genirq: Change force_irqthreads to a static key

With CONFIG_IRQ_FORCED_THREADING=y, testing the boolean force_irqthreads
could incur a cache line miss in invoke_softirq() and other places.

Replace the test with a static key to avoid the potential cache miss.

[ tglx: Dropped the IDE part, removed the export and updated blk-mq ]
Suggested-by: Eric Dumazet <edumazet@google.com>
Signed-off-by: Tanner Love <tannerlove@google.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210602180338.3324213-1-tannerlove.kernel@gmail.com

block/blk-mq.c

@@ -606,7 +606,7 @@ static inline bool blk_mq_complete_need_ipi(struct request *rq)
 	 * This is probably worse than completing the request on a different
 	 * cache domain.
 	 */
-	if (force_irqthreads)
+	if (force_irqthreads())
 		return false;
 
 	/* same CPU or cache domain?  Complete locally */

include/linux/interrupt.h

@@ -13,6 +13,7 @@
 #include <linux/hrtimer.h>
 #include <linux/kref.h>
 #include <linux/workqueue.h>
+#include <linux/jump_label.h>
 
 #include <linux/atomic.h>
 #include <asm/ptrace.h>
@@ -474,12 +475,13 @@ extern int irq_set_irqchip_state(unsigned int irq, enum irqchip_irq_state which,
 
 #ifdef CONFIG_IRQ_FORCED_THREADING
 # ifdef CONFIG_PREEMPT_RT
-#  define force_irqthreads	(true)
+#  define force_irqthreads()	(true)
 # else
-extern bool force_irqthreads;
+DECLARE_STATIC_KEY_FALSE(force_irqthreads_key);
+#  define force_irqthreads()	(static_branch_unlikely(&force_irqthreads_key))
 # endif
 #else
-#define force_irqthreads	(0)
+#define force_irqthreads()	(false)
 #endif
 
 #ifndef local_softirq_pending

kernel/irq/manage.c

@@ -25,12 +25,11 @@
 #include "internals.h"
 
 #if defined(CONFIG_IRQ_FORCED_THREADING) && !defined(CONFIG_PREEMPT_RT)
-__read_mostly bool force_irqthreads;
-EXPORT_SYMBOL_GPL(force_irqthreads);
+DEFINE_STATIC_KEY_FALSE(force_irqthreads_key);
 
 static int __init setup_forced_irqthreads(char *arg)
 {
-	force_irqthreads = true;
+	static_branch_enable(&force_irqthreads_key);
 	return 0;
 }
 early_param("threadirqs", setup_forced_irqthreads);
@@ -1260,8 +1259,8 @@ static int irq_thread(void *data)
 	irqreturn_t (*handler_fn)(struct irq_desc *desc,
 			struct irqaction *action);
 
-	if (force_irqthreads && test_bit(IRQTF_FORCED_THREAD,
-					&action->thread_flags))
+	if (force_irqthreads() && test_bit(IRQTF_FORCED_THREAD,
+					   &action->thread_flags))
 		handler_fn = irq_forced_thread_fn;
 	else
 		handler_fn = irq_thread_fn;
@@ -1322,7 +1321,7 @@ EXPORT_SYMBOL_GPL(irq_wake_thread);
 
 static int irq_setup_forced_threading(struct irqaction *new)
 {
-	if (!force_irqthreads)
+	if (!force_irqthreads())
 		return 0;
 	if (new->flags & (IRQF_NO_THREAD | IRQF_PERCPU | IRQF_ONESHOT))
 		return 0;

kernel/softirq.c

@@ -422,7 +422,7 @@ static inline void invoke_softirq(void)
 	if (ksoftirqd_running(local_softirq_pending()))
 		return;
 
-	if (!force_irqthreads || !__this_cpu_read(ksoftirqd)) {
+	if (!force_irqthreads() || !__this_cpu_read(ksoftirqd)) {
 #ifdef CONFIG_HAVE_IRQ_EXIT_ON_IRQ_STACK
 		/*
 		 * We can safely execute softirq on the current stack if