[PATCH] genirq: change force_irqthreads from bool test to static key

[Tanner Love]

From: Tanner Love <tannerlove@xxxxxxxxxx>

With CONFIG_IRQ_FORCED_THREADING=y, testing the bool force_irqthreads
could incur a cache line miss in invoke_softirq().

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

Suggested-by: Eric Dumazet <edumazet@xxxxxxxxxx>
Signed-off-by: Tanner Love <tannerlove@xxxxxxxxxx>
Reviewed-by: Eric Dumazet <edumazet@xxxxxxxxxx>
---
 drivers/ide/ide-iops.c    | 7 +++----
 include/linux/interrupt.h | 4 +++-
 kernel/irq/manage.c       | 6 +++---
 3 files changed, 9 insertions(+), 8 deletions(-)

diff --git a/drivers/ide/ide-iops.c b/drivers/ide/ide-iops.c
index f2be127ee96e..f86cdb8451e6 100644
--- a/drivers/ide/ide-iops.c
+++ b/drivers/ide/ide-iops.c
@@ -109,7 +109,6 @@ int __ide_wait_stat(ide_drive_t *drive, u8 good, u8 bad,
 	ide_hwif_t *hwif = drive->hwif;
 	const struct ide_tp_ops *tp_ops = hwif->tp_ops;
 	unsigned long flags;
-	bool irqs_threaded = force_irqthreads;
 	int i;
 	u8 stat;
 
@@ -117,7 +116,7 @@ int __ide_wait_stat(ide_drive_t *drive, u8 good, u8 bad,
 	stat = tp_ops->read_status(hwif);
 
 	if (stat & ATA_BUSY) {
-		if (!irqs_threaded) {
+		if (!force_irqthreads) {
 			local_save_flags(flags);
 			local_irq_enable_in_hardirq();
 		}
@@ -133,13 +132,13 @@ int __ide_wait_stat(ide_drive_t *drive, u8 good, u8 bad,
 				if ((stat & ATA_BUSY) == 0)
 					break;
 
-				if (!irqs_threaded)
+				if (!force_irqthreads)
 					local_irq_restore(flags);
 				*rstat = stat;
 				return -EBUSY;
 			}
 		}
-		if (!irqs_threaded)
+		if (!force_irqthreads)
 			local_irq_restore(flags);
 	}
 	/*
diff --git a/include/linux/interrupt.h b/include/linux/interrupt.h
index 4777850a6dc7..9e676c351f23 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>
@@ -504,7 +505,8 @@ extern int irq_set_irqchip_state(unsigned int irq, enum irqchip_irq_state which,
 # ifdef CONFIG_PREEMPT_RT
 #  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)
diff --git a/kernel/irq/manage.c b/kernel/irq/manage.c
index 4c14356543d9..395945e54929 100644
--- a/kernel/irq/manage.c
+++ b/kernel/irq/manage.c
@@ -25,12 +25,12 @@
 #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);
+EXPORT_SYMBOL_GPL(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);
-- 
2.32.0.rc0.204.g9fa02ecfa5-goog