Commit 969c7921 authored by Tejun Heo's avatar Tejun Heo
Browse files

sched: replace migration_thread with cpu_stop



Currently migration_thread is serving three purposes - migration
pusher, context to execute active_load_balance() and forced context
switcher for expedited RCU synchronize_sched.  All three roles are
hardcoded into migration_thread() and determining which job is
scheduled is slightly messy.

This patch kills migration_thread and replaces all three uses with
cpu_stop.  The three different roles of migration_thread() are
splitted into three separate cpu_stop callbacks -
migration_cpu_stop(), active_load_balance_cpu_stop() and
synchronize_sched_expedited_cpu_stop() - and each use case now simply
asks cpu_stop to execute the callback as necessary.

synchronize_sched_expedited() was implemented with private
preallocated resources and custom multi-cpu queueing and waiting
logic, both of which are provided by cpu_stop.
synchronize_sched_expedited_count is made atomic and all other shared
resources along with the mutex are dropped.

synchronize_sched_expedited() also implemented a check to detect cases
where not all the callback got executed on their assigned cpus and
fall back to synchronize_sched().  If called with cpu hotplug blocked,
cpu_stop already guarantees that and the condition cannot happen;
otherwise, stop_machine() would break.  However, this patch preserves
the paranoid check using a cpumask to record on which cpus the stopper
ran so that it can serve as a bisection point if something actually
goes wrong theree.

Because the internal execution state is no longer visible,
rcu_expedited_torture_stats() is removed.

This patch also renames cpu_stop threads to from "stopper/%d" to
"migration/%d".  The names of these threads ultimately don't matter
and there's no reason to make unnecessary userland visible changes.

With this patch applied, stop_machine() and sched now share the same
resources.  stop_machine() is faster without wasting any resources and
sched migration users are much cleaner.
Signed-off-by: default avatarTejun Heo <tj@kernel.org>
Acked-by: default avatarPeter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Dipankar Sarma <dipankar@in.ibm.com>
Cc: Josh Triplett <josh@freedesktop.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Dimitri Sivanich <sivanich@sgi.com>
parent 3fc1f1e2
......@@ -182,16 +182,6 @@ Similarly, sched_expedited RCU provides the following:
sched_expedited-torture: Reader Pipe: 12660320201 95875 0 0 0 0 0 0 0 0 0
sched_expedited-torture: Reader Batch: 12660424885 0 0 0 0 0 0 0 0 0 0
sched_expedited-torture: Free-Block Circulation: 1090795 1090795 1090794 1090793 1090792 1090791 1090790 1090789 1090788 1090787 0
state: -1 / 0:0 3:0 4:0
As before, the first four lines are similar to those for RCU.
The last line shows the task-migration state. The first number is
-1 if synchronize_sched_expedited() is idle, -2 if in the process of
posting wakeups to the migration kthreads, and N when waiting on CPU N.
Each of the colon-separated fields following the "/" is a CPU:state pair.
Valid states are "0" for idle, "1" for waiting for quiescent state,
"2" for passed through quiescent state, and "3" when a race with a
CPU-hotplug event forces use of the synchronize_sched() primitive.
USAGE
......
......@@ -60,8 +60,6 @@ static inline long rcu_batches_completed_bh(void)
return 0;
}
extern int rcu_expedited_torture_stats(char *page);
static inline void rcu_force_quiescent_state(void)
{
}
......
......@@ -35,7 +35,6 @@ struct notifier_block;
extern void rcu_sched_qs(int cpu);
extern void rcu_bh_qs(int cpu);
extern int rcu_needs_cpu(int cpu);
extern int rcu_expedited_torture_stats(char *page);
#ifdef CONFIG_TREE_PREEMPT_RCU
......
......@@ -669,7 +669,7 @@ static struct rcu_torture_ops sched_expedited_ops = {
.sync = synchronize_sched_expedited,
.cb_barrier = NULL,
.fqs = rcu_sched_force_quiescent_state,
.stats = rcu_expedited_torture_stats,
.stats = NULL,
.irq_capable = 1,
.name = "sched_expedited"
};
......
