linux-kernel - Re: [RFC PATCH 43/86] sched: enable PREEMPT_COUNT, PREEMPTION for all preemption models

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Date:   Wed, 8 Nov 2023 10:58:16 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Ankur Arora <ankur.a.arora@...cle.com>
Cc:     linux-kernel@...r.kernel.org, tglx@...utronix.de,
        torvalds@...ux-foundation.org, paulmck@...nel.org,
        linux-mm@...ck.org, x86@...nel.org, akpm@...ux-foundation.org,
        luto@...nel.org, bp@...en8.de, dave.hansen@...ux.intel.com,
        hpa@...or.com, mingo@...hat.com, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, willy@...radead.org, mgorman@...e.de,
        jon.grimm@....com, bharata@....com, raghavendra.kt@....com,
        boris.ostrovsky@...cle.com, konrad.wilk@...cle.com,
        jgross@...e.com, andrew.cooper3@...rix.com, mingo@...nel.org,
        bristot@...nel.org, mathieu.desnoyers@...icios.com,
        geert@...ux-m68k.org, glaubitz@...sik.fu-berlin.de,
        anton.ivanov@...bridgegreys.com, mattst88@...il.com,
        krypton@...ich-teichert.org, rostedt@...dmis.org,
        David.Laight@...lab.com, richard@....at, mjguzik@...il.com
Subject: Re: [RFC PATCH 43/86] sched: enable PREEMPT_COUNT, PREEMPTION for
 all preemption models

On Tue, Nov 07, 2023 at 01:57:29PM -0800, Ankur Arora wrote:
> The scheduler uses PREEMPT_COUNT and PREEMPTION to drive
> preemption: the first to demarcate non-preemptible sections and
> the second for the actual mechanics of preemption.
> 
> Enable both for voluntary preemption models.
> 
> In addition, define a new scheduler feature FORCE_PREEMPT which
> can now be used to distinguish between voluntary and full
> preemption models at runtime.
> 
> Originally-by: Thomas Gleixner <tglx@...utronix.de>
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> ---
>  init/Makefile           |  2 +-
>  kernel/Kconfig.preempt  | 12 ++++++++----
>  kernel/entry/common.c   |  3 +--
>  kernel/sched/core.c     | 26 +++++++++++---------------
>  kernel/sched/features.h |  6 ++++++
>  5 files changed, 27 insertions(+), 22 deletions(-)
> 
> diff --git a/init/Makefile b/init/Makefile
> index 385fd80fa2ef..99e480f24cf3 100644
> --- a/init/Makefile
> +++ b/init/Makefile
> @@ -24,7 +24,7 @@ mounts-$(CONFIG_BLK_DEV_INITRD)	+= do_mounts_initrd.o
>  #
>  
>  smp-flag-$(CONFIG_SMP)			:= SMP
> -preempt-flag-$(CONFIG_PREEMPT)          := PREEMPT
> +preempt-flag-$(CONFIG_PREEMPTION)       := PREEMPT_DYNAMIC
>  preempt-flag-$(CONFIG_PREEMPT_RT)	:= PREEMPT_RT
>  
>  build-version = $(or $(KBUILD_BUILD_VERSION), $(build-version-auto))
> diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> index aa87b5cd3ecc..074fe5e253b5 100644
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -6,20 +6,23 @@ choice
>  
>  config PREEMPT_NONE
>  	bool "No Forced Preemption (Server)"
> +	select PREEMPTION
>  	help
>  	  This is the traditional Linux preemption model, geared towards
>  	  throughput. It will still provide good latencies most of the
> -	  time, but there are no guarantees and occasional longer delays
> -	  are possible.
> +	  time, but occasional delays are possible.
>  
>  	  Select this option if you are building a kernel for a server or
>  	  scientific/computation system, or if you want to maximize the
>  	  raw processing power of the kernel, irrespective of scheduling
> -	  latencies.
> +	  latencies. Unless your architecture actively disables preemption,
> +	  you can always switch to one of the other preemption models
> +	  at runtime.


> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index 6433e6c77185..f7f2efabb5b5 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -422,8 +422,7 @@ noinstr void irqentry_exit(struct pt_regs *regs, irqentry_state_t state)
>  		}
>  
>  		instrumentation_begin();
> -		if (IS_ENABLED(CONFIG_PREEMPTION))
> -			irqentry_exit_cond_resched();
> +		irqentry_exit_cond_resched();
>  		/* Covers both tracing and lockdep */
>  		trace_hardirqs_on();
>  		instrumentation_end();

I'm totally confused by the PREEMPT_NONE changes here. How does that
make sense?