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Subject[PATCH 2/3] stop_machine: simplify
Date
stop_machine creates a kthread which creates kernel threads.  We can
create those threads directly and simplify things a little. Some care
must be taken with CPU hotunplug, which has special needs, but that code
seems more robust than it was in the past.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
---
include/linux/stop_machine.h | 12 -
kernel/cpu.c | 13 -
kernel/stop_machine.c | 299 ++++++++++++++++++-------------------------
3 files changed, 135 insertions(+), 189 deletions(-)

diff --git a/include/linux/stop_machine.h b/include/linux/stop_machine.h
--- a/include/linux/stop_machine.h
+++ b/include/linux/stop_machine.h
@@ -17,13 +17,12 @@
* @data: the data ptr for the @fn()
* @cpu: if @cpu == n, run @fn() on cpu n
* if @cpu == NR_CPUS, run @fn() on any cpu
- * if @cpu == ALL_CPUS, run @fn() first on the calling cpu, and then
- * concurrently on all the other cpus
+ * if @cpu == ALL_CPUS, run @fn() on every online CPU.
*
- * Description: This causes a thread to be scheduled on every other cpu,
- * each of which disables interrupts, and finally interrupts are disabled
- * on the current CPU. The result is that noone is holding a spinlock
- * or inside any other preempt-disabled region when @fn() runs.
+ * Description: This causes a thread to be scheduled on every cpu,
+ * each of which disables interrupts. The result is that noone is
+ * holding a spinlock or inside any other preempt-disabled region when
+ * @fn() runs.
*
* This can be thought of as a very heavy write lock, equivalent to
* grabbing every spinlock in the kernel. */
@@ -35,13 +34,10 @@ int stop_machine_run(int (*fn)(void *),
* @data: the data ptr for the @fn
* @cpu: the cpu to run @fn on (or any, if @cpu == NR_CPUS.
*
- * Description: This is a special version of the above, which returns the
- * thread which has run @fn(): kthread_stop will return the return value
- * of @fn(). Used by hotplug cpu.
+ * Description: This is a special version of the above, which assumes cpus
+ * won't come or go while it's being called. Used by hotplug cpu.
*/
-struct task_struct *__stop_machine_run(int (*fn)(void *), void *data,
- unsigned int cpu);
-
+int __stop_machine_run(int (*fn)(void *), void *data, unsigned int cpu);
#else

static inline int stop_machine_run(int (*fn)(void *), void *data,
diff --git a/kernel/cpu.c b/kernel/cpu.c
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -192,7 +192,6 @@ static int __ref _cpu_down(unsigned int
static int __ref _cpu_down(unsigned int cpu, int tasks_frozen)
{
int err, nr_calls = 0;
- struct task_struct *p;
cpumask_t old_allowed, tmp;
void *hcpu = (void *)(long)cpu;
unsigned long mod = tasks_frozen ? CPU_TASKS_FROZEN : 0;
@@ -226,19 +225,15 @@ static int __ref _cpu_down(unsigned int
cpu_clear(cpu, tmp);
set_cpus_allowed_ptr(current, &tmp);

- p = __stop_machine_run(take_cpu_down, &tcd_param, cpu);
+ err = __stop_machine_run(take_cpu_down, &tcd_param, cpu);

- if (IS_ERR(p) || cpu_online(cpu)) {
+ if (err || cpu_online(cpu)) {
/* CPU didn't die: tell everyone. Can't complain. */
if (raw_notifier_call_chain(&cpu_chain, CPU_DOWN_FAILED | mod,
hcpu) == NOTIFY_BAD)
BUG();

- if (IS_ERR(p)) {
- err = PTR_ERR(p);
- goto out_allowed;
- }
- goto out_thread;
+ goto out_allowed;
}

/* Wait for it to sleep (leaving idle task). */
@@ -255,8 +250,6 @@ static int __ref _cpu_down(unsigned int

check_for_tasks(cpu);

-out_thread:
- err = kthread_stop(p);
out_allowed:
set_cpus_allowed_ptr(current, &old_allowed);
out_release:
diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -1,4 +1,4 @@
-/* Copyright 2005 Rusty Russell rusty@rustcorp.com.au IBM Corporation.
+/* Copyright 2008, 2005 Rusty Russell rusty@rustcorp.com.au IBM Corporation.
* GPL v2 and any later version.
*/
#include <linux/cpu.h>
@@ -13,219 +13,176 @@
#include <asm/atomic.h>
#include <asm/uaccess.h>

-/* Since we effect priority and affinity (both of which are visible
- * to, and settable by outside processes) we do indirection via a
- * kthread. */
-
-/* Thread to stop each CPU in user context. */
+/* This controls the threads on each CPU. */
enum stopmachine_state {
- STOPMACHINE_WAIT,
+ /* Dummy starting state for thread. */
+ STOPMACHINE_NONE,
+ /* Awaiting everyone to be scheduled. */
STOPMACHINE_PREPARE,
+ /* Disable interrupts. */
STOPMACHINE_DISABLE_IRQ,
+ /* Run the function */
STOPMACHINE_RUN,
+ /* Exit */
STOPMACHINE_EXIT,
};
+static enum stopmachine_state state;

struct stop_machine_data {
int (*fn)(void *);
void *data;
- struct completion done;
- int run_all;
-} smdata;
+ int fnret;
+};