......@@ -55,9 +55,9 @@
#include <linux/cpu.h>
#include <linux/cpuset.h>
#include <linux/percpu.h>
#include <linux/kthread.h>
#include <linux/proc_fs.h>
#include <linux/seq_file.h>
#include <linux/stop_machine.h>
#include <linux/sysctl.h>
#include <linux/syscalls.h>
#include <linux/times.h>
......@@ -539,15 +539,13 @@ struct rq {
int post_schedule;
int active_balance;
int push_cpu;
struct cpu_stop_work active_balance_work;
/* cpu of this runqueue: */
int cpu;
int online;
unsigned long avg_load_per_task;
struct task_struct *migration_thread;
struct list_head migration_queue;
u64 rt_avg;
u64 age_stamp;
u64 idle_stamp;
......@@ -2037,21 +2035,18 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
__set_task_cpu(p, new_cpu);
}
struct migration_req {
struct list_head list;
struct migration_arg {
struct task_struct *task;
int dest_cpu;
struct completion done;
};
static int migration_cpu_stop(void *data);
/*
* The task's runqueue lock must be held.
* Returns true if you have to wait for migration thread.
*/
static int
migrate_task(struct task_struct *p, int dest_cpu, struct migration_req *req)
static bool migrate_task(struct task_struct *p, int dest_cpu)
{
struct rq *rq = task_rq(p);
......@@ -2059,15 +2054,7 @@ migrate_task(struct task_struct *p, int dest_cpu, struct migration_req *req)
* If the task is not on a runqueue (and not running), then
* the next wake-up will properly place the task.
*/
if (!p->se.on_rq && !task_running(rq, p))
return 0;
init_completion(&req->done);
req->task = p;
req->dest_cpu = dest_cpu;
list_add(&req->list, &rq->migration_queue);
return 1;
return p->se.on_rq || task_running(rq, p);
}
/*
......@@ -3110,7 +3097,6 @@ static void update_cpu_load(struct rq *this_rq)
void sched_exec(void)
{
struct task_struct *p = current;
struct migration_req req;
unsigned long flags;
struct rq *rq;
int dest_cpu;
......@@ -3124,17 +3110,11 @@ void sched_exec(void)
* select_task_rq() can race against ->cpus_allowed
*/
if (cpumask_test_cpu(dest_cpu, &p->cpus_allowed) &&
likely(cpu_active(dest_cpu)) &&
migrate_task(p, dest_cpu, &req)) {
/* Need to wait for migration thread (might exit: take ref). */
struct task_struct *mt = rq->migration_thread;
likely(cpu_active(dest_cpu)) && migrate_task(p, dest_cpu)) {
struct migration_arg arg = { p, dest_cpu };
get_task_struct(mt);
task_rq_unlock(rq, &flags);
wake_up_process(mt);
put_task_struct(mt);
wait_for_completion(&req.done);
stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
return;
}
unlock:
......@@ -5290,17 +5270,15 @@ static inline void sched_init_granularity(void)
/*
* This is how migration works:
*
* 1) we queue a struct migration_req structure in the source CPU's
* runqueue and wake up that CPU's migration thread.
* 2) we down() the locked semaphore => thread blocks.
* 3) migration thread wakes up (implicitly it forces the migrated
* thread off the CPU)
* 4) it gets the migration request and checks whether the migrated
* task is still in the wrong runqueue.
* 5) if it's in the wrong runqueue then the migration thread removes
* 1) we invoke migration_cpu_stop() on the target CPU using
* stop_one_cpu().
* 2) stopper starts to run (implicitly forcing the migrated thread
* off the CPU)
* 3) it checks whether the migrated task is still in the wrong runqueue.
* 4) if it's in the wrong runqueue then the migration thread removes
* it and puts it into the right queue.
* 6) migration thread up()s the semaphore.
* 7) we wake up and the migration is done.
* 5) stopper completes and stop_one_cpu() returns and the migration
* is done.
