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

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Date:   Tue, 16 May 2017 15:06:21 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Mark Rutland <mark.rutland@....com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Subject: Re: [patch V2 01/17] init: Pin init task to boot cpu initially

On Tue, 16 May 2017 20:42:32 +0200
Thomas Gleixner <tglx@...utronix.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@...utronix.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);
>  
>