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

[Robin Holt]

On Tue, Apr 16, 2013 at 09:18:07PM +0530, Srivatsa S. Bhat wrote:
> On 04/16/2013 05:36 PM, Robin Holt wrote:
> > On Tue, Apr 16, 2013 at 01:32:56PM +0200, Ingo Molnar wrote:
> >>
> >> * 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?
> > 
> > Done.
> > 
> >>> +{
> >>> +	/* 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?
> > 
> > Done.
> > 
> > 		reboot_cpu_id = cpumask_first(cpu_online_mask);
> > 
> 
> Let me ask again: if CPU 0 (or whatever the preferred reboot cpu is)
> is offline, then why should we even bother pinning the task to (another)
> CPU? Why not just proceed with the reboot?

No idea.  I copied it from the arch/x86 code.  I can not defend it.

Robin
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