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Date:   Tue, 16 Feb 2021 11:38:12 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Frederic Weisbecker <frederic@kernel.org>
Cc:     linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org,
        Mike Galbraith <efault@gmx.de>, x86@kernel.org
Subject: Re: [tip: sched/core] sched,x86: Allow !PREEMPT_DYNAMIC
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 <20210210141838.GA53130@lothringen>
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On Wed, Feb 10, 2021 at 03:18:38PM +0100, Frederic Weisbecker wrote:
> Also should we add something like this?

I suppose we can do that, but I'd rather have actual numbers to go with
it, I don't think the trampolines are really that terrible.

> From: Frederic Weisbecker <frederic@kernel.org>
> Date: Wed, 10 Feb 2021 15:11:39 +0100
> Subject: [PATCH] preempt/dynamic: Make PREEMPT_DYNAMIC optional
> 
> In order not to make the small trampoline overhead mandatory for archs
> that support HAVE_STATIC_CALL but not HAVE_STATIC_CALL_INLINE, make
> PREEMPT_DYNAMIC optional.
> 
> Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
> ---
>  kernel/Kconfig.preempt | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> index 416017301660..1fe759677907 100644
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -40,7 +40,6 @@ config PREEMPT
>  	depends on !ARCH_NO_PREEMPT
>  	select PREEMPTION
>  	select UNINLINE_SPIN_UNLOCK if !ARCH_INLINE_SPIN_UNLOCK
> -	select PREEMPT_DYNAMIC if HAVE_PREEMPT_DYNAMIC
>  	help
>  	  This option reduces the latency of the kernel by making
>  	  all kernel code (that is not executing in a critical section)
> @@ -83,11 +82,13 @@ config PREEMPTION
>         select PREEMPT_COUNT
>  
>  config PREEMPT_DYNAMIC
> -	bool
> +	bool "Override preemption flavour at boot time"
> +	depends on HAVE_PREEMPT_DYNAMIC && PREEMPT
> +	default HAVE_STATIC_CALL_INLINE
>  	help
>  	  This option allows to define the preemption model on the kernel
> -	  command line parameter and thus override the default preemption
> -	  model defined during compile time.
> +	  command line parameter "preempt=" and thus override the default
> +	  preemption model defined during compile time.
>  
>  	  The feature is primarily interesting for Linux distributions which
>  	  provide a pre-built kernel binary to reduce the number of kernel
> @@ -99,3 +100,5 @@ config PREEMPT_DYNAMIC
>  
>  	  Interesting if you want the same pre-built kernel should be used for
>  	  both Server and Desktop workloads.
> +
> +	  Say Y if you have CONFIG_HAVE_STATIC_CALL_INLINE.
> -- 
> 2.25.1
>