[tip: irq/core] genirq: Change force_irqthreads to a static key

[tip-bot2 for Tanner Love]

The following commit has been merged into the irq/core branch of tip:

Commit-ID:     91cc470e797828d779cd4c1efbe8519bcb358bae
Gitweb:        https://git.kernel.org/tip/91cc470e797828d779cd4c1efbe8519bcb358bae
Author:        Tanner Love <tannerlove@xxxxxxxxxx>
AuthorDate:    Wed, 02 Jun 2021 14:03:38 -04:00
Committer:     Thomas Gleixner <tglx@xxxxxxxxxxxxx>
CommitterDate: Tue, 10 Aug 2021 22:50:07 +02:00

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@xxxxxxxxxx>
Signed-off-by: Tanner Love <tannerlove@xxxxxxxxxx>
Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Link: https://lore.kernel.org/r/20210602180338.3324213-1-tannerlove.kernel@xxxxxxxxx

---
 block/blk-mq.c            |  2 +-
 include/linux/interrupt.h |  8 +++++---
 kernel/irq/manage.c       | 11 +++++------
 kernel/softirq.c          |  2 +-
 4 files changed, 12 insertions(+), 11 deletions(-)

diff --git a/block/blk-mq.c b/block/blk-mq.c
index 2c4ac51..572d8ab 100644
--- a/block/blk-mq.c
+++ b/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 */
diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 2ed65b0..1f22a30 100644
--- a/include/linux/interrupt.h
+++ b/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
diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 766468a..34a66c4 100644
--- a/kernel/irq/manage.c
+++ b/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;
diff --git a/kernel/softirq.c b/kernel/softirq.c
index f3a0121..322b65d 100644
--- a/kernel/softirq.c
+++ b/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