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From: Tejun Heo on 31 May 2010 15:00
Currently, when a cpu goes down, cpu_active is cleared before
CPU_DOWN_PREPARE starts and cpuset configuration is updated from a
default priority cpu notifier. When a cpu is coming up, it's set
before CPU_ONLINE but cpuset configuration again is updated from the
same cpu notifier.

For cpu notifiers, this presents an inconsistent state. Threads which
a CPU_DOWN_PREPARE notifier expects to be bound to the CPU can be
migrated to other cpus because the cpu is no more inactive.

Fix it by updating cpu_active in the highest priority cpu notifier and
cpuset configuration in the second highest when a cpu is coming up.
Down path is updated similarly. This guarantees that all other cpu
notifiers see consistent cpu_active and cpuset configuration.

This problem is triggered by cmwq. During CPU_DOWN_PREPARE, hotplug
callback creates a kthread and kthread_bind()s it to the target cpu,
and the thread is expected to run on that cpu.

Signed-off-by: Tejun Heo <tj(a)kernel.org>
Cc: Rusty Russell <rusty(a)rustcorp.com.au>
Cc: Peter Zijlstra <a.p.zijlstra(a)chello.nl>
Cc: Ingo Molnar <mingo(a)elte.hu>
Cc: Paul Menage <menage(a)google.com>
---
include/linux/cpu.h | 16 ++++++++++++++++
kernel/cpu.c | 6 ------
kernel/cpuset.c | 46 ++++++++++++++++++++++++++++------------------
kernel/sched.c | 31 ++++++++++++++++++++++++++++++-
4 files changed, 74 insertions(+), 25 deletions(-)

diff --git a/include/linux/cpu.h b/include/linux/cpu.h
index 2d90738..de6b172 100644
--- a/include/linux/cpu.h
+++ b/include/linux/cpu.h
@@ -52,6 +52,22 @@ struct notifier_block;
* CPU notifier priorities.
*/
enum {
+ /*
+ * SCHED_ACTIVE marks a cpu which is coming up active during
+ * CPU_ONLINE and CPU_DOWN_FAILED and must be the first
+ * notifier. CPUSET_ACTIVE adjusts cpuset according to
+ * cpu_active mask right after SCHED_ACTIVE. During
+ * CPU_DOWN_PREPARE, SCHED_INACTIVE and CPUSET_INACTIVE are
+ * ordered in the similar way.
+ *
+ * This ordering guarantees consistent cpu_active mask and
+ * migration behavior to all cpu notifiers.
+ */
+ CPU_PRI_SCHED_ACTIVE = INT_MAX,
+ CPU_PRI_CPUSET_ACTIVE = INT_MAX - 1,
+ CPU_PRI_SCHED_INACTIVE = INT_MIN + 1,
+ CPU_PRI_CPUSET_INACTIVE = INT_MIN,
+
/* migration should happen before other stuff but after perf */
CPU_PRI_PERF = 20,
CPU_PRI_MIGRATION = 10,
diff --git a/kernel/cpu.c b/kernel/cpu.c
index 5457775..69d25a7 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -211,12 +211,9 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
return -EINVAL;

cpu_hotplug_begin();
- set_cpu_active(cpu, false);
err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE | mod,
hcpu, -1, &nr_calls);
if (err == NOTIFY_BAD) {
- set_cpu_active(cpu, true);
-
nr_calls--;
__raw_notifier_call_chain(&cpu_chain, CPU_DOWN_FAILED | mod,
hcpu, nr_calls, NULL);
@@ -228,7 +225,6 @@ static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)

err = __stop_machine(take_cpu_down, &tcd_param, cpumask_of(cpu));
if (err) {
- set_cpu_active(cpu, true);
/* CPU didn't die: tell everyone. Can't complain. */
if (raw_notifier_call_chain(&cpu_chain, CPU_DOWN_FAILED | mod,
hcpu) == NOTIFY_BAD)
@@ -309,8 +305,6 @@ static int __cpuinit _cpu_up(unsigned int cpu, int tasks_frozen)
goto out_notify;
BUG_ON(!cpu_online(cpu));

- set_cpu_active(cpu, true);
-
/* Now call notifier in preparation. */
raw_notifier_call_chain(&cpu_chain, CPU_ONLINE | mod, hcpu);

diff --git a/kernel/cpuset.c b/kernel/cpuset.c
index 9a50c5f..2ad660c 100644
--- a/kernel/cpuset.c
+++ b/kernel/cpuset.c
@@ -2071,31 +2071,17 @@ static void scan_for_empty_cpusets(struct cpuset *root)
* but making no active use of cpusets.
*
* This routine ensures that top_cpuset.cpus_allowed tracks
- * cpu_online_map on each CPU hotplug (cpuhp) event.
+ * cpu_active_mask on each CPU hotplug (cpuhp) event.
*
* Called within get_online_cpus(). Needs to call cgroup_lock()
* before calling generate_sched_domains().
*/
-static int cpuset_track_online_cpus(struct notifier_block *unused_nb,
- unsigned long phase, void *unused_cpu)
+static void cpuset_update_active_cpus(void)
{
struct sched_domain_attr *attr;
cpumask_var_t *doms;
int ndoms;

