Re: [PATCH v3 2/4] preempt/dynamic: Introduce preemption model accessors

[Marco Elver <elver@google.com>]

On Fri, 12 Nov 2021 at 19:52, Valentin Schneider
<valentin.schneider@arm.com> wrote:
>
> CONFIG_PREEMPT{_NONE, _VOLUNTARY} designate either:
> o The build-time preemption model when !PREEMPT_DYNAMIC
> o The default boot-time preemption model when PREEMPT_DYNAMIC
>
> IOW, using those on PREEMPT_DYNAMIC kernels is meaningless - the actual
> model could have been set to something else by the "preempt=foo" cmdline
> parameter. Same problem applies to CONFIG_PREEMPTION.
>
> Introduce a set of helpers to determine the actual preemption model used by
> the live kernel.
>
> Suggested-by: Marco Elver <elver@google.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>

Looks sane.

Reviewed-by: Marco Elver <elver@google.com>

> ---
>  include/linux/sched.h | 41 +++++++++++++++++++++++++++++++++++++++++
>  kernel/sched/core.c   | 12 ++++++++++++
>  2 files changed, 53 insertions(+)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 5f8db54226af..e8e884ee6e8b 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2073,6 +2073,47 @@ static inline void cond_resched_rcu(void)
>  #endif
>  }
>
> +#ifdef CONFIG_PREEMPT_DYNAMIC
> +
> +extern bool preempt_model_none(void);
> +extern bool preempt_model_voluntary(void);
> +extern bool preempt_model_full(void);
> +
> +#else
> +
> +static inline bool preempt_model_none(void)
> +{
> +       return IS_ENABLED(CONFIG_PREEMPT_NONE);
> +}
> +static inline bool preempt_model_voluntary(void)
> +{
> +       return IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY);
> +}
> +static inline bool preempt_model_full(void)
> +{
> +       return IS_ENABLED(CONFIG_PREEMPT);
> +}
> +
> +#endif
> +
> +static inline bool preempt_model_rt(void)
> +{
> +       return IS_ENABLED(CONFIG_PREEMPT_RT);
> +}
> +
> +/*
> + * Does the preemption model allow non-cooperative preemption?
> + *
> + * For !CONFIG_PREEMPT_DYNAMIC kernels this is an exact match with
> + * CONFIG_PREEMPTION; for CONFIG_PREEMPT_DYNAMIC this doesn't work as the
> + * kernel is *built* with CONFIG_PREEMPTION=y but may run with e.g. the
> + * PREEMPT_NONE model.
> + */
> +static inline bool preempt_model_preemptible(void)
> +{
> +       return preempt_model_full() || preempt_model_rt();
> +}
> +
>  /*
>   * Does a critical section need to be broken due to another
>   * task waiting?: (technically does not depend on CONFIG_PREEMPTION,
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 97047aa7b6c2..e2502b8643b4 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6638,6 +6638,18 @@ static void __init preempt_dynamic_init(void)
>         }
>  }
>
> +#define PREEMPT_MODEL_ACCESSOR(mode) \
> +       bool preempt_model_##mode(void)                                          \
> +       {                                                                        \
> +               WARN_ON_ONCE(preempt_dynamic_mode == preempt_dynamic_undefined); \
> +               return preempt_dynamic_mode == preempt_dynamic_##mode;           \
> +       }                                                                        \
> +       EXPORT_SYMBOL_GPL(preempt_model_##mode)
> +
> +PREEMPT_MODEL_ACCESSOR(none);
> +PREEMPT_MODEL_ACCESSOR(voluntary);
> +PREEMPT_MODEL_ACCESSOR(full);
> +
>  #else /* !CONFIG_PREEMPT_DYNAMIC */
>
>  static inline void preempt_dynamic_init(void) { }
> --
> 2.25.1
>