linux-kernel - Re: [PATCH v2] keep rd->online and cpu_online

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Date:	Mon, 10 Mar 2008 16:36:13 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Gregory Haskins <ghaskins@...ell.com>
Cc:	suresh.b.siddha@...el.com, ego@...ibm.com, rjw@...k.pl,
	dmitry.adamushko@...il.com, mingo@...e.hu, oleg@...n.ru,
	yi.y.yang@...el.com, linux-kernel@...r.kernel.org,
	tglx@...utronix.de
Subject: Re: [PATCH v2] keep rd->online and cpu_online_map in sync

On Mon, 10 Mar 2008 17:59:11 -0400 Gregory Haskins <ghaskins@...ell.com> wrote:

> >>> On Mon, Mar 10, 2008 at  6:10 PM, in message
> <20080310221014.GB27329@...ux-os.sc.intel.com>, Suresh Siddha
> <suresh.b.siddha@...el.com> wrote: 
> > On Mon, Mar 10, 2008 at 04:00:28PM -0600, Gregory Haskins wrote:
> >> >>> On Mon, Mar 10, 2008 at  6:03 PM, in message 
> > <200803102303.28660.rjw@...k.pl>,
> >> "Rafael J. Wysocki" <rjw@...k.pl> wrote: 
> >> > On Monday, 10 of March 2008, Suresh Siddha wrote:
> >> >> >  
> >> >> > -	case CPU_DOWN_PREPARE:
> >> >> > +	case CPU_DYING:
> >> >> 
> >> >> Don't we need to take care of CPU_DYING_FROZEN aswell?
> >> > 
> >> > Well, I'd say we do.
> >> 
> >> Should I add that to the patch as well then?
> > 
> > Yes please.
> 
> Here is v2 with the suggested improvement
> 
> -Greg
> 
> ------------------------
> keep rd->online and cpu_online_map in sync
> 
> It is possible to allow the root-domain cache of online cpus to
> become out of sync with the global cpu_online_map.  This is because we
> currently trigger removal of cpus too early in the notifier chain.
> Other DOWN_PREPARE handlers may in fact run and reconfigure the
> root-domain topology, thereby stomping on our own offline handling.
> 
> The end result is that rd->online may become out of sync with
> cpu_online_map, which results in potential task misrouting.
> 
> So change the offline handling to be more tightly coupled with the
> global offline process by triggering on CPU_DYING intead of
> CPU_DOWN_PREPARE.
> 
> Signed-off-by: Gregory Haskins <ghaskins@...ell.com>
> Cc: Gautham R Shenoy <ego@...ibm.com>
> Cc: "Siddha, Suresh B" <suresh.b.siddha@...el.com>
> Cc: Ingo Molnar <mingo@...e.hu>
> Cc: "Rafael J. Wysocki" <rjw@...k.pl>
> Cc: Andrew Morton <akpm@...ux-foundation.org>
> ---
> 
>  kernel/sched.c |    3 ++-
>  1 files changed, 2 insertions(+), 1 deletions(-)
> 
> diff --git a/kernel/sched.c b/kernel/sched.c
> index 52b9867..1cb53fb 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -5881,7 +5881,8 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
>  		spin_unlock_irq(&rq->lock);
>  		break;
>  
> -	case CPU_DOWN_PREPARE:
> +	case CPU_DYING:
> +	case CPU_DYING_FROZEN:
>  		/* Update our root-domain */
>  		rq = cpu_rq(cpu);
>  		spin_lock_irqsave(&rq->lock, flags);

Does this make
cpu-hotplug-register-update_sched_domains-notifier-with-higher-prio.patch
(below) obsolete, or do we want both?  

--- a/kernel/sched.c~cpu-hotplug-register-update_sched_domains-notifier-with-higher-prio
+++ a/kernel/sched.c
@@ -7096,8 +7096,16 @@ void __init sched_init_smp(void)
 	if (cpus_empty(non_isolated_cpus))
 		cpu_set(smp_processor_id(), non_isolated_cpus);
 	put_online_cpus();
-	/* XXX: Theoretical race here - CPU may be hotplugged now */
-	hotcpu_notifier(update_sched_domains, 0);
+	/*
+	 * XXX: Theoretical race here - CPU may be hotplugged now
+	 *
+	 * We register the notifier with priority 11, which means that
+	 * update_sched_domains() will be called just before migration_call().
+	 *
+	 * This is necessary to ensure that the rt wake up logic works fine
+	 * and the rq->rd->online_map remains in sync with the cpu_online_map.
+	 */
+	hotcpu_notifier(update_sched_domains, 11);
 
 	/* Move init over to a non-isolated CPU */
 	if (set_cpus_allowed(current, non_isolated_cpus) < 0)
_

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