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

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Date:   Tue, 5 Jan 2021 22:40:48 +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 10:37 PM Lai Jiangshan <jiangshanlai@...il.com> wrote:
>
> On Tue, Jan 5, 2021 at 9:17 PM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Tue, Jan 05, 2021 at 04:23:44PM +0800, Lai Jiangshan wrote:
> > > 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.)
> >
> > But it is protected by the hotplug lock, which is really all you need
> > afaict.
> >
> > If the worker thread gets spawned before workqueue_offline_cpu(), said
> > function will observe it and adjust the mask, if it gets spawned after
> > it, it must observe a 'reduced' cpu_active_mask.
>
> Making the workqueue set workers' cpumask correctly is easy.
> The hard part is how to suppress the warning.
>
> It is true that said function will observe it and adjust the mask,
> but the warning is already issued.
>
> >
> > > >
> > > > 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,
> > >  };
> > >
> >
> > That's waay to late, by then userspace is long running and expecting
> > things to 'just-work'.
>
> I don't like this way either, I just list three ways I can think of.
> I prefer the way that __set_cpus_allowed_ptr() doesn't warn
> for kworkers.
>
> >
> > But afaict, things will mostly work for you when you use cpu_active_mask
> > on cpu-down and cpu_online_mask on cpu-up.
> >
> > But I think I see the problem, it is spawning a new worker after
> > workqueue_online_cpu() but before sched_cpu_activate(), right? That
> > wants to have the wider mask set.
> >
> > To solve that, the spawning of workers thing needs to know where we are
> > in the hotplug process, and it can track that using
> > workqueue_{on,off}line_cpu(). If it happens after offline, it needs to
> > use cpu_active_mask, if it happens after online cpu_online_mask is your
> > guy.
> >
> > Does that make sense?
>
> There are six stages we need to know when spawning a worker:
>
> stageA ap_deactive stageB workqueue_offline stageC
> stageD workqueue_online stageE ap_active stageF
>
> I don't think create_worker()/worker_attach_to_pool() can know where
> it is in the hotplug process unless it uses get_online_cpus() so that
> it knows it is not in the hotplug process.  There is no way to maintain
> needed information since there are no workqueue callbacks in the proper
> stages in the hotplug process.
>
> Again, making the workqueue set workers' cpumask correctly is easy.
> But we can't distinguish stageA&B or stageE&F to suppress the warning
> in __set_cpus_allowed_ptr() for new unbound workers when pool->attr->cpumask
> has only one cpu online&!active since there is no way to keep
> cpu_active_mask stable except get_online_cpus().

when pool->attr->cpumask has multi cpus but only one cpu online&!active.