Re: [PATCH resend ftrace] Asynchronous grace period for register_ftrace_direct()

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

On Wed, 1 May 2024 16:12:37 -0700
"Paul E. McKenney" <paulmck@kernel.org> wrote:

> Note that the immediate pressure for this patch should be relieved by the
> NAPI patch series [1], but this sort of problem could easily arise again.
> 
> When running heavy test workloads with KASAN enabled, RCU Tasks grace
> periods can extend for many tens of seconds, significantly slowing
> trace registration.  Therefore, make the registration-side RCU Tasks
> grace period be asynchronous via call_rcu_tasks().
> 

Good catch! AFAICS, there is no reason to wait for synchronization
when adding a new direct trampoline.
This looks good to me.

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

Thank you,

> [1] https://lore.kernel.org/all/cover.1710877680.git.yan@cloudflare.com/
> 
> Reported-by: Jakub Kicinski <kuba@kernel.org>
> Reported-by: Alexei Starovoitov <ast@kernel.org>
> Reported-by: Chris Mason <clm@fb.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Masami Hiramatsu <mhiramat@kernel.org>
> Cc: Mark Rutland <mark.rutland@arm.com>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: <linux-trace-kernel@vger.kernel.org>
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 6c96b30f3d63b..32ea92934268c 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -5365,6 +5365,13 @@ static void remove_direct_functions_hash(struct ftrace_hash *hash, unsigned long
>  	}
>  }
>  
> +static void register_ftrace_direct_cb(struct rcu_head *rhp)
> +{
> +	struct ftrace_hash *fhp = container_of(rhp, struct ftrace_hash, rcu);
> +
> +	free_ftrace_hash(fhp);
> +}
> +
>  /**
>   * register_ftrace_direct - Call a custom trampoline directly
>   * for multiple functions registered in @ops
> @@ -5463,10 +5470,8 @@ int register_ftrace_direct(struct ftrace_ops *ops, unsigned long addr)
>   out_unlock:
>  	mutex_unlock(&direct_mutex);
>  
> -	if (free_hash && free_hash != EMPTY_HASH) {
> -		synchronize_rcu_tasks();
> -		free_ftrace_hash(free_hash);
> -	}
> +	if (free_hash && free_hash != EMPTY_HASH)
> +		call_rcu_tasks(&free_hash->rcu, register_ftrace_direct_cb);
>  
>  	if (new_hash)
>  		free_ftrace_hash(new_hash);


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