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

[Robin Holt <holt@sgi.com>]

On Tue, Apr 16, 2013 at 01:32:56PM +0200, Ingo Molnar wrote:
> 
> * Robin Holt <holt@sgi.com> 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@sgi.com>
> > Tested-by: Shawn Guo <shawn.guo@linaro.org>
> > To: Ingo Molnar <mingo@redhat.com>
> > To: Russ Anderson <rja@sgi.com>
> > To: Oleg Nesterov <oleg@redhat.com>
> > Cc: Andrew Morton <akpm@linux-foundation.org>
> > Cc: "H. Peter Anvin" <hpa@zytor.com>
> > Cc: Lai Jiangshan <laijs@cn.fujitsu.com>
> > Cc: Linus Torvalds <torvalds@linux-foundation.org>
> > Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
> > Cc: Michel Lespinasse <walken@google.com>
> > Cc: Oleg Nesterov <oleg@redhat.com>
> > Cc: "Paul E. McKenney" <paulmck@linux.vnet.ibm.com>
> > Cc: Paul Mackerras <paulus@samba.org>
> > Cc: Peter Zijlstra <peterz@infradead.org>
> > Cc: Robin Holt <holt@sgi.com>
> > Cc: "rusty@rustcorp.com.au" <rusty@rustcorp.com.au>
> > Cc: Tejun Heo <tj@kernel.org>
> > Cc: the arch/x86 maintainers <x86@kernel.org>
> > Cc: Thomas Gleixner <tglx@linutronix.de>
> > Cc: <stable@vger.kernel.org>
> > 
> > ---
> > 
> > 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);

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

Not sure.  I have not considered hotplug.  I will look that over when I
am in the office.

> ( 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.)

Done.

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

I will resubmit when I have the hotplug stuff understood and after giving
the set some more time for comments.

Robin