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

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Date:	Mon, 15 Apr 2013 12:16:44 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Robin Holt <holt@....com>
Cc:	"Srivatsa S. Bhat" <srivatsa.bhat@...ux.vnet.ibm.com>,
	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>
Subject: Re: Bulk CPU Hotplug (Was Re: [PATCH] Do not force shutdown/reboot
 to boot cpu.)


* Robin Holt <holt@....com> wrote:

> From 1767003c943325e52ac78cac6fdbaf2ab638888d Mon Sep 17 00:00:00 2001
> From: Robin Holt <holt@....com>
> Date: Wed, 3 Apr 2013 13:52:00 -0500
> Subject: [PATCH] Migrate shutdown/reboot to boot cpu.
> 
> We recently noticed that reboot of a 1024 cpu machine takes approx 16
> minutes of just stopping the cpus.  The slowdown was tracked to commit
> f96972f.
> 
> The current implementation does all the work of hot removing the cpus
> before halting the system.  We are switching to just migrating to the
> boot cpu and then calling continuing with shutdown/reboot.
> 
> This also has the effect of not breaking x86's command line parameter for
> specifying the reboot cpu.  Note, this code was shamelessly copied from
> arch/x86/kernel/reboot.c with bits removed pertaining to the reboot_cpu
> command line parameter.
> 
> Signed-off-by: Robin Holt <holt@....com>
> To: Shawn Guo <shawn.guo@...aro.org>
> To: Ingo Molnar <mingo@...hat.com>
> To: Russ Anderson <rja@....com>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> Cc: "H. Peter Anvin" <hpa@...or.com>
> Cc: Lai Jiangshan <laijs@...fujitsu.com>
> Cc: Linus Torvalds <torvalds@...ux-foundation.org>
> Cc: Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
> Cc: Michel Lespinasse <walken@...gle.com>
> Cc: Oleg Nesterov <oleg@...hat.com>
> Cc: "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
> Cc: Paul Mackerras <paulus@...ba.org>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Robin Holt <holt@....com>
> Cc: "rusty@...tcorp.com.au" <rusty@...tcorp.com.au>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: the arch/x86 maintainers <x86@...nel.org>
> Cc: Thomas Gleixner <tglx@...utronix.de>
> Cc: <stable@...r.kernel.org>
> ---
>  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();
> +
> +	/* Make certain I only run on the appropriate processor */
> +	set_cpus_allowed_ptr(current, cpumask_of(reboot_cpu_id));
> +}

I guess you suspect what I'm going to suggest next? :-)

While I think something like this commit would be acceptable as a minimal 
regression fix, it would be really lovely to add a second patch as well, which 
removes the same code from arch/x86/kernel/reboot.c:native_machine_shutdown() and 
merged it with the kernel/sys.c version? That way all platforms gained a 
reboot_cpu command line, and we'd have less code duplication as well. Win-win.

( While at it, it might make sense to move the reboot/shutdown related bits from 
  kernel/sys.c to a new kernel/shutdown.c or kernel/reboot.c file or so. )

Thanks,

	Ingo
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