linux-kernel - Re: [PATCH v4 5/7] sched/preempt: add PREEMPT

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Date:   Fri, 18 Mar 2022 09:49:25 +0100
From:   Sven Schnelle <svens@...ux.ibm.com>
To:     Mark Rutland <mark.rutland@....com>
Cc:     linux-arm-kernel@...ts.infradead.org, ardb@...nel.org,
        bp@...en8.de, catalin.marinas@....com, dave.hansen@...ux.intel.com,
        frederic@...nel.org, james.morse@....com, joey.gouly@....com,
        juri.lelli@...hat.com, linux-kernel@...r.kernel.org,
        luto@...nel.org, mingo@...hat.com, peterz@...radead.org,
        tglx@...utronix.de, valentin.schneider@....com, will@...nel.org
Subject: Re: [PATCH v4 5/7] sched/preempt: add PREEMPT_DYNAMIC using static
 keys

Hi Mark,

Mark Rutland <mark.rutland@....com> writes:

> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index 1739ca79613b..b145249ad91a 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -3,6 +3,7 @@
>  #include <linux/context_tracking.h>
>  #include <linux/entry-common.h>
>  #include <linux/highmem.h>
> +#include <linux/jump_label.h>
>  #include <linux/livepatch.h>
>  #include <linux/audit.h>
>  #include <linux/tick.h>
> @@ -392,7 +393,17 @@ void raw_irqentry_exit_cond_resched(void)
>  	}
>  }
>  #ifdef CONFIG_PREEMPT_DYNAMIC
> +#if defined(CONFIG_HAVE_PREEMPT_DYNAMIC_CALL)
>  DEFINE_STATIC_CALL(irqentry_exit_cond_resched, raw_irqentry_exit_cond_resched);
> +#elif defined(CONFIG_HAVE_PREEMPT_DYNAMIC_KEY)
> +DEFINE_STATIC_KEY_TRUE(sk_dynamic_irqentry_exit_cond_resched);
> +void dynamic_irqentry_exit_cond_resched(void)
> +{
> +	if (!static_key_unlikely(&sk_dynamic_irqentry_exit_cond_resched))
> +		return;

I think that should be static_branch_unlikely()?

Also can we make dynamic_irqentry_exit_cond_resched static so that the
compiler will inline it? I played a bit with your changes on s390 and
noticed the the compiler will generate an empty function. Declaring it
static inlines it and saves one unnecessary function call.

Thanks!
Sven