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Subject[RFC PATCH v2 06/10] sched, cpu hotplug: Use stable online cpus in try_to_wake_up() & select_task_rq()
Date
Once stop_machine() is gone from the CPU offline path, we won't be able to
depend on preempt_disable() to prevent CPUs from going offline from under us.

Use the get/put_online_cpus_atomic_light() APIs to prevent changes to the
cpu_online_mask, while invoking from atomic context.

Scheduler functions such as try_to_wake_up() and select_task_rq() (and even
select_fallback_rq()) deal with picking new CPUs to run tasks. They care only
about the cpu_online_mask and not really about the actual CPU tear-down. So
they qualify as "light" atomic readers. So use the _light() variant of the APIs.

Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@linux.vnet.ibm.com>
---

kernel/sched/core.c | 14 ++++++++++++++
1 file changed, 14 insertions(+)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 2d8927f..c9a331e 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1103,6 +1103,10 @@ EXPORT_SYMBOL_GPL(kick_process);
#ifdef CONFIG_SMP
/*
* ->cpus_allowed is protected by both rq->lock and p->pi_lock
+ *
+ * Must be called under get/put_online_cpus_atomic_light() or
+ * equivalent, to avoid CPUs from going offline from underneath
+ * us.
*/
static int select_fallback_rq(int cpu, struct task_struct *p)
{
@@ -1166,6 +1170,9 @@ out:

/*
* The caller (fork, wakeup) owns p->pi_lock, ->cpus_allowed is stable.
+ *
+ * Must be called under get/put_online_cpus_atomic_light(), to prevent
+ * CPUs from going offline from underneath us.
*/
static inline
int select_task_rq(struct task_struct *p, int sd_flags, int wake_flags)
@@ -1406,6 +1413,7 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
int cpu, success = 0;

smp_wmb();
+ get_online_cpus_atomic_light();
raw_spin_lock_irqsave(&p->pi_lock, flags);
if (!(p->state & state))
goto out;
@@ -1446,6 +1454,7 @@ stat:
ttwu_stat(p, cpu, wake_flags);
out:
raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+ put_online_cpus_atomic_light();

return success;
}
@@ -1624,6 +1633,7 @@ void wake_up_new_task(struct task_struct *p)
unsigned long flags;
struct rq *rq;

+ get_online_cpus_atomic_light();
raw_spin_lock_irqsave(&p->pi_lock, flags);
#ifdef CONFIG_SMP
/*
@@ -1644,6 +1654,7 @@ void wake_up_new_task(struct task_struct *p)
p->sched_class->task_woken(rq, p);
#endif
task_rq_unlock(rq, p, &flags);
+ put_online_cpus_atomic_light();
}

#ifdef CONFIG_PREEMPT_NOTIFIERS
@@ -2541,6 +2552,7 @@ void sched_exec(void)
unsigned long flags;
int dest_cpu;

+ get_online_cpus_atomic_light();
raw_spin_lock_irqsave(&p->pi_lock, flags);
dest_cpu = p->sched_class->select_task_rq(p, SD_BALANCE_EXEC, 0);
if (dest_cpu == smp_processor_id())
@@ -2550,11 +2562,13 @@ void sched_exec(void)
struct migration_arg arg = { p, dest_cpu };

raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+ put_online_cpus_atomic_light();
stop_one_cpu(task_cpu(p), migration_cpu_stop, &arg);
return;
}
unlock:
raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+ put_online_cpus_atomic_light();
}

#endif


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