linux-kernel - Re: [patch V2 09/28] x86/Kconfig: Select SCHED

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Date:   Thu, 29 Nov 2018 09:44:00 -0500
From:   Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Andy Lutomirski <luto@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jkosina@...e.cz>,
        Tom Lendacky <thomas.lendacky@....com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Casey Schaufler <casey.schaufler@...el.com>,
        Asit Mallick <asit.k.mallick@...el.com>,
        Arjan van de Ven <arjan@...ux.intel.com>,
        Jon Masters <jcm@...hat.com>,
        Waiman Long <longman9394@...il.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Dave Stewart <david.c.stewart@...el.com>,
        Kees Cook <keescook@...omium.org>
Subject: Re: [patch V2 09/28] x86/Kconfig: Select SCHED_SMT if SMP enabled

On Sun, Nov 25, 2018 at 07:33:37PM +0100, Thomas Gleixner wrote:
> CONFIG_SCHED_SMT is enabled by all distros, so there is not a real point to
> have it configurable. The runtime overhead in the core scheduler code is
> minimal because the actual SMT scheduling parts are conditional on a static
> key.
> 
> This allows to expose the scheduler's SMT state static key to the
> speculation control code. Alternatively the scheduler's static key could be
> made always available when CONFIG_SMP is enabled, but that's just adding an
> unused static key to every other architecture for nothing.
> 
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>

Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>


> 
> ---
>  arch/x86/Kconfig |    8 +-------
>  1 file changed, 1 insertion(+), 7 deletions(-)
> 
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1001,13 +1001,7 @@ config NR_CPUS
>  	  to the kernel image.
>  
>  config SCHED_SMT
> -	bool "SMT (Hyperthreading) scheduler support"
> -	depends on SMP
> -	---help---
> -	  SMT scheduler support improves the CPU scheduler's decision making
> -	  when dealing with Intel Pentium 4 chips with HyperThreading at a
> -	  cost of slightly increased overhead in some places. If unsure say
> -	  N here.
> +	def_bool y if SMP
>  
>  config SCHED_MC
>  	def_bool y
> 
>