Messages in this thread Patch in this message | | | From | Christian Borntraeger <> | Subject | [PATCH v2] NOHZ: fix nohz on cpu unplug | Date | Mon, 2 Feb 2009 13:23:43 +0100 |
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After some testing, I think this patch is better: [PATCH] NOHZ: fix nohz on cpu unplug
From: Christian Borntraeger <borntraeger@de.ibm.com>
After using cpu unplug I have seen one cpu with full ticks, even on a idle systems. It turns out that nohz.cpu_mask is not updated on cpu unplug. 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.
This patch clear the nohz bits in the migration_call, a function that is already called by the cpu hotplug notifier.
Opinions?
Signed-off-by: Christian Borntraeger <borntraeger@de.ibm.com> --- kernel/sched.c | 1 + 1 file changed, 1 insertion(+)
Index: kvm/kernel/sched.c =================================================================== --- kvm.orig/kernel/sched.c +++ kvm/kernel/sched.c @@ -6696,6 +6696,7 @@ migration_call(struct notifier_block *nf rq->idle->sched_class = &idle_sched_class; migrate_dead_tasks(cpu); spin_unlock_irq(&rq->lock); + cpumask_clear_cpu(cpu, nohz.cpu_mask); cpuset_unlock(); migrate_nr_uninterruptible(rq); BUG_ON(rq->nr_running != 0);
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