*/
/*
......@@ -5314,9 +5292,9 @@ static inline void sched_init_granularity(void)
*/
int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
{
struct migration_req req;
unsigned long flags;
struct rq *rq;
unsigned int dest_cpu;
int ret = 0;
/*
......@@ -5354,15 +5332,12 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
if (cpumask_test_cpu(task_cpu(p), new_mask))
goto out;
if (migrate_task(p, cpumask_any_and(cpu_active_mask, new_mask), &req)) {
dest_cpu = cpumask_any_and(cpu_active_mask, new_mask);
if (migrate_task(p, dest_cpu)) {
struct migration_arg arg = { p, dest_cpu };
/* Need help from migration thread: drop lock and wait. */
struct task_struct *mt = rq->migration_thread;
get_task_struct(mt);
task_rq_unlock(rq, &flags);
wake_up_process(mt);
put_task_struct(mt);
wait_for_completion(&req.done);
stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
tlb_migrate_finish(p->mm);
return 0;
}
......@@ -5420,70 +5395,22 @@ static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
return ret;
}
#define RCU_MIGRATION_IDLE 0
#define RCU_MIGRATION_NEED_QS 1
#define RCU_MIGRATION_GOT_QS 2
#define RCU_MIGRATION_MUST_SYNC 3
/*
* migration_thread - this is a highprio system thread that performs
* thread migration by bumping thread off CPU then 'pushing' onto
* another runqueue.
* migration_cpu_stop - this will be executed by a highprio stopper thread
* and performs thread migration by bumping thread off CPU then
* 'pushing' onto another runqueue.
*/
static int migration_thread(void *data)
static int migration_cpu_stop(void *data)
{
int badcpu;
int cpu = (long)data;
struct rq *rq;
rq = cpu_rq(cpu);
BUG_ON(rq->migration_thread != current);
set_current_state(TASK_INTERRUPTIBLE);
while (!kthread_should_stop()) {
struct migration_req *req;
struct list_head *head;
raw_spin_lock_irq(&rq->lock);
if (cpu_is_offline(cpu)) {
raw_spin_unlock_irq(&rq->lock);
break;
}
if (rq->active_balance) {
active_load_balance(rq, cpu);
rq->active_balance = 0;
}
head = &rq->migration_queue;
if (list_empty(head)) {
raw_spin_unlock_irq(&rq->lock);
schedule();
set_current_state(TASK_INTERRUPTIBLE);
continue;
}
req = list_entry(head->next, struct migration_req, list);
list_del_init(head->next);
if (req->task != NULL) {
raw_spin_unlock(&rq->lock);
__migrate_task(req->task, cpu, req->dest_cpu);
} else if (likely(cpu == (badcpu = smp_processor_id()))) {
req->dest_cpu = RCU_MIGRATION_GOT_QS;
raw_spin_unlock(&rq->lock);
} else {
req->dest_cpu = RCU_MIGRATION_MUST_SYNC;
raw_spin_unlock(&rq->lock);
WARN_ONCE(1, "migration_thread() on CPU %d, expected %d\n", badcpu, cpu);
}
local_irq_enable();
complete(&req->done);
}
__set_current_state(TASK_RUNNING);
struct migration_arg *arg = data;
/*
* The original target cpu might have gone down and we might
* be on another cpu but it doesn't matter.