-static enum stopmachine_state stopmachine_state;
-static unsigned int stopmachine_num_threads;
-static atomic_t stopmachine_thread_ack;
+/* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
+static unsigned int num_threads;
+static atomic_t thread_ack;
+static struct completion finished;
+static DEFINE_MUTEX(lock);

-static int stopmachine(void *cpu)
+static void set_state(enum stopmachine_state newstate)
{
- int irqs_disabled = 0;
- int prepared = 0;
- int ran = 0;
+ /* Reset ack counter. */
+ atomic_set(&thread_ack, num_threads);
+ smp_wmb();
+ state = newstate;
+}

- set_cpus_allowed_ptr(current, &cpumask_of_cpu((int)(long)cpu));
+/* Last one to ack a state moves to the next state. */
+static void ack_state(void)
+{
+ if (atomic_dec_and_test(&thread_ack)) {
+ /* If we're the last one to ack the EXIT, we're finished. */
+ if (state == STOPMACHINE_EXIT)
+ complete(&finished);
+ else
+ set_state(state + 1);
+ }
+}

- /* Ack: we are alive */
- smp_mb(); /* Theoretically the ack = 0 might not be on this CPU yet. */
- atomic_inc(&stopmachine_thread_ack);
+/* This is the actual thread which stops the CPU. It exits by itself rather
+ * than waiting for kthread_stop(), because it's easier for hotplug CPU. */
+static int stop_cpu(struct stop_machine_data *smdata)
+{
+ enum stopmachine_state curstate = STOPMACHINE_NONE;
+ int uninitialized_var(ret);

/* Simple state machine */
- while (stopmachine_state != STOPMACHINE_EXIT) {
- if (stopmachine_state == STOPMACHINE_DISABLE_IRQ
- && !irqs_disabled) {
- local_irq_disable();
- hard_irq_disable();
- irqs_disabled = 1;
- /* Ack: irqs disabled. */
- smp_mb(); /* Must read state first. */
- atomic_inc(&stopmachine_thread_ack);
- } else if (stopmachine_state == STOPMACHINE_PREPARE
- && !prepared) {
- /* Everyone is in place, hold CPU. */
- preempt_disable();
- prepared = 1;
- smp_mb(); /* Must read state first. */
- atomic_inc(&stopmachine_thread_ack);
- } else if (stopmachine_state == STOPMACHINE_RUN && !ran) {
- smdata.fn(smdata.data);
- ran = 1;
- smp_mb(); /* Must read state first. */
- atomic_inc(&stopmachine_thread_ack);
+ do {
+ /* Chill out and ensure we re-read stopmachine_state. */
+ cpu_relax();
+ if (state != curstate) {
+ curstate = state;
+ switch (curstate) {
+ case STOPMACHINE_DISABLE_IRQ:
+ local_irq_disable();
+ hard_irq_disable();
+ break;
+ case STOPMACHINE_RUN:
+ /* |= allows error detection if functions on
+ * multiple CPUs. */
+ smdata->fnret |= smdata->fn(smdata->data);
+ break;
+ default:
+ break;
+ }
+ ack_state();
}
- /* Yield in first stage: migration threads need to
- * help our sisters onto their CPUs. */
- if (!prepared && !irqs_disabled)
- yield();
- cpu_relax();
- }
+ } while (curstate != STOPMACHINE_EXIT);

- /* Ack: we are exiting. */
- smp_mb(); /* Must read state first. */
- atomic_inc(&stopmachine_thread_ack);
+ local_irq_enable();
+ do_exit(0);
+}

- if (irqs_disabled)
- local_irq_enable();
- if (prepared)
- preempt_enable();
-
+/* Callback for CPUs which aren't supposed to do anything. */
+static int chill(void *unused)
+{
return 0;
}

-/* Change the thread state */
-static void stopmachine_set_state(enum stopmachine_state state)
+int __stop_machine_run(int (*fn)(void *), void *data, unsigned int cpu)
{
- atomic_set(&stopmachine_thread_ack, 0);
- smp_wmb();
- stopmachine_state = state;
- while (atomic_read(&stopmachine_thread_ack) != stopmachine_num_threads)
- cpu_relax();
-}
+ int i, err;
+ struct stop_machine_data active, idle;
+ struct task_struct **threads;

-static int stop_machine(void)
-{
- int i, ret = 0;
+ active.fn = fn;
+ active.data = data;
+ active.fnret = 0;
+ idle.fn = chill;
+ idle.data = NULL;

