Re: [PATCH -tip V3 3/8] workqueue: introduce wq_online_cpumask

[Lai Jiangshan]

On Tue, Jan 5, 2021 at 10:41 AM Lai Jiangshan <jiangshanlai@xxxxxxxxx> wrote:
>
> On Mon, Jan 4, 2021 at 9:56 PM Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> >
> > On Sat, Dec 26, 2020 at 10:51:11AM +0800, Lai Jiangshan wrote:
> > > From: Lai Jiangshan <laijs@xxxxxxxxxxxxxxxxx>
> > >
> > > 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             |
>                                   |  remove bit from cpu_active_mask
>                                   |  no cpu in pool->attrs->cpumask is active
> set pool->attrs->cpumask to worker|
> and hit the warning
>
>
> 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.

To have replied too soon, let me think about it again.