Re: [PATCH v2 2/5] preempt/dynamic: Introduce preempt mode accessors

From: Christophe Leroy <christophe.leroy@csgroup.eu>
To: Valentin Schneider <valentin.schneider@arm.com>,
	linux-kernel@vger.kernel.org, kasan-dev@googlegroups.com,
	linuxppc-dev@lists.ozlabs.org, linux-kbuild@vger.kernel.org
Cc: Marco Elver <elver@google.com>,
	Michal Marek <michal.lkml@markovi.net>,
	Peter Zijlstra <peterz@infradead.org>,
	Frederic Weisbecker <frederic@kernel.org>,
	Mike Galbraith <efault@gmx.de>,
	Nick Desaulniers <ndesaulniers@google.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Paul Mackerras <paulus@samba.org>,
	Masahiro Yamada <masahiroy@kernel.org>,
	Ingo Molnar <mingo@kernel.org>,
	Dmitry Vyukov <dvyukov@google.com>
Subject: Re: [PATCH v2 2/5] preempt/dynamic: Introduce preempt mode accessors
Date: Tue, 16 Nov 2021 14:29:51 +0100	[thread overview]
Message-ID: <2f22c57d-9bf0-3cc1-f0f1-61ecdf5dfa52@csgroup.eu> (raw)
In-Reply-To: <20211110202448.4054153-3-valentin.schneider@arm.com>

Le 10/11/2021 à 21:24, Valentin Schneider a écrit :
> 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.
> 
> Introduce a set of helpers to determine the actual preemption mode used by
> the live kernel.
> 
> Suggested-by: Marco Elver <elver@google.com>
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
>   include/linux/sched.h | 16 ++++++++++++++++
>   kernel/sched/core.c   | 11 +++++++++++
>   2 files changed, 27 insertions(+)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 5f8db54226af..0640d5622496 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -2073,6 +2073,22 @@ static inline void cond_resched_rcu(void)
>   #endif
>   }
>   
> +#ifdef CONFIG_PREEMPT_DYNAMIC
> +
> +extern bool is_preempt_none(void);
> +extern bool is_preempt_voluntary(void);
> +extern bool is_preempt_full(void);

Those are trivial tests supposed to be used in fast pathes. They should 
be static inlines in order to minimise the overhead.

> +
> +#else
> +
> +#define is_preempt_none() IS_ENABLED(CONFIG_PREEMPT_NONE)
> +#define is_preempt_voluntary() IS_ENABLED(CONFIG_PREEMPT_VOLUNTARY)
> +#define is_preempt_full() IS_ENABLED(CONFIG_PREEMPT)

Would be better to use static inlines here as well instead of macros.

> +
> +#endif
> +
> +#define is_preempt_rt() IS_ENABLED(CONFIG_PREEMPT_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..9db7f77e53c3 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6638,6 +6638,17 @@ static void __init preempt_dynamic_init(void)
>   	}
>   }
>   
> +#define PREEMPT_MODE_ACCESSOR(mode) \
> +	bool is_preempt_##mode(void)						 \
> +	{									 \
> +		WARN_ON_ONCE(preempt_dynamic_mode == preempt_dynamic_undefined); \

Not sure using WARN_ON is a good idea here, as it may be called very 
early, see comment on powerpc patch.

> +		return preempt_dynamic_mode == preempt_dynamic_##mode;		 \
> +	}

I'm not sure that's worth a macro. You only have 3 accessors, 2 lines of 
code each. Just define all 3 in plain text.

CONFIG_PREEMPT_DYNAMIC is based on using strategies like static_calls in 
order to minimise the overhead. For those accessors you should use the 
same kind of approach and use things like jump_labels in order to not 
redo the test at each time and minimise overhead as much as possible.

> +
> +PREEMPT_MODE_ACCESSOR(none)
> +PREEMPT_MODE_ACCESSOR(voluntary)
> +PREEMPT_MODE_ACCESSOR(full)
> +
>   #else /* !CONFIG_PREEMPT_DYNAMIC */
>   
>   static inline void preempt_dynamic_init(void) { }
> 