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SubjectRe: [PATCH v2] NOHZ: fix nohz on cpu unplug
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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@de.ibm.com>
> Tested-by: Christian Borntraeger <borntraeger@de.ibm.com>

Thanks Christian for testing the fix. Ingo, appended is the updated patch. Thanks.
---
From: Suresh Siddha <suresh.b.siddha@intel.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@de.ibm.com>
Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com>
Tested-by: Christian Borntraeger <borntraeger@de.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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