On Sun, Jan 05, 2025 at 12:22:33AM +0100, Frederic Weisbecker wrote:
> Le Fri, Jan 03, 2025 at 03:27:03PM +0000, Will Deacon a écrit :
> > 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?
>
> Good point! How is the following updated version?
That's exactly what I had in mind, thanks!
Acked-by: Will Deacon <will@xxxxxxxxxx>
Will
> ---
> From: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Date: Fri, 27 Sep 2024 00:48:59 +0200
> Subject: [PATCH 2/2] sched,arm64: Handle CPU isolation on last resort fallback
> rq selection
>
> 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@xxxxxxxx>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> ---
> arch/arm64/include/asm/cpufeature.h | 1 +
> arch/arm64/include/asm/mmu_context.h | 14 +++++++++++---
> arch/arm64/kernel/cpufeature.c | 5 +++++
> include/linux/mmu_context.h | 1 +
> kernel/sched/core.c | 2 +-
> 5 files changed, 19 insertions(+), 4 deletions(-)
>
> 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..0dbe3b29049b 100644
> --- a/arch/arm64/include/asm/mmu_context.h
> +++ b/arch/arm64/include/asm/mmu_context.h
> @@ -271,18 +271,26 @@ switch_mm(struct mm_struct *prev, struct mm_struct *next,
> }
>
> static inline const struct cpumask *
> -task_cpu_possible_mask(struct task_struct *p)
> +__task_cpu_possible_mask(struct task_struct *p, const struct cpumask *mask)
> {
> if (!static_branch_unlikely(&arm64_mismatched_32bit_el0))
> - return cpu_possible_mask;
> + return mask;
>
> if (!is_compat_thread(task_thread_info(p)))
> - return cpu_possible_mask;
> + return mask;
>
> return system_32bit_el0_cpumask();
> }
> +
> +static inline const struct cpumask *
> +task_cpu_possible_mask(struct task_struct *p)
> +{
> + return __task_cpu_possible_mask(p, cpu_possible_mask);
> +}
> #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 3c87659c14db..a983e8660987 100644
> --- a/arch/arm64/kernel/cpufeature.c
> +++ b/arch/arm64/kernel/cpufeature.c
> @@ -1642,6 +1642,11 @@ const struct cpumask *system_32bit_el0_cpumask(void)
> return cpu_possible_mask;
> }
>
> +const struct cpumask *task_cpu_fallback_mask(struct task_struct *p)
> +{
> + return __task_cpu_possible_mask(p, housekeeping_cpumask(HK_TYPE_TICK));
> +}
> +
> static int __init parse_32bit_el0_param(char *str)
> {
> allow_mismatched_32bit_el0 = true;
> diff --git a/include/linux/mmu_context.h b/include/linux/mmu_context.h
> index bbaec80c78c5..ac01dc4eb2ce 100644
> --- a/include/linux/mmu_context.h
> +++ b/include/linux/mmu_context.h
> @@ -24,6 +24,7 @@ static inline void leave_mm(void) { }
> #ifndef task_cpu_possible_mask
> # define task_cpu_possible_mask(p) cpu_possible_mask
> # define task_cpu_possible(cpu, p) true
> +# define task_cpu_fallback_mask(p) housekeeping_cpumask(HK_TYPE_TICK)
> #else
> # define task_cpu_possible(cpu, p) cpumask_test_cpu((cpu), task_cpu_possible_mask(p))
> #endif
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 95e40895a519..233b50b0e123 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3534,7 +3534,7 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
> *
> * More yuck to audit.
> */
> - do_set_cpus_allowed(p, task_cpu_possible_mask(p));
> + do_set_cpus_allowed(p, task_cpu_fallback_mask(p));
> state = fail;
> break;
> case fail:
> --
> 2.46.0
>
