[RFC 3/3] softirq: participate in cpuidle polling

[Stefan Hajnoczi <stefanha@redhat.com>]

Normally softirqs are invoked when exiting irqs. When polling in the
cpuidle driver there may be no irq. Therefore pending softirqs go
unnoticed and polling continues without invoking them.

Add a softirq_poll() function to explicitly check for and invoke
softirqs.

Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
---
This commit is not needed for virtio-blk. I added it when I realized
virtio-net's NAPI scheduling might not be detected by the cpuidle busy
wait loop because it is unaware of softirqs. However, even after doing
this virtio-net's NAPI polling doesn't combine with cpuidle haltpoll.

Perhaps this patch is still desirable for cpuidle poll_state in case a
softirq is raised?
---
 include/linux/interrupt.h     |  2 ++
 drivers/cpuidle/poll_source.c |  3 +++
 kernel/softirq.c              | 14 ++++++++++++++
 3 files changed, 19 insertions(+)

diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 4777850a6dc7..9bfdcc466ba8 100644
--- a/include/linux/interrupt.h
+++ b/include/linux/interrupt.h
@@ -573,6 +573,8 @@ struct softirq_action
 asmlinkage void do_softirq(void);
 asmlinkage void __do_softirq(void);
 
+extern void softirq_poll(void);
+
 extern void open_softirq(int nr, void (*action)(struct softirq_action *));
 extern void softirq_init(void);
 extern void __raise_softirq_irqoff(unsigned int nr);
diff --git a/drivers/cpuidle/poll_source.c b/drivers/cpuidle/poll_source.c
index 46100e5a71e4..ed200feb0daa 100644
--- a/drivers/cpuidle/poll_source.c
+++ b/drivers/cpuidle/poll_source.c
@@ -6,6 +6,7 @@
 #include <linux/lockdep.h>
 #include <linux/percpu.h>
 #include <linux/poll_source.h>
+#include <linux/interrupt.h>
 
 /* The per-cpu list of registered poll sources */
 DEFINE_PER_CPU(struct list_head, poll_source_list);
@@ -26,6 +27,8 @@ void poll_source_run_once(void)
 
 	list_for_each_entry(src, this_cpu_ptr(&poll_source_list), node)
 		src->ops->poll(src);
+
+	softirq_poll();
 }
 
 /* Called from idle task with TIF_POLLING_NRFLAG set and irqs enabled */
diff --git a/kernel/softirq.c b/kernel/softirq.c
index 4992853ef53d..f45bf0204218 100644
--- a/kernel/softirq.c
+++ b/kernel/softirq.c
@@ -611,6 +611,20 @@ void irq_enter(void)
 	irq_enter_rcu();
 }
 
+/**
+ * softirq_poll() - invoke pending softirqs
+ *
+ * Normally it is not necessary to explicitly poll for softirqs, but in the
+ * cpuidle driver a polling function may have raised a softirq with no irq exit
+ * to invoke it. Therefore it is necessary to poll for pending softirqs and
+ * invoke them explicitly.
+ */
+void softirq_poll(void)
+{
+	if (!in_interrupt() && local_softirq_pending())
+		invoke_softirq();
+}
+
 static inline void tick_irq_exit(void)
 {
 #ifdef CONFIG_NO_HZ_COMMON
-- 
2.31.1