linux-kernel - Re: [PATCH v2 1/1] timers/migration: add 'tmigr' kernel parameter to optionally disable timer migration

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Message-ID: <3b011d8c-33f7-4189-ac48-d298bf377988@infradead.org>
Date: Tue, 7 Oct 2025 11:38:54 -0700
From: Randy Dunlap <rdunlap@...radead.org>
To: Sehee Jeong <sehee1.jeong@...sung.com>, anna-maria@...utronix.de,
 frederic@...nel.org, tglx@...utronix.de, corbet@....net
Cc: linux-kernel@...r.kernel.org, linux-doc@...r.kernel.org
Subject: Re: [PATCH v2 1/1] timers/migration: add 'tmigr' kernel parameter to
 optionally disable timer migration

Hi,

On 9/10/25 12:42 AM, Sehee Jeong wrote:
> On heterogeneous systems with big.LITTLE architectures, timer migration
> may cause timers from little cores to run on big cores, or vice versa,
> because core type differences are not considered in the current timer
> migration logic. This can be undesirable in systems that require
> strict power management.
> 
> For example, if timers are frequently migrated to a big CPU, it must
> handle callbacks that could have run on a little CPU. This reduces the
> big CPU's idle residency and increases overall energy consumption due to
> higher power draw on the big CPU.
> 
> To avoid this issue, introduce an early boot parameter to optionally
> disable timer migration:
> 
>     tmigr=on|off	(default: on)
> 
> When set to "off", timer migration initialization is skipped entirely.
> 
> Signed-off-by: Sehee Jeong <sehee1.jeong@...sung.com>
> ---
>  .../admin-guide/kernel-parameters.txt         |  4 ++++
>  kernel/time/timer_migration.c                 | 19 +++++++++++++++++++
>  2 files changed, 23 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index ab4c049faba9..0f5d42c046bb 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -6783,6 +6783,10 @@
>  			Force threading of all interrupt handlers except those
>  			marked explicitly IRQF_NO_THREAD.
>  
> +	tmigr		[KNL,EARLY] Enable/disable timer migration

	tmigr=
please.

> +			Valid parameters: on, off
> +			Default: on
> +
>  	topology=	[S390,EARLY]
>  			Format: {off | on}
>  			Specify if the kernel should make use of the cpu
> diff --git a/kernel/time/timer_migration.c b/kernel/time/timer_migration.c
> index 72538baa7a1f..00e3740aec98 100644
> --- a/kernel/time/timer_migration.c
> +++ b/kernel/time/timer_migration.c
> @@ -422,6 +422,8 @@ static unsigned int tmigr_crossnode_level __read_mostly;
>  
>  static DEFINE_PER_CPU(struct tmigr_cpu, tmigr_cpu);
>  
> +static bool tmigr_enabled = true;
> +
>  #define TMIGR_NONE	0xFF
>  #define BIT_CNT		8
>  
> @@ -1790,6 +1792,9 @@ static int __init tmigr_init(void)
>  
>  	BUILD_BUG_ON_NOT_POWER_OF_2(TMIGR_CHILDREN_PER_GROUP);
>  
> +	if (!tmigr_enabled)
> +		return 0;
> +
>  	/* Nothing to do if running on UP */
>  	if (ncpus == 1)
>  		return 0;
> @@ -1854,3 +1859,17 @@ static int __init tmigr_init(void)
>  	return ret;
>  }
>  early_initcall(tmigr_init);
> +
> +static int __init tmigr_setup(char *str)
> +{
> +	if (!str)
> +		return 0;
> +
> +	if (!strcmp(str, "off"))
> +		tmigr_enabled = false;
> +	else if (!strcmp(str, "on"))
> +		tmigr_enabled = true;
> +
> +	return 0;
> +}
> +early_param("tmigr", tmigr_setup);