linux-kernel - Re: [patch 01/18] init: Pin init task to boot cpu initially

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Message-ID: <20170515101029.580e88b2@gandalf.local.home>
Date:   Mon, 15 May 2017 10:10:29 -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>
Subject: Re: [patch 01/18] init: Pin init task to boot cpu initially

On Sun, 14 May 2017 20:27:17 +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>

Reviewed-by: Steven Rostedt (VMware) <rostedt@...dmis.org>

-- Steve

> ---
>  init/main.c |    7 +++++--
>  1 file changed, 5 insertions(+), 2 deletions(-)
> 
> --- a/init/main.c
> +++ b/init/main.c
> @@ -1015,10 +1015,13 @@ 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.
> +	 * 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.
>  	 */
> -	set_cpus_allowed_ptr(current, cpu_all_mask);
> +	set_cpus_allowed_ptr(current, cpumask_of(raw_smp_processor_id()));
>  
>  	cad_pid = task_pid(current);
>  
>