[PATCH rcu 4/9] rcu-tasks: Make Tasks RCU account for userspace execution

["Paul E. McKenney" <paulmck@kernel.org>]

The main Tasks RCU quiescent state is voluntary context switch.  However,
userspace execution is also a valid quiescent state, and is a valuable one
for userspace applications that spin repeatedly executing light-weight
non-sleeping system calls.  Currently, such an application can delay a
Tasks RCU grace period for many tens of seconds.

This commit therefore enlists the aid of the scheduler-clock interrupt to
provide a Tasks RCU quiescent state when it interrupted a task executing
in userspace.

[ paulmck: Apply feedback from kernel test robot. ]

Cc: Martin KaFai Lau <kafai@fb.com>
Cc: Neil Spring <ntspring@fb.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
 include/linux/rcupdate.h | 1 +
 kernel/rcu/tree.c        | 2 ++
 2 files changed, 3 insertions(+)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index e7c39c200e2b..1a32036c918c 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -196,6 +196,7 @@ void synchronize_rcu_tasks_rude(void);
 void exit_tasks_rcu_start(void);
 void exit_tasks_rcu_finish(void);
 #else /* #ifdef CONFIG_TASKS_RCU_GENERIC */
+#define rcu_tasks_classic_qs(t, preempt) do { } while (0)
 #define rcu_tasks_qs(t, preempt) do { } while (0)
 #define rcu_note_voluntary_context_switch(t) do { } while (0)
 #define call_rcu_tasks call_rcu
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a4b8189455d5..8dbfb63f0391 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2624,6 +2624,8 @@ void rcu_sched_clock_irq(int user)
 	rcu_flavor_sched_clock_irq(user);
 	if (rcu_pending(user))
 		invoke_rcu_core();
+	if (user)
+		rcu_tasks_classic_qs(current, false);
 	lockdep_assert_irqs_disabled();
 
 	trace_rcu_utilization(TPS("End scheduler-tick"));
-- 
2.31.1.189.g2e36527f23