On Tue, Nov 15, 2022 at 09:19:26PM +0800, Zqiang wrote: > For CONFIG_PREEMPT_RT=y kernel, the "use_softirq=0" will be set, the > RCU_SOFTIRQ processing is moved to per-CPU rcuc kthreads which created > by smpboot_register_percpu_thread(). when CPU is going offline, the > corresponding rcu_data.rcu_cpu_kthread_status is set RCU_KTHREAD_OFFCPU, > and the rcuc kthreads enter TASK_PARKED state, kthreads in TASK_PARKED > state only accept kthread_unpark() to wakeup. > > Therefore, This commit avoid invoke wake_up_process() to rcuc kthreads > in TASK_PARKED state. > > Signed-off-by: Zqiang <qiang1.zhang@xxxxxxxxx> > --- > kernel/rcu/tree.c | 3 ++- > 1 file changed, 2 insertions(+), 1 deletion(-) > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c > index 3ccad468887e..49dd87356851 100644 > --- a/kernel/rcu/tree.c > +++ b/kernel/rcu/tree.c > @@ -2375,7 +2375,8 @@ static void rcu_wake_cond(struct task_struct *t, int status) > * If the thread is yielding, only wake it when this > * is invoked from idle > */ > - if (t && (status != RCU_KTHREAD_YIELDING || is_idle_task(current))) > + if (t && (status != RCU_KTHREAD_YIELDING || is_idle_task(current)) && > + status != RCU_KTHREAD_OFFCPU) > wake_up_process(t); There is a tiny window where this can happen (between CPUHP_TEARDOWN_CPU and CPUHP_AP_SMPBOOT_THREADS) and it can't cause a spurious unpark because wake_up_process() only wakes up from TASK_[UN]INTERRUPTIBLE states. And even if it did, the KTHREAD_SHOULD_PARK bit would still be on. And more important! On unpark time RCU_KTHREAD_OFFCPU isn't cleared. Only the rcuc kthread does it, and after your patch it couldn't be awaken to perform that, unless rcuc is lucky enough to have rcu_data.rcu_cpu_has_work = 1 by the time it unparks and that isn't guaranteed. So rcuc may sleep forever. OTOH one cleanup that could be done is to make rcu_cpu_kthread_park() to use __this_cpu_write as it's guaranteed that cpu == smp_processor_id(). Thanks.
