From ebb2bdcef8a00d59b27d3532c423110559821e1d Mon Sep 17 00:00:00 2001 From: Petr Mladek Date: Mon, 14 Dec 2020 19:03:18 -0800 Subject: kthread_worker: document CPU hotplug handling The kthread worker API is simple. In short, it allows to create, use, and destroy workers. kthread_create_worker_on_cpu() just allows to bind a newly created worker to a given CPU. It is up to the API user how to handle CPU hotplug. They have to decide how to handle pending work items, prevent queuing new ones, and restore the functionality when the CPU goes off and on. There are few catches: + The CPU affinity gets lost when it is scheduled on an offline CPU. + The worker might not exist when the CPU was off when the user created the workers. A good practice is to implement two CPU hotplug callbacks and destroy/create the worker when CPU goes down/up. Mention this in the function description. [akpm@linux-foundation.org: grammar tweaks] Link: https://lore.kernel.org/r/20201028073031.4536-1-qiang.zhang@windriver.com Link: https://lkml.kernel.org/r/20201102101039.19227-1-pmladek@suse.com Reported-by: Zhang Qiang Signed-off-by: Petr Mladek Cc: Tejun Heo Cc: Thomas Gleixner Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- kernel/kthread.c | 20 +++++++++++++++++++- 1 file changed, 19 insertions(+), 1 deletion(-) (limited to 'kernel') diff --git a/kernel/kthread.c b/kernel/kthread.c index 34516b0a6eb7..97e053ade74a 100644 --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -793,7 +793,25 @@ EXPORT_SYMBOL(kthread_create_worker); * A good practice is to add the cpu number also into the worker name. * For example, use kthread_create_worker_on_cpu(cpu, "helper/%d", cpu). * - * Returns a pointer to the allocated worker on success, ERR_PTR(-ENOMEM) + * CPU hotplug: + * The kthread worker API is simple and generic. It just provides a way + * to create, use, and destroy workers. + * + * It is up to the API user how to handle CPU hotplug. They have to decide + * how to handle pending work items, prevent queuing new ones, and + * restore the functionality when the CPU goes off and on. There are a + * few catches: + * + * - CPU affinity gets lost when it is scheduled on an offline CPU. + * + * - The worker might not exist when the CPU was off when the user + * created the workers. + * + * Good practice is to implement two CPU hotplug callbacks and to + * destroy/create the worker when the CPU goes down/up. + * + * Return: + * The pointer to the allocated worker on success, ERR_PTR(-ENOMEM) * when the needed structures could not get allocated, and ERR_PTR(-EINTR) * when the worker was SIGKILLed. */ -- cgit v1.2.3