linux-kernel - Re: [PATCH 09/19] arm64: Exclude nohz

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Message-ID: <20250108154905.GA9662@willie-the-truck>
Date: Wed, 8 Jan 2025 15:49:06 +0000
From: Will Deacon <will@...nel.org>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: LKML <linux-kernel@...r.kernel.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	Ard Biesheuvel <ardb@...nel.org>,
	Mark Rutland <mark.rutland@....com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH 09/19] arm64: Exclude nohz_full CPUs from 32bits el0
 support

On Sun, Jan 05, 2025 at 12:18:40AM +0100, Frederic Weisbecker wrote:
> Le Fri, Jan 03, 2025 at 03:20:44PM +0000, Will Deacon a écrit :
> > I think it would be helpful to emit a diagnostic when a 32-bit CPU is
> > ignored because of the housekeeping check. e.g.
> > 
> > 
> > 	if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0)) {
> > 		if (!housekeeping_cpu(cpu, HK_TYPE_TICK))
> > 			pr_info("Treating adaptive-ticks CPU %u as 64-bit only\n", cpu);
> > 		else
> > 			cpu_32bit = true;
> > 	}
> > 
> > 
> > It's a bit of a bummer that this will fire on hardware that isn't
> > asymmetric, but I suppose that's easily resolved by not passing the
> > 'allow_mismatched_32bit_el0' option in that case.
> 
> Right! How is the following updated version?

Thank you, that looks slightly better to me!

Acked-by: Will Deacon <will@...nel.org>

Will

> From: Frederic Weisbecker <frederic@...nel.org>
> Date: Fri, 25 Oct 2024 00:12:59 +0200
> Subject: [PATCH 1/2] arm64: Exclude nohz_full CPUs from 32bits el0 support
> 
> Nohz full CPUs are not a desirable fallback target to run 32bits el0
> applications. If present, prefer a set of housekeeping CPUs that can do
> the job instead. Otherwise just don't support el0 32 bits. Should the
> need arise, appropriate support can be introduced in the future.
> 
> Suggested-by: Will Deacon <will@...nel.org>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> ---
>  Documentation/arch/arm64/asymmetric-32bit.rst |  8 ++++++++
>  arch/arm64/kernel/cpufeature.c                | 10 +++++++++-
>  2 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/Documentation/arch/arm64/asymmetric-32bit.rst b/Documentation/arch/arm64/asymmetric-32bit.rst
> index 64a0b505da7d..1ca2b359a907 100644
> --- a/Documentation/arch/arm64/asymmetric-32bit.rst
> +++ b/Documentation/arch/arm64/asymmetric-32bit.rst
> @@ -153,3 +153,11 @@ asymmetric system, a broken guest at EL1 could still attempt to execute
>  mode will return to host userspace with an ``exit_reason`` of
>  ``KVM_EXIT_FAIL_ENTRY`` and will remain non-runnable until successfully
>  re-initialised by a subsequent ``KVM_ARM_VCPU_INIT`` operation.
> +
> +NOHZ FULL
> +---------
> +
> +To avoid perturbing an adaptive-ticks CPU (specified using
> +``nohz_full=``) when a 32-bit task is forcefully migrated, these CPUs
> +are treated as 64-bit-only when support for asymmetric 32-bit systems
> +is enabled.
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 6ce71f444ed8..3c87659c14db 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -75,6 +75,7 @@
>  #include <linux/cpu.h>
>  #include <linux/kasan.h>
>  #include <linux/percpu.h>
> +#include <linux/sched/isolation.h>
>  
>  #include <asm/cpu.h>
>  #include <asm/cpufeature.h>
> @@ -3742,7 +3743,14 @@ static int enable_mismatched_32bit_el0(unsigned int cpu)
>  	static int lucky_winner = -1;
>  
>  	struct cpuinfo_arm64 *info = &per_cpu(cpu_data, cpu);
> -	bool cpu_32bit = id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0);
> +	bool cpu_32bit = false;
> +
> +	if (id_aa64pfr0_32bit_el0(info->reg_id_aa64pfr0)) {
> +		if (!housekeeping_cpu(cpu, HK_TYPE_TICK))
> +			pr_info("Treating adaptive-ticks CPU %u as 64-bit only\n", cpu);
> +		else
> +			cpu_32bit = true;
> +	}
>  
>  	if (cpu_32bit) {
>  		cpumask_set_cpu(cpu, cpu_32bit_el0_mask);
> -- 
> 2.46.0
>