Mailing List Archive

This patch implements process_freezer based cpu-hotplug
core.
The sailent features are:
o No more (un)lock_cpu_hotplug.
o No more CPU_LOCK_ACQUIRE and CPU_LOCK_RELEASE. Hence no per-subsystem
hotcpu mutexes.
o Calls freeze_process/thaw_processes at the beginning/end of
the hotplug operation.
o Splits CPU_DEAD into two events namely
- CPU_DEAD: which will be handled while the processes are still
frozen.

- CPU_DEAD_KILL_THREADS: To be handled after we thaw_processes.

This split is required because stopping of the per-cpu threads
using kthread_stop cannot be done while the system is in the frozen
state. Hence we need subsystems which have created per-cpu threads
have to stop them once thaw_processes have been called.

o Handles CPU_DEAD and CPU_DEAD_KILL_THREADS for subsystems which
create per-cpu threads.

Points to ponder:
o Can calling CPU_DOWN_PREPARE/CPU_UP_PREPARE in the
frozen context create any dirty dependencies in the future?

o Can the SYSTEM_RUNNING hack in _cpu_up be avoided by some cleaner means.

Signed-off-by : Srivatsa Vaddagiri <vatsa@in.ibm.com>
Signed-off-by : Gautham R Shenoy <ego@in.ibm.com>
--
include/linux/notifier.h | 3 --
kernel/cpu.c | 68 +++++++++++++++++++----------------------------
kernel/sched.c | 7 +++-
kernel/softirq.c | 10 ++++--
kernel/softlockup.c | 3 +-
kernel/workqueue.c | 16 ++++++++++-
6 files changed, 58 insertions(+), 49 deletions(-)

Index: hotplug/kernel/cpu.c
===================================================================
--- hotplug.orig/kernel/cpu.c
+++ hotplug/kernel/cpu.c
@@ -14,6 +14,7 @@
#include <linux/kthread.h>
#include <linux/stop_machine.h>
#include <linux/mutex.h>
+#include <linux/freezer.h>

/* This protects CPUs going up and down... */
static DEFINE_MUTEX(cpu_add_remove_lock);
@@ -28,38 +29,15 @@ static int cpu_hotplug_disabled;

#ifdef CONFIG_HOTPLUG_CPU

-/* Crappy recursive lock-takers in cpufreq! Complain loudly about idiots */
-static struct task_struct *recursive;
-static int recursive_depth;
-
void lock_cpu_hotplug(void)
{
- struct task_struct *tsk = current;
-
- if (tsk == recursive) {
- static int warnings = 10;
- if (warnings) {
- printk(KERN_ERR "Lukewarm IQ detected in hotplug locking\n");
- WARN_ON(1);
- warnings--;
- }
- recursive_depth++;
- return;
- }
- mutex_lock(&cpu_bitmask_lock);
- recursive = tsk;
+ return;
}
EXPORT_SYMBOL_GPL(lock_cpu_hotplug);

void unlock_cpu_hotplug(void)
{
- WARN_ON(recursive != current);
- if (recursive_depth) {
- recursive_depth--;
- return;
- }
- recursive = NULL;
- mutex_unlock(&cpu_bitmask_lock);
+ return;
}
EXPORT_SYMBOL_GPL(unlock_cpu_hotplug);

@@ -133,7 +111,11 @@ static int _cpu_down(unsigned int cpu)
if (!cpu_online(cpu))
return -EINVAL;

- raw_notifier_call_chain(&cpu_chain, CPU_LOCK_ACQUIRE, hcpu);
+ if (freeze_processes()) {
+ thaw_processes();
+ return -EBUSY;
+ }
+
err = __raw_notifier_call_chain(&cpu_chain, CPU_DOWN_PREPARE,
hcpu, -1, &nr_calls);
if (err == NOTIFY_BAD) {
@@ -141,8 +123,8 @@ static int _cpu_down(unsigned int cpu)
nr_calls, NULL);
printk("%s: attempt to take down CPU %u failed\n",
__FUNCTION__, cpu);
- err = -EINVAL;
- goto out_release;
+ thaw_processes();
+ return -EINVAL;
}

/* Ensure that we are not runnable on dying cpu */
@@ -151,9 +133,7 @@ static int _cpu_down(unsigned int cpu)
cpu_clear(cpu, tmp);
set_cpus_allowed(current, tmp);