*/
local_irq_disable();
__migrate_task(arg->task, raw_smp_processor_id(), arg->dest_cpu);
local_irq_enable();
return 0;
}
......@@ -5850,35 +5777,20 @@ static void set_rq_offline(struct rq *rq)
static int __cpuinit
migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
{
struct task_struct *p;
int cpu = (long)hcpu;
unsigned long flags;
struct rq *rq;
struct rq *rq = cpu_rq(cpu);
switch (action) {
case CPU_UP_PREPARE:
case CPU_UP_PREPARE_FROZEN:
p = kthread_create(migration_thread, hcpu, "migration/%d", cpu);
if (IS_ERR(p))
return NOTIFY_BAD;
kthread_bind(p, cpu);
/* Must be high prio: stop_machine expects to yield to it. */
rq = task_rq_lock(p, &flags);
__setscheduler(rq, p, SCHED_FIFO, MAX_RT_PRIO-1);
task_rq_unlock(rq, &flags);
get_task_struct(p);
cpu_rq(cpu)->migration_thread = p;
rq->calc_load_update = calc_load_update;
break;
case CPU_ONLINE:
case CPU_ONLINE_FROZEN:
/* Strictly unnecessary, as first user will wake it. */
wake_up_process(cpu_rq(cpu)->migration_thread);
/* Update our root-domain */
rq = cpu_rq(cpu);
raw_spin_lock_irqsave(&rq->lock, flags);
if (rq->rd) {
BUG_ON(!cpumask_test_cpu(cpu, rq->rd->span));
......@@ -5889,25 +5801,9 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
break;
#ifdef CONFIG_HOTPLUG_CPU
case CPU_UP_CANCELED:
case CPU_UP_CANCELED_FROZEN:
if (!cpu_rq(cpu)->migration_thread)
break;
/* Unbind it from offline cpu so it can run. Fall thru. */
kthread_bind(cpu_rq(cpu)->migration_thread,
cpumask_any(cpu_online_mask));
kthread_stop(cpu_rq(cpu)->migration_thread);
put_task_struct(cpu_rq(cpu)->migration_thread);
cpu_rq(cpu)->migration_thread = NULL;
break;
case CPU_DEAD:
case CPU_DEAD_FROZEN:
migrate_live_tasks(cpu);
rq = cpu_rq(cpu);
kthread_stop(rq->migration_thread);
put_task_struct(rq->migration_thread);
rq->migration_thread = NULL;
/* Idle task back to normal (off runqueue, low prio) */
raw_spin_lock_irq(&rq->lock);
deactivate_task(rq, rq->idle, 0);
......@@ -5918,29 +5814,11 @@ migration_call(struct notifier_block *nfb, unsigned long action, void *hcpu)
migrate_nr_uninterruptible(rq);
BUG_ON(rq->nr_running != 0);
calc_global_load_remove(rq);
/*
* No need to migrate the tasks: it was best-effort if
* they didn't take sched_hotcpu_mutex. Just wake up
* the requestors.
*/
raw_spin_lock_irq(&rq->lock);
while (!list_empty(&rq->migration_queue)) {
struct migration_req *req;
req = list_entry(rq->migration_queue.next,
struct migration_req, list);
list_del_init(&req->list);
raw_spin_unlock_irq(&rq->lock);
complete(&req->done);
raw_spin_lock_irq(&rq->lock);
}
raw_spin_unlock_irq(&rq->lock);
break;
case CPU_DYING:
case CPU_DYING_FROZEN:
/* Update our root-domain */
rq = cpu_rq(cpu);
raw_spin_lock_irqsave(&rq->lock, flags);
if (rq->rd) {
BUG_ON(!cpumask_test_cpu(cpu, rq->rd->span));
......@@ -7757,10 +7635,8 @@ void __init sched_init(void)
rq->push_cpu = 0;
rq->cpu = i;
rq->online = 0;
rq->migration_thread = NULL;
rq->idle_stamp = 0;
rq->avg_idle = 2*sysctl_sched_migration_cost;
INIT_LIST_HEAD(&rq->migration_queue);
rq_attach_root(rq, &def_root_domain);
#endif
init_rq_hrtick(rq);
......@@ -9054,43 +8930,39 @@ struct cgroup_subsys cpuacct_subsys = {
#ifndef CONFIG_SMP
int rcu_expedited_torture_stats(char *page)
{
return 0;
}
EXPORT_SYMBOL_GPL(rcu_expedited_torture_stats);
void synchronize_sched_expedited(void)
{
/*
* There must be a full memory barrier on each affected CPU
* between the time that try_stop_cpus() is called and the
* time that it returns.
*
* In the current initial implementation of cpu_stop, the
* above condition is already met when the control reaches
* this point and the following smp_mb() is not strictly
* necessary. Do smp_mb() anyway for documentation and
* robustness against future implementation changes.
