Re: [PATCH v2] ftrace: simplify ftrace hash lookup code

[Steven Rostedt]

On Mon,  9 Sep 2019 08:31:59 +0800
Changbin Du <changbin.du@xxxxxxxxx> wrote:

> Function ftrace_lookup_ip() will check empty hash table. So we don't
> need extra check outside.
> 
> Signed-off-by: Changbin Du <changbin.du@xxxxxxxxx>
> 
> ---
> v2: fix incorrect code remove.
> ---
>  kernel/trace/ftrace.c | 9 ++-------
>  1 file changed, 2 insertions(+), 7 deletions(-)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index f9821a3374e9..92aab854d3b1 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -1463,8 +1463,7 @@ static bool hash_contains_ip(unsigned long ip,
>  	 */
>  	return (ftrace_hash_empty(hash->filter_hash) ||
>  		__ftrace_lookup_ip(hash->filter_hash, ip)) &&
> -		(ftrace_hash_empty(hash->notrace_hash) ||
> -		 !__ftrace_lookup_ip(hash->notrace_hash, ip));
> +	       !ftrace_lookup_ip(hash->notrace_hash, ip);

I don't care for this part. I've nacked this change in the past. Why?
let's compare the changes:

	return (ftrace_hash_empty(hash->filter_hash) ||
		__ftrace_lookup_ip(hash->filter_hash, ip)) &&
		(ftrace_hash_empty(hash->notrace_hash) ||
		 !__ftrace_lookup_ip(hash->notrace_hash, ip));

 vs:

	return (ftrace_hash_empty(hash->filter_hash) ||
		__ftrace_lookup_ip(hash->filter_hash, ip)) &&
		!ftrace_lookup_ip(hash->notrace_hash, ip);

The issue I have with this is that it abstracts out the difference
between the filter_hash and the notrace_hash. Sometimes open coded
works better if it is compared to something that is similar.

The current code I see:

	Return true if (filter_hash is empty or ip exists in filter_hash
		 and notrace_hash is empty or it does not exist in notrace_hash

With your update I see:

	Return true if filter_hash is empty or ip exists in filter_hash
                and ip does not exist in notrace_hash

It makes it not easy to see if what happens if notrace_hash is empty

Hmm, come to think of it, perhaps we should change ftrace_lookup_ip()
to include what to do on empty.

Maybe:

bool ftrace_lookup_ip(struct ftrace_hash *hash, unsigned long ip, bool empty_result)
{
	if (ftrace_hash_empty(hash))
		return empty_result;

	return __ftrace_lookup_ip(hash, ip);
}

Then we can change the above to:

	return ftrace_lookup_ip(hash->filter_hash, ip, true) &&
	       !ftrace_lookup_ip(hash->notrace_hash, ip, false);

That would probably work better.

Want to send that update?

-- Steve


>  }
>  
>  /*
> @@ -6036,11 +6035,7 @@ clear_func_from_hash(struct ftrace_init_func
> *func, struct ftrace_hash *hash) {
>  	struct ftrace_func_entry *entry;
>  
> -	if (ftrace_hash_empty(hash))
> -		return;
> -
> -	entry = __ftrace_lookup_ip(hash, func->ip);
> -
> +	entry = ftrace_lookup_ip(hash, func->ip);
>  	/*
>  	 * Do not allow this rec to match again.
>  	 * Yeah, it may waste some memory, but will be removed