- mutex_lock(&cpu_bitmask_lock);
p = __stop_machine_run(take_cpu_down, NULL, cpu);
- mutex_unlock(&cpu_bitmask_lock);

if (IS_ERR(p) || cpu_online(cpu)) {
/* CPU didn't die: tell everyone. Can't complain. */
@@ -161,9 +141,12 @@ static int _cpu_down(unsigned int cpu)
hcpu) == NOTIFY_BAD)
BUG();

+ set_cpus_allowed(current, old_allowed);
+ thaw_processes();
+
if (IS_ERR(p)) {
err = PTR_ERR(p);
- goto out_allowed;
+ return err;
}
goto out_thread;
}
@@ -185,13 +168,12 @@ static int _cpu_down(unsigned int cpu)

check_for_tasks(cpu);

+ thaw_processes();
+
+ if (raw_notifier_call_chain(&cpu_chain, CPU_DEAD_KILL_THREADS, hcpu) == NOTIFY_BAD)
+ BUG();
out_thread:
err = kthread_stop(p);
-out_allowed:
- set_cpus_allowed(current, old_allowed);
-out_release:
- raw_notifier_call_chain(&cpu_chain, CPU_LOCK_RELEASE,
- (void *)(long)cpu);
return err;
}

@@ -219,7 +201,12 @@ static int __cpuinit _cpu_up(unsigned in
if (cpu_online(cpu) || !cpu_present(cpu))
return -EINVAL;

- raw_notifier_call_chain(&cpu_chain, CPU_LOCK_ACQUIRE, hcpu);
+ if (system_state == SYSTEM_RUNNING && freeze_processes()) {
+ if (system_state == SYSTEM_RUNNING)
+ thaw_processes();
+ return -EBUSY;
+ }
+
ret = __raw_notifier_call_chain(&cpu_chain, CPU_UP_PREPARE, hcpu,
-1, &nr_calls);
if (ret == NOTIFY_BAD) {
@@ -229,10 +216,9 @@ static int __cpuinit _cpu_up(unsigned in
goto out_notify;
}

+
/* Arch-specific enabling code. */
- mutex_lock(&cpu_bitmask_lock);
ret = __cpu_up(cpu);
- mutex_unlock(&cpu_bitmask_lock);
if (ret != 0)
goto out_notify;
BUG_ON(!cpu_online(cpu));
@@ -241,10 +227,12 @@ static int __cpuinit _cpu_up(unsigned in
raw_notifier_call_chain(&cpu_chain, CPU_ONLINE, hcpu);

out_notify:
+ if (system_state == SYSTEM_RUNNING)
+ thaw_processes();
+
if (ret != 0)
__raw_notifier_call_chain(&cpu_chain,
CPU_UP_CANCELED, hcpu, nr_calls, NULL);
- raw_notifier_call_chain(&cpu_chain, CPU_LOCK_RELEASE, hcpu);

return ret;
}
Index: hotplug/kernel/softirq.c
===================================================================
--- hotplug.orig/kernel/softirq.c
+++ hotplug/kernel/softirq.c
@@ -18,6 +18,7 @@
#include <linux/rcupdate.h>
#include <linux/smp.h>
#include <linux/tick.h>
+#include <linux/freezer.h>

#include <asm/irq.h>
/*
@@ -489,7 +490,6 @@ void __init softirq_init(void)
static int ksoftirqd(void * __bind_cpu)
{
set_user_nice(current, 19);
- current->flags |= PF_NOFREEZE;

set_current_state(TASK_INTERRUPTIBLE);

