[PATCH v16 04/13] Add try_get_task_struct_on_cpu() to scheduler for task isolation

[Chris Metcalf]

Task isolation wants to be able to verify that a remote core is
running an isolated task to determine if it should generate a
diagnostic, and also possibly interrupt it.

This API returns a pointer to the task_struct of the task that
was running on the specified core at the moment of the request;
it uses try_get_task_struct() to increment the ref count on the
returned task_struct so that the caller can examine it even if
the actual remote task has already exited by that point.

Signed-off-by: Chris Metcalf <cmetcalf@xxxxxxxxxxxx>
---
 include/linux/sched/task.h |  1 +
 kernel/sched/core.c        | 11 +++++++++++
 2 files changed, 12 insertions(+)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 270ff76d43d9..6785db926857 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -97,6 +97,7 @@ static inline void put_task_struct(struct task_struct *t)
 
 struct task_struct *task_rcu_dereference(struct task_struct **ptask);
 struct task_struct *try_get_task_struct(struct task_struct **ptask);
+struct task_struct *try_get_task_struct_on_cpu(int cpu);
 
 
 #ifdef CONFIG_ARCH_WANTS_DYNAMIC_TASK_STRUCT
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index d17c5da523a0..2728154057ae 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -670,6 +670,17 @@ bool sched_can_stop_tick(struct rq *rq)
 }
 #endif /* CONFIG_NO_HZ_FULL */
 
+/*
+ * Return a pointer to the task_struct for the task that is running on
+ * the specified cpu at the time of the call (note that the task may have
+ * exited by the time the caller inspects the resulting task_struct).
+ * Caller must put_task_struct() with the pointer when finished with it.
+ */
+struct task_struct *try_get_task_struct_on_cpu(int cpu)
+{
+	return try_get_task_struct(&cpu_rq(cpu)->curr);
+}
+
 void sched_avg_update(struct rq *rq)
 {
 	s64 period = sched_avg_period();
-- 
2.1.2