Messages in this thread | | | From | Lai Jiangshan <> | Date | Tue, 5 Jan 2021 10:41:07 +0800 | Subject | Re: [PATCH -tip V3 3/8] workqueue: introduce wq_online_cpumask |
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On Mon, Jan 4, 2021 at 9:56 PM Peter Zijlstra <peterz@infradead.org> wrote: > > On Sat, Dec 26, 2020 at 10:51:11AM +0800, Lai Jiangshan wrote: > > From: Lai Jiangshan <laijs@linux.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 | | 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.
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