linux-kernel - Re: [PATCH v2 05/35] thread_info: selector for TIF_NEED_RESCHED[

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Date: Tue, 28 May 2024 17:55:43 +0200
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,
	rostedt@...dmis.org, mark.rutland@....com, juri.lelli@...hat.com,
	joel@...lfernandes.org, raghavendra.kt@....com,
	sshegde@...ux.ibm.com, boris.ostrovsky@...cle.com,
	konrad.wilk@...cle.com
Subject: Re: [PATCH v2 05/35] thread_info: selector for
 TIF_NEED_RESCHED[_LAZY]

On Mon, May 27, 2024 at 05:34:51PM -0700, Ankur Arora wrote:
> Define tif_resched() to serve as selector for the specific
> need-resched flag: with tif_resched() mapping to TIF_NEED_RESCHED
> or to TIF_NEED_RESCHED_LAZY.
> 
> For !CONFIG_PREEMPT_AUTO, tif_resched() always evaluates
> to TIF_NEED_RESCHED, preserving existing scheduling behaviour.
> 
> Cc: Peter Ziljstra <peterz@...radead.org>
> Originally-by: Thomas Gleixner <tglx@...utronix.de>
> Link: https://lore.kernel.org/lkml/87jzshhexi.ffs@tglx/
> Signed-off-by: Ankur Arora <ankur.a.arora@...cle.com>
> ---
>  include/linux/thread_info.h | 25 +++++++++++++++++++++++++
>  1 file changed, 25 insertions(+)
> 
> diff --git a/include/linux/thread_info.h b/include/linux/thread_info.h
> index 06e13e7acbe2..65e5beedc915 100644
> --- a/include/linux/thread_info.h
> +++ b/include/linux/thread_info.h
> @@ -71,6 +71,31 @@ enum syscall_work_bit {
>  #define _TIF_NEED_RESCHED_LAZY _TIF_NEED_RESCHED
>  #endif
>  
> +typedef enum {
> +	RESCHED_NOW = 0,
> +	RESCHED_LAZY = 1,
> +} resched_t;
> +
> +/*
> + * tif_resched(r) maps to TIF_NEED_RESCHED[_LAZY] with CONFIG_PREEMPT_AUTO.
> + *
> + * For !CONFIG_PREEMPT_AUTO, both tif_resched(RESCHED_NOW) and
> + * tif_resched(RESCHED_LAZY) reduce to the same value (TIF_NEED_RESCHED)
> + * leaving any scheduling behaviour unchanged.
> + */
> +static __always_inline int tif_resched(resched_t rs)
> +{
> +	if (IS_ENABLED(CONFIG_PREEMPT_AUTO))
> +		return (rs == RESCHED_NOW) ? TIF_NEED_RESCHED : TIF_NEED_RESCHED_LAZY;
> +	else
> +		return TIF_NEED_RESCHED;
> +}

Perhaps:

	if (IS_ENABLED(CONFIG_PREEMPT_AUTO) && rs == RESCHED_LAZY)
		return TIF_NEED_RESCHED_LAZY;

	return TIF_NEED_RESCHED;

hmm?

> +
> +static __always_inline int _tif_resched(resched_t rs)
> +{
> +	return 1 << tif_resched(rs);
> +}
> +
>  #ifdef __KERNEL__
>  
>  #ifndef arch_set_restart_data
> -- 
> 2.31.1
>