[PATCH 01/10] irq_work: Introduce void irq work

[Frederic Weisbecker <fweisbec@gmail.com>]

Being able to trigger an empty IPI appears to be useless in the first
place. Yet it is expected to be very useful for callers who just need
to execute irq_enter() or irq_exit() to a remote target.

More precisely this is going to be useful for the nohz subsystem which
often needs a remote CPU to reschedule its tick when some state changed
and require periodicity for any reason. Similar cases have been handled
with random IPIs until now. But they surely bring unwanted overhead
all along since they are all dedicated for specific tasks.

Triggering an irq work/smp_call_function IPI should be enough to solve
that. If competing and spurious IPIs become a problem, we can still
optimize that later.

Cc: Ingo Molnar <mingo@kernel.org>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Frederic Weisbecker <fweisbec@gmail.com>
---
 include/linux/irq_work.h |  1 +
 kernel/irq_work.c        | 21 +++++++++++++++++++++
 2 files changed, 22 insertions(+)

diff --git a/include/linux/irq_work.h b/include/linux/irq_work.h
index bf9422c..b2ad065 100644
--- a/include/linux/irq_work.h
+++ b/include/linux/irq_work.h
@@ -36,6 +36,7 @@ bool irq_work_queue(struct irq_work *work);
 
 #ifdef CONFIG_SMP
 bool irq_work_queue_on(struct irq_work *work, int cpu);
+void irq_work_void_on(int cpu);
 #endif
 
 void irq_work_run(void);
diff --git a/kernel/irq_work.c b/kernel/irq_work.c
index 4b0a890..36b7fb2 100644
--- a/kernel/irq_work.c
+++ b/kernel/irq_work.c
@@ -81,6 +81,27 @@ bool irq_work_queue_on(struct irq_work *work, int cpu)
 	return true;
 }
 EXPORT_SYMBOL_GPL(irq_work_queue_on);
+
+/**
+ * irq_work_void_on(): Run a void IRQ on the target
+ * @cpu: The cpu to run the IRQ on
+ *
+ * Run a void IRQ for its own sake on the target. It's generally
+ * useful for callers which want to run irq_enter() or irq_exit()
+ * on a remote CPU.
+ */
+void irq_work_void_on(int cpu)
+{
+	/*
+	 * NOTE: we could optimize that and spare some IPIs
+	 * after checking that raised_list isn't empty before raising.
+	 * This can't be done properly without cmpxchg() though so
+	 * it may make things worse after all. But lets leave that
+	 * possibility open in case people report such issue in the
+	 * future.
+	 */
+	arch_send_call_function_single_ipi(cpu);
+}
 #endif
 
 /* Enqueue the irq work @work on the current CPU */
-- 
1.8.3.1