*/
smp_mb();
}
EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
#else /* #ifndef CONFIG_SMP */
static DEFINE_PER_CPU(struct migration_req, rcu_migration_req);
static DEFINE_MUTEX(rcu_sched_expedited_mutex);
#define RCU_EXPEDITED_STATE_POST -2
#define RCU_EXPEDITED_STATE_IDLE -1
static atomic_t synchronize_sched_expedited_count = ATOMIC_INIT(0);
static int rcu_expedited_state = RCU_EXPEDITED_STATE_IDLE;
int rcu_expedited_torture_stats(char *page)
static int synchronize_sched_expedited_cpu_stop(void *data)
{
int cnt = 0;
int cpu;
static DEFINE_SPINLOCK(done_mask_lock);
struct cpumask *done_mask = data;
cnt += sprintf(&page[cnt], "state: %d /", rcu_expedited_state);
for_each_online_cpu(cpu) {
cnt += sprintf(&page[cnt], " %d:%d",
cpu, per_cpu(rcu_migration_req, cpu).dest_cpu);
if (done_mask) {
spin_lock(&done_mask_lock);
cpumask_set_cpu(smp_processor_id(), done_mask);
spin_unlock(&done_mask_lock);
}
cnt += sprintf(&page[cnt], "\n");
return cnt;
return 0;
}
EXPORT_SYMBOL_GPL(rcu_expedited_torture_stats);
static long synchronize_sched_expedited_count;
/*
* Wait for an rcu-sched grace period to elapse, but use "big hammer"
......@@ -9104,60 +8976,55 @@ static long synchronize_sched_expedited_count;
*/
void synchronize_sched_expedited(void)
{
int cpu;
unsigned long flags;
bool need_full_sync = 0;
struct rq *rq;
struct migration_req *req;
long snap;
int trycount = 0;
cpumask_var_t done_mask_var;
struct cpumask *done_mask = NULL;
int snap, trycount = 0;
/*
* done_mask is used to check that all cpus actually have
* finished running the stopper, which is guaranteed by
* stop_cpus() if it's called with cpu hotplug blocked. Keep
* the paranoia for now but it's best effort if cpumask is off
* stack.
*/
if (zalloc_cpumask_var(&done_mask_var, GFP_ATOMIC))
done_mask = done_mask_var;
smp_mb(); /* ensure prior mod happens before capturing snap. */
snap = ACCESS_ONCE(synchronize_sched_expedited_count) + 1;
snap = atomic_read(&synchronize_sched_expedited_count) + 1;
get_online_cpus();
while (!mutex_trylock(&rcu_sched_expedited_mutex)) {
while (try_stop_cpus(cpu_online_mask,
synchronize_sched_expedited_cpu_stop,
done_mask) == -EAGAIN) {
put_online_cpus();
if (trycount++ < 10)
udelay(trycount * num_online_cpus());
else {
synchronize_sched();
return;
goto free_out;
}
if (ACCESS_ONCE(synchronize_sched_expedited_count) - snap > 0) {
if (atomic_read(&synchronize_sched_expedited_count) - snap > 0) {
smp_mb(); /* ensure test happens before caller kfree */
return;
goto free_out;
}
get_online_cpus();
}
rcu_expedited_state = RCU_EXPEDITED_STATE_POST;
for_each_online_cpu(cpu) {
rq = cpu_rq(cpu);
req = &per_cpu(rcu_migration_req, cpu);
init_completion(&req->done);
req->task = NULL;
req->dest_cpu = RCU_MIGRATION_NEED_QS;
raw_spin_lock_irqsave(&rq->lock, flags);
list_add(&req->list, &rq->migration_queue);
raw_spin_unlock_irqrestore(&rq->lock, flags);
wake_up_process(rq->migration_thread);
}
for_each_online_cpu(cpu) {
rcu_expedited_state = cpu;
req = &per_cpu(rcu_migration_req, cpu);
rq = cpu_rq(cpu);
wait_for_completion(&req->done);
raw_spin_lock_irqsave(&rq->lock, flags);
if (unlikely(req->dest_cpu == RCU_MIGRATION_MUST_SYNC))
need_full_sync = 1;
req->dest_cpu = RCU_MIGRATION_IDLE;
raw_spin_unlock_irqrestore(&rq->lock, flags);
}
rcu_expedited_state = RCU_EXPEDITED_STATE_IDLE;
synchronize_sched_expedited_count++;
mutex_unlock(&rcu_sched_expedited_mutex);
atomic_inc(&synchronize_sched_expedited_count);
if (done_mask)
cpumask_xor(done_mask, done_mask, cpu_online_mask);
put_online_cpus();
if (need_full_sync)
/* paranoia - this can't happen */
if (done_mask && cpumask_weight(done_mask)) {
char buf[80];
cpulist_scnprintf(buf, sizeof(buf), done_mask);
WARN_ONCE(1, "synchronize_sched_expedited: cpu online and done masks disagree on %d cpus: %s\n",
cpumask_weight(done_mask), buf);
synchronize_sched();
}
free_out:
free_cpumask_var(done_mask_var);
}
EXPORT_SYMBOL_GPL(synchronize_sched_expedited);
......