- atomic_set(&stopmachine_thread_ack, 0);
- stopmachine_num_threads = 0;
- stopmachine_state = STOPMACHINE_WAIT;
+ /* If they don't care which cpu fn runs on, just pick one. */
+ if (cpu == NR_CPUS)
+ cpu = any_online_cpu(cpu_online_map);
+
+ /* This could be too big for stack on large machines. */
+ threads = kcalloc(NR_CPUS, sizeof(threads[0]), GFP_KERNEL);
+ if (!threads)
+ return -ENOMEM;
+
+ /* Set up initial state. */
+ mutex_lock(&lock);
+ init_completion(&finished);
+ num_threads = num_online_cpus();
+ set_state(STOPMACHINE_PREPARE);

for_each_online_cpu(i) {
- if (i == raw_smp_processor_id())
- continue;
- ret = kernel_thread(stopmachine, (void *)(long)i,CLONE_KERNEL);
- if (ret < 0)
- break;
- stopmachine_num_threads++;
+ struct stop_machine_data *smdata;
+ struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
+
+ if (cpu == ALL_CPUS || i == cpu)
+ smdata = &active;
+ else
+ smdata = &idle;
+
+ threads[i] = kthread_create(stop_cpu, smdata, "kstop%u", i);
+ if (IS_ERR(threads[i])) {
+ err = PTR_ERR(threads[i]);
+ threads[i] = NULL;
+ goto kill_threads;
+ }
+
+ /* Place it onto correct cpu. */
+ kthread_bind(threads[i], i);
+
+ /* Make it highest prio. */
+ if (sched_setscheduler_nocheck(threads[i], SCHED_FIFO, &param))
+ BUG();
}

- /* Wait for them all to come to life. */
- while (atomic_read(&stopmachine_thread_ack) != stopmachine_num_threads) {
- yield();
- cpu_relax();
- }
+ /* We've created all the threads. Wake them all: hold this CPU so one
+ * doesn't hit this CPU until we're ready. */
+ cpu = get_cpu();
+ for_each_online_cpu(i)
+ wake_up_process(threads[i]);

- /* If some failed, kill them all. */
- if (ret < 0) {
- stopmachine_set_state(STOPMACHINE_EXIT);
- return ret;
- }
+ /* This will release the thread on our CPU. */
+ put_cpu();
+ wait_for_completion(&finished);
+ mutex_unlock(&lock);

- /* Now they are all started, make them hold the CPUs, ready. */
- preempt_disable();
- stopmachine_set_state(STOPMACHINE_PREPARE);
+ kfree(threads);

- /* Make them disable irqs. */
- local_irq_disable();
- hard_irq_disable();
- stopmachine_set_state(STOPMACHINE_DISABLE_IRQ);
+ return active.fnret;

- return 0;
-}
+kill_threads:
+ for_each_online_cpu(i)
+ if (threads[i])
+ kthread_stop(threads[i]);
+ mutex_unlock(&lock);

-static void restart_machine(void)
-{
- stopmachine_set_state(STOPMACHINE_EXIT);
- local_irq_enable();
- preempt_enable_no_resched();
-}
-
-static void run_other_cpus(void)
-{
- stopmachine_set_state(STOPMACHINE_RUN);
-}
-
-static int do_stop(void *_smdata)
-{
- struct stop_machine_data *smdata = _smdata;
- int ret;
-
- ret = stop_machine();
- if (ret == 0) {
- ret = smdata->fn(smdata->data);
- if (smdata->run_all)
- run_other_cpus();
- restart_machine();
- }
-
- /* We're done: you can kthread_stop us now */
- complete(&smdata->done);
-
- /* 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 ret;
-}
-
-struct task_struct *__stop_machine_run(int (*fn)(void *), void *data,
- unsigned int cpu)
-{
- static DEFINE_MUTEX(stopmachine_mutex);
- struct stop_machine_data smdata;
- struct task_struct *p;
-
- mutex_lock(&stopmachine_mutex);
-
- smdata.fn = fn;
- smdata.data = data;
- smdata.run_all = (cpu == ALL_CPUS) ? 1 : 0;
- init_completion(&smdata.done);
-
- smp_wmb(); /* make sure other cpus see smdata updates */
-
- /* If they don't care which CPU fn runs on, bind to any online one. */
- if (cpu == NR_CPUS || cpu == ALL_CPUS)
- cpu = raw_smp_processor_id();
-
- p = kthread_create(do_stop, &smdata, "kstopmachine");
- if (!IS_ERR(p)) {
- struct sched_param param = { .sched_priority = MAX_RT_PRIO-1 };
-
- /* One high-prio thread per cpu. We'll do this one. */
- sched_setscheduler_nocheck(p, SCHED_FIFO, &param);
- kthread_bind(p, cpu);
- wake_up_process(p);
- wait_for_completion(&smdata.done);
- }
- mutex_unlock(&stopmachine_mutex);
- return p;
+ kfree(threads);
+ return err;
}

int stop_machine_run(int (*fn)(void *), void *data, unsigned int cpu)
{
- struct task_struct *p;
int ret;

/* No CPUs can come up or down during this. */
get_online_cpus();
- p = __stop_machine_run(fn, data, cpu);
- if (!IS_ERR(p))
- ret = kthread_stop(p);
- else
- ret = PTR_ERR(p);
+ ret = __stop_machine_run(fn, data, cpu);
put_online_cpus();

return ret;

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