There is a need to distinguish geniune per-cpu kthreads from kthreads that happen to have a single CPU affinity.
Geniune per-cpu kthreads are kthreads that are CPU affine for correctness, these will obviously have PF_KTHREAD set, but must also have PF_NO_SETAFFINITY set, lest userspace modify their affinity and ruins things. However, these two things are not sufficient, PF_NO_SETAFFINITY is also set on other tasks that have their affinities controlled through other means, like for instance workqueues. Therefore another bit is needed; it turns out kthread_create_per_cpu() already has such a bit: KTHREAD_IS_PER_CPU, which is used to make kthread_park()/kthread_unpark() work correctly. Expose this flag and remove the implicit setting of it from kthread_create_on_cpu(); the io_uring usage of it seems dubious at best. Signed-off-by: Peter Zijlstra (Intel) <[email protected]> Tested-by: Paul E. McKenney <[email protected]> --- include/linux/kthread.h | 3 +++ kernel/kthread.c | 25 ++++++++++++++++++++++++- kernel/sched/core.c | 2 +- kernel/sched/sched.h | 4 ++-- kernel/smpboot.c | 1 + kernel/workqueue.c | 11 +++++++++-- 6 files changed, 40 insertions(+), 6 deletions(-) --- a/include/linux/kthread.h +++ b/include/linux/kthread.h @@ -33,6 +33,9 @@ struct task_struct *kthread_create_on_cp unsigned int cpu, const char *namefmt); +void kthread_set_per_cpu(struct task_struct *k, bool set); +bool kthread_is_per_cpu(struct task_struct *k); + /** * kthread_run - create and wake a thread. * @threadfn: the function to run until signal_pending(current). --- a/kernel/kthread.c +++ b/kernel/kthread.c @@ -493,11 +493,34 @@ struct task_struct *kthread_create_on_cp return p; kthread_bind(p, cpu); /* CPU hotplug need to bind once again when unparking the thread. */ - set_bit(KTHREAD_IS_PER_CPU, &to_kthread(p)->flags); to_kthread(p)->cpu = cpu; return p; } +void kthread_set_per_cpu(struct task_struct *k, bool set) +{ + struct kthread *kthread = to_kthread(k); + if (!kthread) + return; + + if (set) { + WARN_ON_ONCE(!(k->flags & PF_NO_SETAFFINITY)); + WARN_ON_ONCE(k->nr_cpus_allowed != 1); + set_bit(KTHREAD_IS_PER_CPU, &kthread->flags); + } else { + clear_bit(KTHREAD_IS_PER_CPU, &kthread->flags); + } +} + +bool kthread_is_per_cpu(struct task_struct *k) +{ + struct kthread *kthread = to_kthread(k); + if (!kthread) + return false; + + return test_bit(KTHREAD_IS_PER_CPU, &kthread->flags); +} + /** * kthread_unpark - unpark a thread created by kthread_create(). * @k: thread created by kthread_create(). --- a/kernel/smpboot.c +++ b/kernel/smpboot.c @@ -188,6 +188,7 @@ __smpboot_create_thread(struct smp_hotpl kfree(td); return PTR_ERR(tsk); } + kthread_set_per_cpu(tsk, true); /* * Park the thread so that it could start right on the CPU * when it is available.
