linux-kernel - Re: [PATCH] arm64: Avoid pointless schedule_preempt

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Date:   Wed, 17 Jul 2019 10:54:09 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        LAK <linux-arm-kernel@...ts.infradead.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] arm64: Avoid pointless schedule_preempt_irq() invocations

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@...utronix.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
>