Re: [Patch -v4 1/4] Migrate shutdown/reboot to boot cpu.

[Ingo Molnar]

* Robin Holt <holt@xxxxxxx> wrote:

> 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 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@xxxxxxx>
> Tested-by: Shawn Guo <shawn.guo@xxxxxxxxxx>
> To: Ingo Molnar <mingo@xxxxxxxxxx>
> To: Russ Anderson <rja@xxxxxxx>
> To: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: Lai Jiangshan <laijs@xxxxxxxxxxxxxx>
> Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Cc: Linux Kernel Mailing List <linux-kernel@xxxxxxxxxxxxxxx>
> Cc: Michel Lespinasse <walken@xxxxxxxxxx>
> Cc: Oleg Nesterov <oleg@xxxxxxxxxx>
> Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
> Cc: Paul Mackerras <paulus@xxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Robin Holt <holt@xxxxxxx>
> Cc: "rusty@xxxxxxxxxxxxxxx" <rusty@xxxxxxxxxxxxxxx>
> Cc: Tejun Heo <tj@xxxxxxxxxx>
> Cc: the arch/x86 maintainers <x86@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx>
> 
> ---
> 
> Changes since -v1.
> - Set PF_THREAD_BOUND before migrating to eliminate potential race.
> - Modified kernel_power_off to also migrate instead of using
>   disable_nonboot_cpus().
> ---
>  kernel/sys.c | 22 +++++++++++++++++++---
>  1 file changed, 19 insertions(+), 3 deletions(-)
> 
> diff --git a/kernel/sys.c b/kernel/sys.c
> index 0da73cf..5ef7aa2 100644
> --- a/kernel/sys.c
> +++ b/kernel/sys.c
> @@ -357,6 +357,22 @@ int unregister_reboot_notifier(struct notifier_block *nb)
>  }
>  EXPORT_SYMBOL(unregister_reboot_notifier);
>  
> +void migrate_to_reboot_cpu(void)

It appears to be file-scope, so should be static I guess?

> +{
> +	/* 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();

Shouldn't we pick the first online CPU instead, to make it deterministic?

Also, does this codepath prevent hotplug from going on in parallel?

( Plus, the smp_processor_id() is in a preemptible section AFAICS, so it will 
  throw a warning with preempt debug on. )

> +
> +	/* Prevent races with other tasks migrating this task. */

(I guess the colon can be dropped here, like in the other comments.)

> +	current->flags |= PF_THREAD_BOUND;
> +
> +	/* Make certain I only run on the appropriate processor */
> +	set_cpus_allowed_ptr(current, cpumask_of(reboot_cpu_id));
> +}

Thanks,

	Ingo
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