linux-kernel - Re: [PATCH 10/19] sched,arm64: Handle CPU isolation on last resort fallback rq selection

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Message-ID: <20250103152702.GB3816@willie-the-truck>
Date: Fri, 3 Jan 2025 15:27:03 +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>,
	Peter Zijlstra <peterz@...radead.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Vlastimil Babka <vbabka@...e.cz>,
	"Paul E. McKenney" <paulmck@...nel.org>,
	Neeraj Upadhyay <neeraj.upadhyay@...nel.org>,
	Joel Fernandes <joel@...lfernandes.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>,
	Uladzislau Rezki <urezki@...il.com>, rcu@...r.kernel.org,
	Michal Hocko <mhocko@...e.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 10/19] sched,arm64: Handle CPU isolation on last resort
 fallback rq selection

On Wed, Dec 11, 2024 at 04:40:23PM +0100, Frederic Weisbecker wrote:
> When a kthread or any other task has an affinity mask that is fully
> offline or unallowed, the scheduler reaffines the task to all possible
> CPUs as a last resort.
> 
> This default decision doesn't mix up very well with nohz_full CPUs that
> are part of the possible cpumask but don't want to be disturbed by
> unbound kthreads or even detached pinned user tasks.
> 
> Make the fallback affinity setting aware of nohz_full.
> 
> Suggested-by: Michal Hocko <mhocko@...e.com>
> Signed-off-by: Frederic Weisbecker <frederic@...nel.org>
> ---
>  arch/arm64/include/asm/cpufeature.h  |  1 +
>  arch/arm64/include/asm/mmu_context.h |  2 ++
>  arch/arm64/kernel/cpufeature.c       | 11 +++++++++++
>  include/linux/mmu_context.h          |  1 +
>  kernel/sched/core.c                  |  2 +-
>  5 files changed, 16 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/arm64/include/asm/cpufeature.h b/arch/arm64/include/asm/cpufeature.h
> index 8b4e5a3cd24c..cac5efc836c0 100644
> --- a/arch/arm64/include/asm/cpufeature.h
> +++ b/arch/arm64/include/asm/cpufeature.h
> @@ -671,6 +671,7 @@ static inline bool supports_clearbhb(int scope)
>  }
>  
>  const struct cpumask *system_32bit_el0_cpumask(void);
> +const struct cpumask *fallback_32bit_el0_cpumask(void);
>  DECLARE_STATIC_KEY_FALSE(arm64_mismatched_32bit_el0);
>  
>  static inline bool system_supports_32bit_el0(void)
> diff --git a/arch/arm64/include/asm/mmu_context.h b/arch/arm64/include/asm/mmu_context.h
> index 48b3d9553b67..7883abd6b29a 100644
> --- a/arch/arm64/include/asm/mmu_context.h
> +++ b/arch/arm64/include/asm/mmu_context.h
> @@ -283,6 +283,8 @@ task_cpu_possible_mask(struct task_struct *p)
>  }
>  #define task_cpu_possible_mask	task_cpu_possible_mask
>  
> +const struct cpumask *task_cpu_fallback_mask(struct task_struct *p);
> +
>  void verify_cpu_asid_bits(void);
>  void post_ttbr_update_workaround(void);
>  
> diff --git a/arch/arm64/kernel/cpufeature.c b/arch/arm64/kernel/cpufeature.c
> index 7ce1b8ab417f..2b7aa32bf436 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1642,6 +1642,17 @@ const struct cpumask *system_32bit_el0_cpumask(void)
>  	return cpu_possible_mask;
>  }
>  
> +const struct cpumask *task_cpu_fallback_mask(struct task_struct *p)
> +{
> +	if (!static_branch_unlikely(&arm64_mismatched_32bit_el0))
> +		return housekeeping_cpumask(HK_TYPE_TICK);
> +
> +	if (!is_compat_thread(task_thread_info(p)))
> +		return housekeeping_cpumask(HK_TYPE_TICK);
> +
> +	return system_32bit_el0_cpumask();
> +}

I think this is correct, but damn what we really want to ask for is the
intersection of task_cpu_possible_mask(p) and
housekeeping_cpumask(HK_TYPE_TICK). It's a shame to duplicate the logic
in task_cpu_possible_mask() here because we don't want to allocate a
temporary mask.

Maybe we could have a helper to consolidate things a little?

static inline const struct cpumask *
__task_cpu_possible_mask(struct task_struct *p, const struct cpumask *mask)
{
	if (!static_branch_unlikely(&arm64_mismatched_32bit_el0))
		return mask;

	if (!is_compat_thread(task_thread_info(p)))
		return mask;

	return system_32bit_el0_cpumask();
}

Then we could call that from both task_cpu_possible_mask() and
task_cpu_fallback_mask(), but passing 'cpu_possible_mask' and
housekeeping_cpumask(HK_TYPE_TICK) for the 'mask' argument respectively?

Will