpu offlining
> check cpu_active_mask             |
check wq_online_cpumask
>                                   |  remove bit from cpu_active_mask
>                                   |  no cpu in pool->attrs->cpumask is active
> set pool->attrs->cpumask to worker|
> and hit the warning
                                    |  remove bit from wq_online_cpumask

Even with the help of wq_online_cpumask, the patchset can't silence
the warning in __set_cpus_allowed_ptr() in this case.  It is indeed
hard to suppress the warning for unbound pools.  Maybe we need something
like this (outmost callback of CPUHP_AP_WORKQUEUE_UNBOUND_ONLINE,
so that workqueue can do preparation when offlining before AP_ACTIVE):

diff --git a/include/linux/cpuhotplug.h b/include/linux/cpuhotplug.h
index 0042ef362511..ac2103deb20b 100644
--- a/include/linux/cpuhotplug.h
+++ b/include/linux/cpuhotplug.h
@@ -20,6 +20,9 @@
  *               |                               ^
  *               v                               |
  *              AP_ACTIVE                      AP_ACTIVE
+ *               |                               ^
+ *               v                               |
+ *              ONLINE                         ONLINE
  */

 enum cpuhp_state {
@@ -194,6 +197,7 @@ enum cpuhp_state {
        CPUHP_AP_X86_HPET_ONLINE,
        CPUHP_AP_X86_KVM_CLK_ONLINE,
        CPUHP_AP_ACTIVE,
+       CPUHP_AP_WORKQUEUE_UNBOUND_ONLINE,
        CPUHP_ONLINE,
 };


The other way is to modify __set_cpus_allowed_ptr() to suppress the
warning for kworkers and believe/let the workqueue handle cpumask correctly.

And the third way is to use get_online_cpus() in worker_attach_to_pool()
which might delay work items to be processed during cpuhotplug and might
be dangerous when someone call flush_work() in cpuhotplug callbacks.

Any thoughts?

Thanks,
Lai

>
>
> And when a cpu is onlining, there may be some workers which were just created
> after the workqueue hotplug callback is finished but before cpu_active_mask
> was updated. workqueue has not call back after cpu_active_mask updated and
> these workers' cpumask is not updated.
>
> For percpu workers, these problems can be handled with the help of
> POOL_DISASSOCIATED which is protected by workqueue mutexs and the
> help of sched/core.c which doesn't warn when per-cpu-kthread.
>
> For unbound workers, the way to handle it without using wq_online_cpumask
> is much more complex when a cpu is going out.