linux-kernel - Re: [PATCH v2] ftrace: simplify ftrace hash lookup code

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Date:   Tue, 10 Sep 2019 08:33:23 +0800
From:   Changbin Du <changbin.du@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Changbin Du <changbin.du@...il.com>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] ftrace: simplify ftrace hash lookup code

On Mon, Sep 09, 2019 at 10:54:24AM -0400, Steven Rostedt wrote:
> On Mon,  9 Sep 2019 08:31:59 +0800
> Changbin Du <changbin.du@...il.com> 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@...il.com>
> > 
> > ---
> > 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);
> }
>
We must add another similar function since ftrace_lookup_ip() returns a pointer.

bool ftrace_contains_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);
}

But after this, it's a little overkill I think. It is not much simpler than before.
Do you still want this then?

> 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
> 

-- 
Cheers,
Changbin Du