Re: [patch V2 01/17] init: Pin init task to boot cpu initially

Steven Rostedt
2017-05-16T19:06:27Z

On Tue, 16 May 2017 20:42:32 +0200
Thomas Gleixner <tglx@linutronix.de> wrote:

> Some of the boot code in init_kernel_freeable() which runs before SMP
> bringup assumes (rightfully) that it runs on the boot cpu and therefor can
> use smp_processor_id() in preemptible context.
>
> That works so far because the smp_processor_id() check starts to be
> effective after smp bringup. That's just wrong. Starting with SMP bringup
> and the ability to move threads around, smp_processor_id() in preemptible
> context is broken.
>
> Aside of that it does not make sense to allow init to run on all cpus
> before sched_smp_init() has been run.
>
> Pin the init to the boot cpu so the existing code can continue to use
> smp_processor_id() without triggering the checks when the enabling of those
> checks starts earlier.
>
> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
> ---
> init/main.c | 17 ++++++++++++-----
> 1 file changed, 12 insertions(+), 5 deletions(-)
>
> --- a/init/main.c
> +++ b/init/main.c
> @@ -389,6 +389,7 @@ static __initdata DECLARE_COMPLETION(kth
>
> static noinline void __ref rest_init(void)
> {
> + struct task_struct *tsk;
> int pid;
>
> rcu_scheduler_starting();
> @@ -397,7 +398,17 @@ static noinline void __ref rest_init(voi
> * the init task will end up wanting to create kthreads, which, if
> * we schedule it before we create kthreadd, will OOPS.
> */
> - kernel_thread(kernel_init, NULL, CLONE_FS);
> + pid = kernel_thread(kernel_init, NULL, CLONE_FS);
> + /*
> + * Pin init on the boot cpu. Task migration is not properly working
> + * until sched_init_smp() has been run. It will set the allowed
> + * cpus for init to the non isolated cpus.
> + */
> + rcu_read_lock();
> + tsk = find_task_by_pid_ns(pid, &init_pid_ns);

Should we have a:

BUG_ON(!tsk);

Just to be paranoid?

-- Steve

> + set_cpus_allowed_ptr(tsk, cpumask_of(smp_processor_id()));
> + rcu_read_unlock();
> +
> numa_default_policy();
> pid = kernel_thread(kthreadd, NULL, CLONE_FS | CLONE_FILES);
> rcu_read_lock();
> @@ -1015,10 +1026,6 @@ static noinline void __init kernel_init_
> * init can allocate pages on any node
> */
> set_mems_allowed(node_states[N_MEMORY]);
> - /*
> - * init can run on any cpu.
> - */
> - set_cpus_allowed_ptr(current, cpu_all_mask);
>
> cad_pid = task_pid(current);
>
>