Hi!
From: Peter Zijlstra peterz@infradead.org
[ Upstream commit ac687e6e8c26181a33270efd1a2e2241377924b0 ]
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.
AFAIK this should not be in 4.19/5.10 as it does not fix anything w/o 5ba2ffba13a1e. Nobody calls kthread_is_per_cpu() in those kernels.
Best regards, Pavel