[PATCH v2] ftrace: simplify ftrace hash lookup code

[PATCH v2] ftrace: simplify ftrace hash lookup code

[Changbin Du]

Function ftrace_lookup_ip() will check empty hash table. So we don't
need extra check outside.

Signed-off-by: Changbin Du <changbin.du@gmail.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);
 }
 
 /*
@@ -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
-- 
2.20.1

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

[Steven Rostedt]

On Mon,  9 Sep 2019 08:31:59 +0800
Changbin Du <changbin.du@gmail.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@gmail.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);
}

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

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

[Changbin Du]

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@gmail.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@gmail.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
>
Yes, I agree with you entirly.

> 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?
> 
Yes, let me update it with your idea. Thanks!

> -- 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

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

[Changbin Du]

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@gmail.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@gmail.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

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

[Steven Rostedt]

On Tue, 10 Sep 2019 08:33:23 +0800
Changbin Du <changbin.du@gmail.com> wrote:

> > 
> > 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?
> 
>

Or...

static struct ftrace_func_entry empty_func_entry;
#define EMPTY_FUNC_ENTRY = &empty_func_entry;

[..]
 * @empty_result: return NULL if false or EMPTY_FUNC_ENTRY on true
[..]
 * @empty_result should be false, unless this is used for testing if the ip
 * exists in the hash, and an empty hash should be considered true.
 * This is useful when the empty hash is considered to contain all addresses.
[..]
struct ftrace_func_entry *
ftrace_lookup_ip(struct ftrace_hash *hash, unsigned long ip, bool empty_result)
{
	if (ftrace_hash_empty(hash))
		return empty_result ? EMPTY_FUNC_ENTRY : NULL;

	return __ftrace_lookup_ip(hash, ip);
}

But looking at this more, I'm going back to not touching the code in
this location, because __ftrace_lookup_ip() is static, where as
ftrace_lookup_ip() is not, and this is in a very fast path, and I
rather keep it open coded.

Lets just drop the first hunk of your patch. The second hunk is fine.


-- Steve

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

[Changbin Du]

On Tue, Sep 10, 2019 at 05:28:04AM -0400, Steven Rostedt wrote:
> On Tue, 10 Sep 2019 08:33:23 +0800
> Changbin Du <changbin.du@gmail.com> wrote:
> 
> > > 
> > > 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?
> > 
> >
> 
> Or...
> 
> static struct ftrace_func_entry empty_func_entry;
> #define EMPTY_FUNC_ENTRY = &empty_func_entry;
> 
> [..]
>  * @empty_result: return NULL if false or EMPTY_FUNC_ENTRY on true
> [..]
>  * @empty_result should be false, unless this is used for testing if the ip
>  * exists in the hash, and an empty hash should be considered true.
>  * This is useful when the empty hash is considered to contain all addresses.
> [..]
> struct ftrace_func_entry *
> ftrace_lookup_ip(struct ftrace_hash *hash, unsigned long ip, bool empty_result)
> {
> 	if (ftrace_hash_empty(hash))
> 		return empty_result ? EMPTY_FUNC_ENTRY : NULL;
> 
> 	return __ftrace_lookup_ip(hash, ip);
> }
> 
> But looking at this more, I'm going back to not touching the code in
> this location, because __ftrace_lookup_ip() is static, where as
> ftrace_lookup_ip() is not, and this is in a very fast path, and I
> rather keep it open coded.
> 
> Lets just drop the first hunk of your patch. The second hunk is fine.
>
Sure, I will send a update short later. Thanks for your suggestions.

> 
> -- Steve

-- 
Cheers,
Changbin Du