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* [PATCH] arm64: Avoid pointless schedule_preempt_irq() invocations
@ 2019-07-17  8:43 Thomas Gleixner
  2019-07-17  9:54 ` Valentin Schneider
  0 siblings, 1 reply; 3+ messages in thread
From: Thomas Gleixner @ 2019-07-17  8:43 UTC (permalink / raw)
  To: LAK; +Cc: Peter Zijlstra, Catalin Marinas, Will Deacon, LKML

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@linutronix.de>
---
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

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2019-07-17  8:43 [PATCH] arm64: Avoid pointless schedule_preempt_irq() invocations Thomas Gleixner
2019-07-17  9:54 ` Valentin Schneider
2019-07-17 10:04   ` Thomas Gleixner

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