* [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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^ permalink raw reply [flat|nested] 3+ messages in thread
* Re: [PATCH] arm64: Avoid pointless schedule_preempt_irq() invocations
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
0 siblings, 1 reply; 3+ messages in thread
From: Valentin Schneider @ 2019-07-17 9:54 UTC (permalink / raw)
To: Thomas Gleixner, LAK; +Cc: Peter Zijlstra, Catalin Marinas, Will Deacon, LKML
On 17/07/2019 09:43, Thomas Gleixner wrote:
> 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>
Don't we have NEED_RESCHED squashed into preempt count?
396244692232 ("arm64: preempt: Provide our own implementation of asm/preempt.h")
So the existing check should cover that, unless I'm missing something?
> ---
> 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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^ permalink raw reply [flat|nested] 3+ messages in thread* Re: [PATCH] arm64: Avoid pointless schedule_preempt_irq() invocations
2019-07-17 9:54 ` Valentin Schneider
@ 2019-07-17 10:04 ` Thomas Gleixner
0 siblings, 0 replies; 3+ messages in thread
From: Thomas Gleixner @ 2019-07-17 10:04 UTC (permalink / raw)
To: Valentin Schneider
Cc: Peter Zijlstra, Catalin Marinas, Will Deacon, LKML, LAK
On Wed, 17 Jul 2019, Valentin Schneider wrote:
> On 17/07/2019 09:43, Thomas Gleixner wrote:
> > 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>
>
> Don't we have NEED_RESCHED squashed into preempt count?
>
> 396244692232 ("arm64: preempt: Provide our own implementation of asm/preempt.h")
>
> So the existing check should cover that, unless I'm missing something?
Right. Ignore me.
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^ permalink raw reply [flat|nested] 3+ messages in thread
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