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SubjectRe: [PATCH 17/18] sched: Retry migration of tasks to CPU on a preferred node

Subject: stop_machine: Introduce stop_two_cpus()
From: Peter Zijlstra <peterz@infradead.org>
Date: Sun Jul 21 12:24:09 CEST 2013

Introduce stop_two_cpus() in order to allow controlled swapping of two
tasks. It repurposes the stop_machine() state machine but only stops
the two cpus which we can do with on-stack structures and avoid
machine wide synchronization issues.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
---
include/linux/stop_machine.h | 1
kernel/stop_machine.c | 243 +++++++++++++++++++++++++------------------
2 files changed, 146 insertions(+), 98 deletions(-)

--- a/include/linux/stop_machine.h
+++ b/include/linux/stop_machine.h
@@ -28,6 +28,7 @@ struct cpu_stop_work {
};

int stop_one_cpu(unsigned int cpu, cpu_stop_fn_t fn, void *arg);
+int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg);
void stop_one_cpu_nowait(unsigned int cpu, cpu_stop_fn_t fn, void *arg,
struct cpu_stop_work *work_buf);
int stop_cpus(const struct cpumask *cpumask, cpu_stop_fn_t fn, void *arg);
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -115,6 +115,137 @@ int stop_one_cpu(unsigned int cpu, cpu_s
return done.executed ? done.ret : -ENOENT;
}

+/* This controls the threads on each CPU. */
+enum multi_stop_state {
+ /* Dummy starting state for thread. */
+ MULTI_STOP_NONE,
+ /* Awaiting everyone to be scheduled. */
+ MULTI_STOP_PREPARE,
+ /* Disable interrupts. */
+ MULTI_STOP_DISABLE_IRQ,
+ /* Run the function */
+ MULTI_STOP_RUN,
+ /* Exit */
+ MULTI_STOP_EXIT,
+};
+
+struct multi_stop_data {
+ int (*fn)(void *);
+ void *data;
+ /* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
+ unsigned int num_threads;
+ const struct cpumask *active_cpus;
+
+ enum multi_stop_state state;
+ atomic_t thread_ack;
+};
+
+static void set_state(struct multi_stop_data *msdata,
+ enum multi_stop_state newstate)
+{
+ /* Reset ack counter. */
+ atomic_set(&msdata->thread_ack, msdata->num_threads);
+ smp_wmb();
+ msdata->state = newstate;
+}
+
+/* Last one to ack a state moves to the next state. */
+static void ack_state(struct multi_stop_data *msdata)
+{
+ if (atomic_dec_and_test(&msdata->thread_ack))
+ set_state(msdata, msdata->state + 1);
+}
+
+/* This is the cpu_stop function which stops the CPU. */
+static int multi_cpu_stop(void *data)
+{
+ struct multi_stop_data *msdata = data;
+ enum multi_stop_state curstate = MULTI_STOP_NONE;
+ int cpu = smp_processor_id(), err = 0;
+ unsigned long flags;
+ bool is_active;
+
+ /*
+ * When called from stop_machine_from_inactive_cpu(), irq might
+ * already be disabled. Save the state and restore it on exit.
+ */
+ local_save_flags(flags);
+
+ if (!msdata->active_cpus)
+ is_active = cpu == cpumask_first(cpu_online_mask);
+ else
+ is_active = cpumask_test_cpu(cpu, msdata->active_cpus);
+
+ /* Simple state machine */
+ do {
+ /* Chill out and ensure we re-read multi_stop_state. */
+ cpu_relax();
+ if (msdata->state != curstate) {
+ curstate = msdata->state;
+ switch (curstate) {
+ case MULTI_STOP_DISABLE_IRQ:
+ local_irq_disable();
+ hard_irq_disable();
+ break;
+ case MULTI_STOP_RUN:
+ if (is_active)
+ err = msdata->fn(msdata->data);
+ break;
+ default:
+ break;
+ }
+ ack_state(msdata);
+ }
+ } while (curstate != MULTI_STOP_EXIT);
+
+ local_irq_restore(flags);
+ return err;
+}
+
+/**
+ * stop_two_cpus - stops two cpus
+ * @cpu1: the cpu to stop
+ * @cpu2: the other cpu to stop
+ * @fn: function to execute
+ * @arg: argument to @fn
+ *
+ * Stops both the current and specified CPU and runs @fn on one of them.
+ *
+ * returns when both are completed.
+ */
+int stop_two_cpus(unsigned int cpu1, unsigned int cpu2, cpu_stop_fn_t fn, void *arg)
+{
+ struct cpu_stop_done done;
+ struct cpu_stop_work work1, work2;
+ struct multi_stop_data msdata = {
+ .fn = fn,
+ .data = arg,
+ .num_threads = 2,
+ .active_cpus = cpumask_of(cpu1),
+ };
+
+ work1 = work2 = (struct cpu_stop_work){
+ .fn = multi_cpu_stop,
+ .arg = &msdata,
+ .done = &done
+ };
+
+ cpu_stop_init_done(&done, 2);
+ set_state(&msdata, MULTI_STOP_PREPARE);
+ /*
+ * Must queue both works with preemption disabled; if cpu1 were
+ * the local cpu we'd never queue the second work, and our fn
+ * might wait forever.
+ */
+ preempt_disable();
+ cpu_stop_queue_work(cpu1, &work1);
+ cpu_stop_queue_work(cpu2, &work2);
+ preempt_enable();
+
+ wait_for_completion(&done.completion);
+ return done.executed ? done.ret : -ENOENT;
+}
+
/**
* stop_one_cpu_nowait - stop a cpu but don't wait for completion
* @cpu: cpu to stop
@@ -359,98 +490,14 @@ early_initcall(cpu_stop_init);

