linux-kernel - Re: Bulk CPU Hotplug (Was Re: [PATCH] Do not force shutdown/reboot to boot cpu.)

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Date:	Sat, 13 Apr 2013 22:31:01 +0530
From:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>
To:	Robin Holt <holt@....com>
CC:	Ingo Molnar <mingo@...nel.org>, Paul Mackerras <paulus@...ba.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Russ Anderson <rja@....com>, Shawn Guo <shawn.guo@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	the arch/x86 maintainers <x86@...nel.org>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Tejun Heo <tj@...nel.org>, Oleg Nesterov <oleg@...hat.com>,
	Lai Jiangshan <laijs@...fujitsu.com>,
	Michel Lespinasse <walken@...gle.com>,
	"rusty@...tcorp.com.au" <rusty@...tcorp.com.au>,
	Peter Zijlstra <peterz@...radead.org>,
	Fenghua Yu <fenghua.yu@...el.com>, lkml@....ca
Subject: Re: Bulk CPU Hotplug (Was Re: [PATCH] Do not force shutdown/reboot
 to boot cpu.)

On 04/12/2013 03:01 PM, Robin Holt wrote:
>  kernel/sys.c | 17 +++++++++++++++--
>  1 file changed, 15 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 0da73cf..4d1047d 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -357,6 +357,19 @@ int unregister_reboot_notifier(struct notifier_block *nb)
>  }
>  EXPORT_SYMBOL(unregister_reboot_notifier);
> 
> +void migrate_to_boot_cpu(void)
> +{
> +	/* The boot cpu is always logical cpu 0 */
> +	int reboot_cpu_id = 0;
> +
> +	/* Make certain the cpu I'm about to reboot on is online */
> +	if (!cpu_online(reboot_cpu_id))
> +		reboot_cpu_id = smp_processor_id();
> +

If CPU 0 is offline, there is no point in binding, right?

[Fenghua (in CC) added the support to offline CPU0 on x86 Intel platforms.
So its possible that CPU0 is offline when you try a reboot.]

> +	/* Make certain I only run on the appropriate processor */
> +	set_cpus_allowed_ptr(current, cpumask_of(reboot_cpu_id));
> +}
> +
>  /**
>   *	kernel_restart - reboot the system
>   *	@cmd: pointer to buffer containing command to execute for restart
> @@ -368,7 +381,7 @@ EXPORT_SYMBOL(unregister_reboot_notifier);
>  void kernel_restart(char *cmd)
>  {
>  	kernel_restart_prepare(cmd);
> -	disable_nonboot_cpus();
> +	migrate_to_boot_cpu();
>  	syscore_shutdown();
>  	if (!cmd)
>  		printk(KERN_EMERG "Restarting system.\n");
> @@ -414,7 +427,7 @@ void kernel_power_off(void)
>  	kernel_shutdown_prepare(SYSTEM_POWER_OFF);
>  	if (pm_power_off_prepare)
>  		pm_power_off_prepare();
> -	disable_nonboot_cpus();
> +	migrate_to_boot_cpu();

Okay, so you are touching poweroff also. Restart was only recently altered
by Shawn, so we can assume that his fix was necessary only to his platform.
However, for poweroff, I see the commit below in the git log, which added
the disable_nonboot_cpus() call.

commit 4047727e5ae33f9b8d2b7766d1994ea6e5ec2991
Author: Mark Lord <lkml@....ca>
Date:   Mon Oct 1 01:20:10 2007 -0700

    Fix SMP poweroff hangs

Its an old commit, so perhaps the issue no longer holds good, but I thought
I should bring this to notice, just in case.

>  	syscore_shutdown();
>  	printk(KERN_EMERG "Power down.\n");
>  	kmsg_dump(KMSG_DUMP_POWEROFF);
> 

Regards,
Srivatsa S. Bhat

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