Re: [PATCH] ftrace: make extra rcu_is_watching() validation check optional

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Fri, 22 Mar 2024 09:03:23 -0700
Andrii Nakryiko <andrii@kernel.org> wrote:

> Introduce CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING config option to
> control whether ftrace low-level code performs additional
> rcu_is_watching()-based validation logic in an attempt to catch noinstr
> violations.
> 
> This check is expected to never be true in practice and would be best
> controlled with extra config to let users decide if they are willing to
> pay the price.

Hmm, for me, it sounds like "WARN_ON(something) never be true in practice
so disable it by default". I think CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING
is OK, but tht should be set to Y by default. If you have already verified
that your system never make it true and you want to optimize your ftrace
path, you can manually set it to N at your own risk.

> 
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Paul E. McKenney <paulmck@kernel.org>
> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---
>  include/linux/trace_recursion.h |  2 +-
>  kernel/trace/Kconfig            | 13 +++++++++++++
>  2 files changed, 14 insertions(+), 1 deletion(-)
> 
> diff --git a/include/linux/trace_recursion.h b/include/linux/trace_recursion.h
> index d48cd92d2364..24ea8ac049b4 100644
> --- a/include/linux/trace_recursion.h
> +++ b/include/linux/trace_recursion.h
> @@ -135,7 +135,7 @@ extern void ftrace_record_recursion(unsigned long ip, unsigned long parent_ip);
>  # define do_ftrace_record_recursion(ip, pip)	do { } while (0)
>  #endif
>  
> -#ifdef CONFIG_ARCH_WANTS_NO_INSTR
> +#ifdef CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING
>  # define trace_warn_on_no_rcu(ip)					\
>  	({								\
>  		bool __ret = !rcu_is_watching();			\

BTW, maybe we can add "unlikely" in the next "if" line?

> diff --git a/kernel/trace/Kconfig b/kernel/trace/Kconfig
> index 61c541c36596..19bce4e217d6 100644
> --- a/kernel/trace/Kconfig
> +++ b/kernel/trace/Kconfig
> @@ -974,6 +974,19 @@ config FTRACE_RECORD_RECURSION_SIZE
>  	  This file can be reset, but the limit can not change in
>  	  size at runtime.
>  
> +config FTRACE_VALIDATE_RCU_IS_WATCHING
> +	bool "Validate RCU is on during ftrace recursion check"
> +	depends on FUNCTION_TRACER
> +	depends on ARCH_WANTS_NO_INSTR

	default y

> +	help
> +	  All callbacks that attach to the function tracing have some sort
> +	  of protection against recursion. This option performs additional
> +	  checks to make sure RCU is on when ftrace callbacks recurse.
> +
> +	  This will add more overhead to all ftrace-based invocations.

	... invocations, but keep it safe.

> +
> +	  If unsure, say N

	If unsure, say Y

Thank you,

> +
>  config RING_BUFFER_RECORD_RECURSION
>  	bool "Record functions that recurse in the ring buffer"
>  	depends on FTRACE_RECORD_RECURSION
> -- 
> 2.43.0
> 


-- 
Masami Hiramatsu (Google) <mhiramat@kernel.org>