@@ -515,6 +515,9 @@ static int ksoftirqd(void * __bind_cpu)
preempt_disable();
}
preempt_enable();
+ try_to_freeze();
+ if (cpu_is_offline((long)__bind_cpu))
+ goto wait_to_die;
set_current_state(TASK_INTERRUPTIBLE);
}
__set_current_state(TASK_RUNNING);
@@ -612,11 +615,12 @@ static int __cpuinit cpu_callback(struct
kthread_bind(per_cpu(ksoftirqd, hotcpu),
any_online_cpu(cpu_online_map));
case CPU_DEAD:
+ takeover_tasklets(hotcpu);
+ break;
+ case CPU_DEAD_KILL_THREADS:
p = per_cpu(ksoftirqd, hotcpu);
per_cpu(ksoftirqd, hotcpu) = NULL;
kthread_stop(p);
- takeover_tasklets(hotcpu);
- break;
#endif /* CONFIG_HOTPLUG_CPU */
}
return NOTIFY_OK;
Index: hotplug/kernel/sched.c
===================================================================
--- hotplug.orig/kernel/sched.c
+++ hotplug/kernel/sched.c
@@ -5519,8 +5519,6 @@ migration_call(struct notifier_block *nf
case CPU_DEAD:
migrate_live_tasks(cpu);
rq = cpu_rq(cpu);
- kthread_stop(rq->migration_thread);
- rq->migration_thread = NULL;
/* Idle task back to normal (off runqueue, low prio) */
rq = task_rq_lock(rq->idle, &flags);
deactivate_task(rq->idle, rq);
@@ -5545,6 +5543,11 @@ migration_call(struct notifier_block *nf
}
spin_unlock_irq(&rq->lock);
break;
+ case CPU_DEAD_KILL_THREADS:
+ rq = cpu_rq(cpu);
+ kthread_stop(rq->migration_thread);
+ rq->migration_thread = NULL;
+ break;
#endif
case CPU_LOCK_RELEASE:
mutex_unlock(&sched_hotcpu_mutex);
Index: hotplug/kernel/softlockup.c
===================================================================
--- hotplug.orig/kernel/softlockup.c
+++ hotplug/kernel/softlockup.c
@@ -13,6 +13,7 @@
#include <linux/kthread.h>
#include <linux/notifier.h>
#include <linux/module.h>
+#include <linux/freezer.h>

static DEFINE_SPINLOCK(print_lock);

@@ -132,7 +133,7 @@ cpu_callback(struct notifier_block *nfb,
/* Unbind so it can run. Fall thru. */
kthread_bind(per_cpu(watchdog_task, hotcpu),
any_online_cpu(cpu_online_map));
- case CPU_DEAD:
+ case CPU_DEAD_KILL_THREADS:
p = per_cpu(watchdog_task, hotcpu);
per_cpu(watchdog_task, hotcpu) = NULL;
kthread_stop(p);
Index: hotplug/kernel/workqueue.c
===================================================================
--- hotplug.orig/kernel/workqueue.c
+++ hotplug/kernel/workqueue.c
@@ -368,6 +368,7 @@ static int worker_thread(void *__cwq)
DEFINE_WAIT(wait);
struct k_sigaction sa;
sigset_t blocked;
+ int bind_cpu = smp_processor_id();

if (!cwq->wq->freezeable)
current->flags |= PF_NOFREEZE;
@@ -392,8 +393,11 @@ static int worker_thread(void *__cwq)
do_sigaction(SIGCHLD, &sa, (struct k_sigaction *)0);

for (;;) {
- if (cwq->wq->freezeable)
+ if (cwq->wq->freezeable) {
try_to_freeze();
+ if (cpu_is_offline(bind_cpu))
+ goto wait_to_die;
+ }

prepare_to_wait(&cwq->more_work, &wait, TASK_INTERRUPTIBLE);
if (!cwq->should_stop && list_empty(&cwq->worklist))
@@ -407,6 +411,16 @@ static int worker_thread(void *__cwq)
}

return 0;
+
+wait_to_die:
+ /* Wait for kthread_stop */
+ set_current_state(TASK_INTERRUPTIBLE);
+ while (!kthread_should_stop()) {
+ schedule();
+ set_current_state(TASK_INTERRUPTIBLE);
+ }
+ __set_current_state(TASK_RUNNING);
+ return 0;
}

struct wq_barrier {
Index: hotplug/include/linux/notifier.h
===================================================================
--- hotplug.orig/include/linux/notifier.h
+++ hotplug/include/linux/notifier.h
@@ -194,8 +194,7 @@ extern int __srcu_notifier_call_chain(st
#define CPU_DOWN_PREPARE 0x0005 /* CPU (unsigned)v going down */
#define CPU_DOWN_FAILED 0x0006 /* CPU (unsigned)v NOT going down */
#define CPU_DEAD 0x0007 /* CPU (unsigned)v dead */
-#define CPU_LOCK_ACQUIRE 0x0008 /* Acquire all hotcpu locks */
-#define CPU_LOCK_RELEASE 0x0009 /* Release all hotcpu locks */
+#define CPU_DEAD_KILL_THREADS 0x0008 /* Kill per-cpu threads after dead */

#endif /* __KERNEL__ */
#endif /* _LINUX_NOTIFIER_H */
--
Gautham R Shenoy
Linux Technology Center
IBM India.
"Freedom comes with a price tag of responsibility, which is still a bargain,
because Freedom is priceless!"
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