#ifdef CONFIG_STOP_MACHINE

-/* This controls the threads on each CPU. */
-enum stopmachine_state {
- /* 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,
-};
-
-struct stop_machine_data {
- int (*fn)(void *);
- void *data;
- /* Like num_online_cpus(), but hotplug cpu uses us, so we need this. */
- unsigned int num_threads;
- const struct cpumask *active_cpus;
-
- enum stopmachine_state state;
- atomic_t thread_ack;
-};
-
-static void set_state(struct stop_machine_data *smdata,
- enum stopmachine_state newstate)
-{
- /* Reset ack counter. */
- atomic_set(&smdata->thread_ack, smdata->num_threads);
- smp_wmb();
- smdata->state = newstate;
-}
-
-/* Last one to ack a state moves to the next state. */
-static void ack_state(struct stop_machine_data *smdata)
-{
- if (atomic_dec_and_test(&smdata->thread_ack))
- set_state(smdata, smdata->state + 1);
-}
-
-/* This is the cpu_stop function which stops the CPU. */
-static int stop_machine_cpu_stop(void *data)
-{
- struct stop_machine_data *smdata = data;
- enum stopmachine_state curstate = STOPMACHINE_NONE;
- int cpu = smp_processor_id(), err = 0;
- unsigned long flags;
- bool is_active;
-
- /*
- * When called from stop_machine_from_inactive_cpu(), irq might
- * already be disabled. Save the state and restore it on exit.
- */
- local_save_flags(flags);
-
- if (!smdata->active_cpus)
- is_active = cpu == cpumask_first(cpu_online_mask);
- else
- is_active = cpumask_test_cpu(cpu, smdata->active_cpus);
-
- /* Simple state machine */
- do {
- /* Chill out and ensure we re-read stopmachine_state. */
- cpu_relax();
- if (smdata->state != curstate) {
- curstate = smdata->state;
- switch (curstate) {
- case STOPMACHINE_DISABLE_IRQ:
- local_irq_disable();
- hard_irq_disable();
- break;
- case STOPMACHINE_RUN:
- if (is_active)
- err = smdata->fn(smdata->data);
- break;
- default:
- break;
- }
- ack_state(smdata);
- }
- } while (curstate != STOPMACHINE_EXIT);
-
- local_irq_restore(flags);
- return err;
-}
-
int __stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
{
- struct stop_machine_data smdata = { .fn = fn, .data = data,
- .num_threads = num_online_cpus(),
- .active_cpus = cpus };
+ struct multi_stop_data msdata = {
+ .fn = fn,
+ .data = data,
+ .num_threads = num_online_cpus(),
+ .active_cpus = cpus,
+ };

if (!stop_machine_initialized) {
/*
@@ -461,7 +508,7 @@ int __stop_machine(int (*fn)(void *), vo
unsigned long flags;
int ret;

- WARN_ON_ONCE(smdata.num_threads != 1);
+ WARN_ON_ONCE(msdata.num_threads != 1);

local_irq_save(flags);
hard_irq_disable();
@@ -472,8 +519,8 @@ int __stop_machine(int (*fn)(void *), vo
}

/* Set the initial state and stop all online cpus. */
- set_state(&smdata, STOPMACHINE_PREPARE);
- return stop_cpus(cpu_online_mask, stop_machine_cpu_stop, &smdata);
+ set_state(&msdata, MULTI_STOP_PREPARE);
+ return stop_cpus(cpu_online_mask, multi_cpu_stop, &msdata);
}

int stop_machine(int (*fn)(void *), void *data, const struct cpumask *cpus)
@@ -513,25 +560,25 @@ EXPORT_SYMBOL_GPL(stop_machine);
int stop_machine_from_inactive_cpu(int (*fn)(void *), void *data,
const struct cpumask *cpus)
{
- struct stop_machine_data smdata = { .fn = fn, .data = data,
+ struct multi_stop_data msdata = { .fn = fn, .data = data,
.active_cpus = cpus };
struct cpu_stop_done done;
int ret;

/* Local CPU must be inactive and CPU hotplug in progress. */
BUG_ON(cpu_active(raw_smp_processor_id()));
- smdata.num_threads = num_active_cpus() + 1; /* +1 for local */
+ msdata.num_threads = num_active_cpus() + 1; /* +1 for local */

/* No proper task established and can't sleep - busy wait for lock. */
while (!mutex_trylock(&stop_cpus_mutex))
cpu_relax();

/* Schedule work on other CPUs and execute directly for local CPU */
- set_state(&smdata, STOPMACHINE_PREPARE);
+ set_state(&msdata, MULTI_STOP_PREPARE);
cpu_stop_init_done(&done, num_active_cpus());
- queue_stop_cpus_work(cpu_active_mask, stop_machine_cpu_stop, &smdata,
+ queue_stop_cpus_work(cpu_active_mask, multi_cpu_stop, &msdata,
&done);
- ret = stop_machine_cpu_stop(&smdata);
+ ret = multi_cpu_stop(&msdata);

/* Busy wait for completion. */
while (!completion_done(&done.completion))

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