rcu-tasks: Make Tasks RCU account for userspace execution

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>

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index e7c39c2..1a32036 100644 (file)
--- 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 a4b8189..8dbfb63 100644 (file)
--- 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"));