linux-kernel - Re: [PATCH v2] NOHZ: fix nohz on cpu unplug

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Date:	Wed, 04 Feb 2009 11:59:44 -0800
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Christian Borntraeger <borntraeger@...ibm.com>
Cc:	Ingo Molnar <mingo@...e.hu>, Thomas Gleixner <tglx@...utronix.de>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Heiko Carstens <heiko.carstens@...ibm.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>
Subject: Re: [PATCH v2] NOHZ: fix nohz on cpu unplug

On Tue, 2009-02-03 at 22:19 -0800, Christian Borntraeger wrote:
> Am Tuesday 03 February 2009 13:28:47 schrieb Ingo Molnar:
> > > > +         if (!cpu_active(cpu) {
> > >
> > > That does not compile...I used the following:
> > >             if (!cpu_active(cpu)) {
> >
> > Could you please resubmit an updated patch?
> 
> Suresh,
> 
> can you send the updated patch? After all, its your work and we need your
> Signed-off-by anyway.
> 
> You can add my
> Reported-by: Christian Borntraeger <borntraeger@...ibm.com>
> Tested-by: Christian Borntraeger <borntraeger@...ibm.com>

Thanks Christian for testing the fix. Ingo, appended is the updated patch. Thanks.
---

From: Suresh Siddha <suresh.b.siddha@...el.com>
Subject: sched: fix nohz load balancer on cpu offline

Christian Borntraeger reports:

> After a logical cpu offline, even on a complete idle system, there
> is one cpu with full ticks. It turns out that nohz.cpu_mask has the
> the offlined cpu still set.
> 
> In select_nohz_load_balancer() we check if the system is completely
> idle to turn of load balancing. We compare cpu_online_map with
> nohz.cpu_mask.  Since cpu_online_map is updated on cpu unplug,
> but nohz.cpu_mask is not, the check fails and the scheduler believes
that
> we need an "idle load balancer" even on a fully idle system. Since the
ilb 
> cpu does not deactivate the timer tick this breaks NOHZ.

Fix the select_nohz_load_balancer() to not set the nohz.cpu_mask
while a cpu is going offline.

Reported-by: Christian Borntraeger <borntraeger@...ibm.com>
Signed-off-by: Suresh Siddha <suresh.b.siddha@...el.com>
Tested-by: Christian Borntraeger <borntraeger@...ibm.com>
---

diff --git a/kernel/sched.c b/kernel/sched.c
index 96439a4..8a0419b 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -4026,19 +4026,24 @@ int select_nohz_load_balancer(int stop_tick)
 	int cpu = smp_processor_id();
 
 	if (stop_tick) {
-		cpumask_set_cpu(cpu, nohz.cpu_mask);
 		cpu_rq(cpu)->in_nohz_recently = 1;
 
-		/*
-		 * If we are going offline and still the leader, give up!
-		 */
-		if (!cpu_active(cpu) &&
-		    atomic_read(&nohz.load_balancer) == cpu) {
+		if (!cpu_active(cpu)) {
+			if (atomic_read(&nohz.load_balancer) != cpu)
+				return 0;
+
+			/*
+			 * If we are going offline and still the leader,
+			 * give up!
+			 */
 			if (atomic_cmpxchg(&nohz.load_balancer, cpu, -1) != cpu)
 				BUG();
+
 			return 0;
 		}
 
+		cpumask_set_cpu(cpu, nohz.cpu_mask);
+
 		/* time for ilb owner also to sleep */
 		if (cpumask_weight(nohz.cpu_mask) == num_online_cpus()) {
 			if (atomic_read(&nohz.load_balancer) == cpu)


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