[PATCH] arm64: Avoid pointless schedule_preempt_irq() invocations

[Thomas Gleixner]

When preempt_count is zero on return from interrupt then
schedule_preempt_irq() is invoked even if TIF_NEED_RESCHED is not set.

That does not make sense because schedule_preempt_irq() has to go through a
full __schedule() for nothing in that case.

Check TIF_NEED_RESCHED and invoke schedule_preempt_irq() only if set.

Signed-off-by: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
---
Found while staring at some RT wrecakge in that area.
---
 arch/arm64/kernel/entry.S |    4 ++++
 1 file changed, 4 insertions(+)

--- a/arch/arm64/kernel/entry.S
+++ b/arch/arm64/kernel/entry.S
@@ -680,6 +680,10 @@ alternative_if ARM64_HAS_IRQ_PRIO_MASKIN
 	orr	x24, x24, x0
 alternative_else_nop_endif
 	cbnz	x24, 1f				// preempt count != 0 || NMI return path
+
+	ldr	x0, [tsk, #TSK_TI_FLAGS]        // get flags
+	tbz	x0, #TIF_NEED_RESCHED, 1f      	// needs rescheduling?
+
 	bl	preempt_schedule_irq		// irq en/disable is done inside
 1:
 #endif