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Subject[tip:sched/urgent] sched: Ensure set_task_cpu() is never called on blocked tasks
Commit-ID:  e2912009fb7b715728311b0d8fe327a1432b3f79
Gitweb: http://git.kernel.org/tip/e2912009fb7b715728311b0d8fe327a1432b3f79
Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
AuthorDate: Wed, 16 Dec 2009 18:04:36 +0100
Committer: Ingo Molnar <mingo@elte.hu>
CommitDate: Wed, 16 Dec 2009 19:01:56 +0100

sched: Ensure set_task_cpu() is never called on blocked tasks

In order to clean up the set_task_cpu() rq dependencies we need
to ensure it is never called on blocked tasks because such usage
does not pair with consistent rq->lock usage.

This puts the migration burden on ttwu().

Furthermore we need to close a race against changing
->cpus_allowed, since select_task_rq() runs with only preemption
disabled.

For sched_fork() this is safe because the child isn't in the
tasklist yet, for wakeup we fix this by synchronizing
set_cpus_allowed_ptr() against TASK_WAKING, which leaves
sched_exec to be a problem

This also closes a hole in (6ad4c1888 sched: Fix balance vs
hotplug race) where ->select_task_rq() doesn't validate the
result against the sched_domain/root_domain.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Mike Galbraith <efault@gmx.de>
LKML-Reference: <20091216170517.807938893@chello.nl>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
---
kernel/sched.c | 85 +++++++++++++++++++++++++++++++++++++++++++------------
1 files changed, 66 insertions(+), 19 deletions(-)
diff --git a/kernel/sched.c b/kernel/sched.c
index 1672823..33d7965 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -2018,22 +2018,15 @@ static inline void check_class_changed(struct rq *rq, struct task_struct *p,
*/
void kthread_bind(struct task_struct *p, unsigned int cpu)
{
- struct rq *rq = cpu_rq(cpu);
- unsigned long flags;
-
/* Must have done schedule() in kthread() before we set_task_cpu */
if (!wait_task_inactive(p, TASK_UNINTERRUPTIBLE)) {
WARN_ON(1);
return;
}

- raw_spin_lock_irqsave(&rq->lock, flags);
- update_rq_clock(rq);
- set_task_cpu(p, cpu);
p->cpus_allowed = cpumask_of_cpu(cpu);
p->rt.nr_cpus_allowed = 1;
p->flags |= PF_THREAD_BOUND;
- raw_spin_unlock_irqrestore(&rq->lock, flags);
}
EXPORT_SYMBOL(kthread_bind);

@@ -2074,6 +2067,14 @@ void set_task_cpu(struct task_struct *p, unsigned int new_cpu)
struct cfs_rq *old_cfsrq = task_cfs_rq(p),
*new_cfsrq = cpu_cfs_rq(old_cfsrq, new_cpu);

+#ifdef CONFIG_SCHED_DEBUG
+ /*
+ * We should never call set_task_cpu() on a blocked task,
+ * ttwu() will sort out the placement.
+ */
+ WARN_ON(p->state != TASK_RUNNING && p->state != TASK_WAKING);
+#endif
+
trace_sched_migrate_task(p, new_cpu);

if (old_cpu != new_cpu) {
@@ -2107,13 +2108,10 @@ 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
- * it is sufficient to simply update the task's cpu field.
+ * the next wake-up will properly place the task.
*/
- if (!p->se.on_rq && !task_running(rq, p)) {
- update_rq_clock(rq);
- set_task_cpu(p, dest_cpu);
+ if (!p->se.on_rq && !task_running(rq, p))
return 0;
- }

init_completion(&req->done);
req->task = p;
@@ -2319,10 +2317,42 @@ void task_oncpu_function_call(struct task_struct *p,
}

#ifdef CONFIG_SMP
+/*
+ * Called from:
+ *
+ * - fork, @p is stable because it isn't on the tasklist yet
+ *
+ * - exec, @p is unstable XXX
+ *
+ * - wake-up, we serialize ->cpus_allowed against TASK_WAKING so
+ * we should be good.
+ */
static inline
int select_task_rq(struct task_struct *p, int sd_flags, int wake_flags)
{
- return p->sched_class->select_task_rq(p, sd_flags, wake_flags);
+ int cpu = p->sched_class->select_task_rq(p, sd_flags, wake_flags);
+
+ /*
+ * In order not to call set_task_cpu() on a blocking task we need
+ * to rely on ttwu() to place the task on a valid ->cpus_allowed
+ * cpu.
+ *
+ * Since this is common to all placement strategies, this lives here.
+ *
+ * [ this allows ->select_task() to simply return task_cpu(p) and
+ * not worry about this generic constraint ]
+ */
+ if (unlikely(!cpumask_test_cpu(cpu, &p->cpus_allowed) ||
+ !cpu_active(cpu))) {
+
+ cpu = cpumask_any_and(&p->cpus_allowed, cpu_active_mask);
+ /*
+ * XXX: race against hot-plug modifying cpu_active_mask
+ */
+ BUG_ON(cpu >= nr_cpu_ids);
+ }
+
+ return cpu;
}
#endif

@@ -7098,7 +7128,23 @@ int set_cpus_allowed_ptr(struct task_struct *p, const struct cpumask *new_mask)
struct rq *rq;
int ret = 0;

+ /*
+ * Since we rely on wake-ups to migrate sleeping tasks, don't change
+ * the ->cpus_allowed mask from under waking tasks, which would be
+ * possible when we change rq->lock in ttwu(), so synchronize against
+ * TASK_WAKING to avoid that.
+ */
+again:
+ while (p->state == TASK_WAKING)
+ cpu_relax();
+
rq = task_rq_lock(p, &flags);
+
+ if (p->state == TASK_WAKING) {
+ task_rq_unlock(rq, &flags);
+ goto again;
+ }
+
if (!cpumask_intersects(new_mask, cpu_active_mask)) {
ret = -EINVAL;
goto out;
@@ -7154,7 +7200,7 @@ EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr);
static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
{
struct rq *rq_dest, *rq_src;
- int ret = 0, on_rq;
+ int ret = 0;

if (unlikely(!cpu_active(dest_cpu)))
return ret;
@@ -7170,12 +7216,13 @@ static int __migrate_task(struct task_struct *p, int src_cpu, int dest_cpu)
if (!cpumask_test_cpu(dest_cpu, &p->cpus_allowed))
goto fail;

- on_rq = p->se.on_rq;
- if (on_rq)
+ /*
+ * If we're not on a rq, the next wake-up will ensure we're
+ * placed properly.
+ */
+ if (p->se.on_rq) {
deactivate_task(rq_src, p, 0);
-
- set_task_cpu(p, dest_cpu);
- if (on_rq) {
+ set_task_cpu(p, dest_cpu);
activate_task(rq_dest, p, 0);
check_preempt_curr(rq_dest, p, 0);
}

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