......@@ -2798,6 +2798,8 @@ static int need_active_balance(struct sched_domain *sd, int sd_idle, int idle)
return unlikely(sd->nr_balance_failed > sd->cache_nice_tries+2);
}
static int active_load_balance_cpu_stop(void *data);
/*
* Check this_cpu to ensure it is balanced within domain. Attempt to move
* tasks if there is an imbalance.
......@@ -2887,8 +2889,9 @@ static int load_balance(int this_cpu, struct rq *this_rq,
if (need_active_balance(sd, sd_idle, idle)) {
raw_spin_lock_irqsave(&busiest->lock, flags);
/* don't kick the migration_thread, if the curr
* task on busiest cpu can't be moved to this_cpu
/* don't kick the active_load_balance_cpu_stop,
* if the curr task on busiest cpu can't be
* moved to this_cpu
*/
if (!cpumask_test_cpu(this_cpu,
&busiest->curr->cpus_allowed)) {
......@@ -2898,14 +2901,22 @@ static int load_balance(int this_cpu, struct rq *this_rq,
goto out_one_pinned;
}
/*
* ->active_balance synchronizes accesses to
* ->active_balance_work. Once set, it's cleared
* only after active load balance is finished.
*/
if (!busiest->active_balance) {
busiest->active_balance = 1;
busiest->push_cpu = this_cpu;
active_balance = 1;
}
raw_spin_unlock_irqrestore(&busiest->lock, flags);
if (active_balance)
wake_up_process(busiest->migration_thread);
stop_one_cpu_nowait(cpu_of(busiest),
active_load_balance_cpu_stop, busiest,
&busiest->active_balance_work);
/*
* We've kicked active balancing, reset the failure
......@@ -3012,24 +3023,29 @@ static void idle_balance(int this_cpu, struct rq *this_rq)
}
/*
* active_load_balance is run by migration threads. It pushes running tasks
* off the busiest CPU onto idle CPUs. It requires at least 1 task to be
* running on each physical CPU where possible, and avoids physical /
* logical imbalances.
*
* Called with busiest_rq locked.
* active_load_balance_cpu_stop is run by cpu stopper. It pushes
* running tasks off the busiest CPU onto idle CPUs. It requires at
* least 1 task to be running on each physical CPU where possible, and
* avoids physical / logical imbalances.
*/
static void active_load_balance(struct rq *busiest_rq, int busiest_cpu)
static int active_load_balance_cpu_stop(void *data)
{
struct rq *busiest_rq = data;
int busiest_cpu = cpu_of(busiest_rq);
int target_cpu = busiest_rq->push_cpu;
struct rq *target_rq = cpu_rq(target_cpu);
struct sched_domain *sd;
struct rq *target_rq;
raw_spin_lock_irq(&busiest_rq->lock);
/* make sure the requested cpu hasn't gone down in the meantime */
if (unlikely(busiest_cpu != smp_processor_id() ||
!busiest_rq->active_balance))
goto out_unlock;
/* Is there any task to move? */
if (busiest_rq->nr_running <= 1)
return;
target_rq = cpu_rq(target_cpu);
goto out_unlock;
/*
* This condition is "impossible", if it occurs
......@@ -3058,6 +3074,10 @@ static void active_load_balance(struct rq *busiest_rq, int busiest_cpu)
schedstat_inc(sd, alb_failed);
}
double_unlock_balance(busiest_rq, target_rq);