Concurrent migrate_disable() and set_cpus_allowed_ptr() has
interesting features. We rely on set_cpus_allowed_ptr() to not return
until the task runs inside the provided mask. This expectation is
exported to userspace.
This means that any set_cpus_allowed_ptr() caller must wait until
migrate_enable() allows migrations.
At the same time, we don't want migrate_enable() to schedule, due to
patterns like:
preempt_disable();
migrate_disable();
...
migrate_enable();
preempt_enable();
And:
raw_spin_lock(&B);
spin_unlock(&A);
this means that when migrate_enable() must restore the affinity
mask, it cannot wait for completion thereof. Luck will have it that
that is exactly the case where there is a pending
set_cpus_allowed_ptr(), so let that provide storage for the async stop
machine.
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>
---
include/linux/sched.h | 1
kernel/sched/core.c | 142 ++++++++++++++++++++++++++++++++++++++++++--------
2 files changed, 123 insertions(+), 20 deletions(-)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -713,6 +713,7 @@ struct task_struct {
int nr_cpus_allowed;
const cpumask_t *cpus_ptr;
cpumask_t cpus_mask;
+ void *migration_pending;
#if defined(CONFIG_SMP) && defined(CONFIG_PREEMPT_RT)
int migration_disabled;
#endif
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1734,15 +1734,28 @@ EXPORT_SYMBOL_GPL(migrate_disable);
void migrate_enable(void)
{
- if (--current->migration_disabled)
- return;
-
- barrier();
+ struct task_struct *p = current;
- if (p->cpus_ptr == &p->cpus_mask)
+ if (p->migration_disabled > 1) {
+ p->migration_disabled--;
return;
+ }
- __set_cpus_allowed_ptr(p, &p->cpus_mask, SCA_MIGRATE_ENABLE);
+ /*
+ * Ensure stop_task runs either before or after this, and that
+ * __set_cpus_allowed_ptr(SCA_MIGRATE_ENABLE) doesn't schedule().
+ */
+ preempt_disable();
+ if (p->cpus_ptr != &p->cpus_mask)
+ __set_cpus_allowed_ptr(p, &p->cpus_mask, SCA_MIGRATE_ENABLE);
+ /*
+ * Mustn't clear migration_disabled() until cpus_ptr points back at the
+ * regular cpus_mask, otherwise things that race (eg.
+ * select_fallback_rq) get confused.
+ */
+ barrier();
+ p->migration_disabled = 0;
+ preempt_enable();
}
EXPORT_SYMBOL_GPL(migrate_enable);
@@ -1818,6 +1831,7 @@ static struct rq *move_queued_task(struc
struct migration_arg {
struct task_struct *task;
int dest_cpu;
+ struct completion *done;
};
/*
@@ -1852,6 +1866,7 @@ static int migration_cpu_stop(void *data
struct migration_arg *arg = data;
struct task_struct *p = arg->task;
struct rq *rq = this_rq();
+ bool complete = false;
struct rq_flags rf;
/*
@@ -1874,15 +1889,27 @@ static int migration_cpu_stop(void *data
* we're holding p->pi_lock.
*/
if (task_rq(p) == rq) {
+ if (is_migration_disabled(p))
+ goto out;
+
if (task_on_rq_queued(p))
rq = __migrate_task(rq, &rf, p, arg->dest_cpu);
else
p->wake_cpu = arg->dest_cpu;
+
+ if (arg->done) {
+ p->migration_pending = NULL;
+ complete = true;
+ }
}
+out:
rq_unlock(rq, &rf);
raw_spin_unlock(&p->pi_lock);
-
local_irq_enable();
+
+ if (complete)
+ complete_all(arg->done);
+
return 0;
}
@@ -1957,6 +1984,92 @@ void do_set_cpus_allowed(struct task_str
__do_set_cpus_allowed(p, new_mask, 0);
}
+struct set_affinity_pending {
+ refcount_t refs;
+ struct completion done;
+ struct cpu_stop_work stop_work;
+ struct migration_arg arg;
+};
+
+static int move_task(struct rq *rq, struct rq_flags *rf, struct task_struct *p,
+ int dest_cpu, unsigned int flags)
+{
+ struct set_affinity_pending my_pending, *pending = NULL;
+ struct migration_arg arg = {
+ .task = p,
+ .dest_cpu = dest_cpu,
+ };
+
+ if (!(flags & SCA_MIGRATE_ENABLE)) {
+ /* serialized by p->pi_lock */
+ if (!p->migration_pending) {
+ refcount_set(&my_pending.refs, 1);
+ init_completion(&my_pending.done);
+ p->migration_pending = &my_pending;
+ } else {
+ pending = p->migration_pending;
+ refcount_inc(&pending->refs);
+ }
+ }
+ pending = p->migration_pending;
+ /*
+ * - !MIGRATE_ENABLE:
+ * we'll have installed a pending if there wasn't one already.
+ *
+ * - MIGRATE_ENABLE:
+ * we're here because the current CPU isn't matching anymore,
+ * the only way that can happen is because of a concurrent
+ * set_cpus_allowed_ptr() call, which should then still be
+ * pending completion.
+ *
+ * Either way, we really should have a @pending here.
+ */
+ if (WARN_ON_ONCE(!pending))
+ return -EINVAL;
+
+ arg.done = &pending->done;
+
+ if (flags & SCA_MIGRATE_ENABLE) {
+
+ task_rq_unlock(rq, p, rf);
+ pending->arg = arg;
+ stop_one_cpu_nowait(cpu_of(rq), migration_cpu_stop,
+ &pending->arg, &pending->stop_work);
+
+ return 0;
+ }
+
+ if (task_running(rq, p) || p->state == TASK_WAKING) {
+
+ task_rq_unlock(rq, p, rf);
+ stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
+
+ } else {
+ bool complete = false;
+
+ if (!is_migration_disabled(p)) {
+ if (task_on_rq_queued(p))
+ rq = move_queued_task(rq, rf, p, dest_cpu);
+
+ p->migration_pending = NULL;
+ complete = true;
+ }
+ task_rq_unlock(rq, p, rf);
+
+ if (complete)
+ complete_all(&pending->done);
+ }
+
+ wait_for_completion(&pending->done);
+
+ if (refcount_dec_and_test(&pending->refs))
+ wake_up_var(&pending->refs);
+
+ wait_var_event(&my_pending.refs, !refcount_read(&my_pending.refs));
+
+ return 0;
+}
+
/*
* Change a given task's CPU affinity. Migrate the thread to a
* proper CPU and schedule it away if the CPU it's executing on
@@ -2025,19 +2138,8 @@ static int __set_cpus_allowed_ptr(struct
if (cpumask_test_cpu(task_cpu(p), new_mask))
goto out;
- if (task_running(rq, p) || p->state == TASK_WAKING) {
- struct migration_arg arg = { p, dest_cpu };
- /* Need help from migration thread: drop lock and wait. */
- task_rq_unlock(rq, p, &rf);
- stop_one_cpu(cpu_of(rq), migration_cpu_stop, &arg);
- return 0;
- } else if (task_on_rq_queued(p)) {
- /*
- * OK, since we're going to drop the lock immediately
- * afterwards anyway.
- */
- rq = move_queued_task(rq, &rf, p, dest_cpu);
- }
+ return move_task(rq, &rf, p, dest_cpu, flags);
+
out:
task_rq_unlock(rq, p, &rf);
