Commit 483b4ee6 authored Feb 04, 2009 by Suresh Siddha Committed by Ingo Molnar Feb 04, 2009

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>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

parent 35626129

kernel/sched.c

+11 −6

Original line number	Diff line number	Diff line
		@@ -3890,19 +3890,24 @@ 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 (!cpu_active(cpu)) {
		if (atomic_read(&nohz.load_balancer) != cpu)
		return 0;

		/*
		* If we are going offline and still the leader, give up!
		* If we are going offline and still the leader,
		* give up!
		*/
		if (!cpu_active(cpu) &&
		atomic_read(&nohz.load_balancer) == cpu) {
		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)