- switch (phase) {
- case CPU_ONLINE:
- case CPU_ONLINE_FROZEN:
- case CPU_DOWN_PREPARE:
- case CPU_DOWN_PREPARE_FROZEN:
- case CPU_DOWN_FAILED:
- case CPU_DOWN_FAILED_FROZEN:
- break;
-
- default:
- return NOTIFY_DONE;
- }
-
cgroup_lock();
mutex_lock(&callback_mutex);
cpumask_copy(top_cpuset.cpus_allowed, cpu_active_mask);
@@ -2106,8 +2092,31 @@ static int cpuset_track_online_cpus(struct notifier_block *unused_nb,

/* Have scheduler rebuild the domains */
partition_sched_domains(ndoms, doms, attr);
+}

- return NOTIFY_OK;
+static int __cpuinit cpuset_cpu_active(struct notifier_block *nfb,
+ unsigned long action, void *hcpu)
+{
+ switch (action & ~CPU_TASKS_FROZEN) {
+ case CPU_ONLINE:
+ case CPU_DOWN_FAILED:
+ cpuset_update_active_cpus();
+ return NOTIFY_OK;
+ default:
+ return NOTIFY_DONE;
+ }
+}
+
+static int __cpuexit cpuset_cpu_inactive(struct notifier_block *nfb,
+ unsigned long action, void *hcpu)
+{
+ switch (action & ~CPU_TASKS_FROZEN) {
+ case CPU_DOWN_PREPARE:
+ cpuset_update_active_cpus();
+ return NOTIFY_OK;
+ default:
+ return NOTIFY_DONE;
+ }
}

#ifdef CONFIG_MEMORY_HOTPLUG
@@ -2161,7 +2170,8 @@ void __init cpuset_init_smp(void)
cpumask_copy(top_cpuset.cpus_allowed, cpu_active_mask);
top_cpuset.mems_allowed = node_states[N_HIGH_MEMORY];

- hotcpu_notifier(cpuset_track_online_cpus, 0);
+ hotcpu_notifier(cpuset_cpu_active, CPU_PRI_CPUSET_ACTIVE);
+ hotcpu_notifier(cpuset_cpu_inactive, CPU_PRI_CPUSET_INACTIVE);
hotplug_memory_notifier(cpuset_track_online_nodes, 10);

cpuset_wq = create_singlethread_workqueue("cpuset");
diff --git a/kernel/sched.c b/kernel/sched.c
index 7f07048..80a806e 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -5840,17 +5840,46 @@ static struct notifier_block __cpuinitdata migration_notifier = {
.priority = CPU_PRI_MIGRATION,
};

+static int __cpuinit sched_cpu_active(struct notifier_block *nfb,
+ unsigned long action, void *hcpu)
+{
+ switch (action & ~CPU_TASKS_FROZEN) {
+ case CPU_ONLINE:
+ case CPU_DOWN_FAILED:
+ set_cpu_active((long)hcpu, true);
+ return NOTIFY_OK;
+ default:
+ return NOTIFY_DONE;
+ }
+}
+
+static int __cpuexit sched_cpu_inactive(struct notifier_block *nfb,
+ unsigned long action, void *hcpu)
+{
+ switch (action & ~CPU_TASKS_FROZEN) {
+ case CPU_DOWN_PREPARE:
+ set_cpu_active((long)hcpu, false);
+ return NOTIFY_OK;
+ default:
+ return NOTIFY_DONE;
+ }
+}
+
static int __init migration_init(void)
{
void *cpu = (void *)(long)smp_processor_id();
int err;

- /* Start one for the boot CPU: */
+ /* Initialize migration for the boot CPU */
err = migration_call(&migration_notifier, CPU_UP_PREPARE, cpu);
BUG_ON(err == NOTIFY_BAD);
migration_call(&migration_notifier, CPU_ONLINE, cpu);
register_cpu_notifier(&migration_notifier);

+ /* Register cpu active notifiers */
+ cpu_notifier(sched_cpu_active, CPU_PRI_SCHED_ACTIVE);
+ cpu_notifier(sched_cpu_inactive, CPU_PRI_SCHED_INACTIVE);
+
return 0;
}
early_initcall(migration_init);
--
1.6.4.2

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Prev: [PATCHSET sched/core] sched: prepare for cmwq, take#2
Next: input: Tone down logging from S3C touchscreen driver