Re: [PATCH v3 2/2] rethook: honor CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING in rethook_try_get()

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

On Wed,  3 Apr 2024 15:03:28 -0700
Andrii Nakryiko <andrii@kernel.org> wrote:

> Take into account CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING when validating
> that RCU is watching when trying to setup rethooko on a function entry.
> 
> This further (in addition to improvements in the previous patch)
> improves BPF multi-kretprobe (which rely on rethook) runtime throughput
> by 2.3%, according to BPF benchmarks ([0]).
> 
>   [0] https://lore.kernel.org/bpf/CAEf4BzauQ2WKMjZdc9s0rBWa01BYbgwHN6aNDXQSHYia47pQ-w@mail.gmail.com/
> 

Hi Andrii,

Can you make this part depends on !KPROBE_EVENTS_ON_NOTRACE (with this
option, kretprobes can be used without ftrace, but with original int3) ?
This option should be set N on production system because of safety,
just for testing raw kretprobes.

Thank you,

> Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
> ---
>  kernel/trace/rethook.c | 2 ++
>  1 file changed, 2 insertions(+)
> 
> diff --git a/kernel/trace/rethook.c b/kernel/trace/rethook.c
> index fa03094e9e69..15b8aa4048d9 100644
> --- a/kernel/trace/rethook.c
> +++ b/kernel/trace/rethook.c
> @@ -166,6 +166,7 @@ struct rethook_node *rethook_try_get(struct rethook *rh)
>  	if (unlikely(!handler))
>  		return NULL;
>  
> +#ifdef CONFIG_FTRACE_VALIDATE_RCU_IS_WATCHING
>  	/*
>  	 * This expects the caller will set up a rethook on a function entry.
>  	 * When the function returns, the rethook will eventually be reclaimed
> @@ -174,6 +175,7 @@ struct rethook_node *rethook_try_get(struct rethook *rh)
>  	 */
>  	if (unlikely(!rcu_is_watching()))
>  		return NULL;
> +#endif
>  
>  	return (struct rethook_node *)objpool_pop(&rh->pool);
>  }
> -- 
> 2.43.0
> 


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