Patchwork skbuff: remove pointless conditional before kfree_skb()

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Submitter Wei Yongjun
Date Aug. 28, 2012, 1:10 p.m.
Message ID <CAPgLHd9m2VbFFXt5Q3MMrK6BPXvMrT-ptq+RfgBToXOpxAKOOw@mail.gmail.com>
Download mbox | patch
Permalink /patch/180483/
State Rejected
Delegated to: David Miller
Headers show

Comments

Wei Yongjun - Aug. 28, 2012, 1:10 p.m.
From: Wei Yongjun <yongjun_wei@trendmicro.com.cn>

Remove pointless conditional before kfree_skb().

Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
---
 include/linux/skbuff.h | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)



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Eric Dumazet - Aug. 28, 2012, 2:12 p.m.
On Tue, 2012-08-28 at 21:10 +0800, Wei Yongjun wrote:
> From: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
> 
> Remove pointless conditional before kfree_skb().
> 
> Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
> ---
>  include/linux/skbuff.h | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> index 7632c87..0b846d9 100644
> --- a/include/linux/skbuff.h
> +++ b/include/linux/skbuff.h
> @@ -2464,8 +2464,7 @@ static inline void nf_conntrack_get_reasm(struct sk_buff *skb)
>  }
>  static inline void nf_conntrack_put_reasm(struct sk_buff *skb)
>  {
> -	if (skb)
> -		kfree_skb(skb);
> +	kfree_skb(skb);
>  }
>  #endif
>  #ifdef CONFIG_BRIDGE_NETFILTER
> 


Its not exactly pointless.

Its a tradeoff between kernel code size, and ability for cpu to predict
a branch in kfree_skb()

This test is in hot path, and therefore this patch can potentially have
a performance impact.

I really think most kfree_skb() calls are done with a non NULL param,
so the branch prediction is good.

But after this patch, things are totally different.

Therefore, I am against it.



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Flavio Leitner - Aug. 28, 2012, 7:17 p.m.
On Tue, 28 Aug 2012 07:12:34 -0700
Eric Dumazet <eric.dumazet@gmail.com> wrote:

> On Tue, 2012-08-28 at 21:10 +0800, Wei Yongjun wrote:
> > From: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
> > 
> > Remove pointless conditional before kfree_skb().
> > 
> > Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
> > ---
> >  include/linux/skbuff.h | 3 +--
> >  1 file changed, 1 insertion(+), 2 deletions(-)
> > 
> > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > index 7632c87..0b846d9 100644
> > --- a/include/linux/skbuff.h
> > +++ b/include/linux/skbuff.h
> > @@ -2464,8 +2464,7 @@ static inline void nf_conntrack_get_reasm(struct sk_buff *skb)
> >  }
> >  static inline void nf_conntrack_put_reasm(struct sk_buff *skb)
> >  {
> > -	if (skb)
> > -		kfree_skb(skb);
> > +	kfree_skb(skb);
> >  }
> >  #endif
> >  #ifdef CONFIG_BRIDGE_NETFILTER
> > 
> 
> 
> Its not exactly pointless.
> 
> Its a tradeoff between kernel code size, and ability for cpu to predict
> a branch in kfree_skb()
> 
> This test is in hot path, and therefore this patch can potentially have
> a performance impact.
> 
> I really think most kfree_skb() calls are done with a non NULL param,
> so the branch prediction is good.
> 
> But after this patch, things are totally different.
>

But then the kfree_skb() is somewhat misleading because it does
check for NULL argument. One would have to remember if it's in
hot path or not. So, what about a new macro to pair with
kfree_skb()? That would document the code and would also
make easier to remember about the performance issue.

For instance:
/* kfree_skb() version to be used in hot code path
 * as the branch prediction can improve performance
 */
#define kfree_skb_hot(skb)	\
	if (skb)		\
		kfree_skb(skb)	\


fbl
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Eric Dumazet - Aug. 28, 2012, 8:09 p.m.
On Tue, 2012-08-28 at 16:17 -0300, Flavio Leitner wrote:
> On Tue, 28 Aug 2012 07:12:34 -0700
> Eric Dumazet <eric.dumazet@gmail.com> wrote:
> 
> > On Tue, 2012-08-28 at 21:10 +0800, Wei Yongjun wrote:
> > > From: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
> > > 
> > > Remove pointless conditional before kfree_skb().
> > > 
> > > Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
> > > ---
> > >  include/linux/skbuff.h | 3 +--
> > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > > 
> > > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > > index 7632c87..0b846d9 100644
> > > --- a/include/linux/skbuff.h
> > > +++ b/include/linux/skbuff.h
> > > @@ -2464,8 +2464,7 @@ static inline void nf_conntrack_get_reasm(struct sk_buff *skb)
> > >  }
> > >  static inline void nf_conntrack_put_reasm(struct sk_buff *skb)
> > >  {
> > > -	if (skb)
> > > -		kfree_skb(skb);
> > > +	kfree_skb(skb);
> > >  }
> > >  #endif
> > >  #ifdef CONFIG_BRIDGE_NETFILTER
> > > 
> > 
> > 
> > Its not exactly pointless.
> > 
> > Its a tradeoff between kernel code size, and ability for cpu to predict
> > a branch in kfree_skb()
> > 
> > This test is in hot path, and therefore this patch can potentially have
> > a performance impact.
> > 
> > I really think most kfree_skb() calls are done with a non NULL param,
> > so the branch prediction is good.
> > 
> > But after this patch, things are totally different.
> >
> 
> But then the kfree_skb() is somewhat misleading because it does
> check for NULL argument. One would have to remember if it's in
> hot path or not. So, what about a new macro to pair with
> kfree_skb()? That would document the code and would also
> make easier to remember about the performance issue.
> 
> For instance:
> /* kfree_skb() version to be used in hot code path
>  * as the branch prediction can improve performance
>  */
> #define kfree_skb_hot(skb)	\
> 	if (skb)		\
> 		kfree_skb(skb)	\

Really kfree_skb() is not misleading at all :

if (unlikely(!skb))
	return;

So while its _perfectly_ valid to call kfree_skb(NULL), this code
expect callers to not abuse this facility.

And nf_conntrack_put_reasm() is called from skb_release_head_state()

We know in this code that most of the time, skb will be NULL.

I dont think we need to add another API for this case and see one
hundred patches coming _trying_ to use this new API.

Just review patches and shout if something bad happens.



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Flavio Leitner - Aug. 28, 2012, 8:39 p.m.
On Tue, 28 Aug 2012 13:09:58 -0700
Eric Dumazet <eric.dumazet@gmail.com> wrote:

> On Tue, 2012-08-28 at 16:17 -0300, Flavio Leitner wrote:
> > On Tue, 28 Aug 2012 07:12:34 -0700
> > Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > 
> > > On Tue, 2012-08-28 at 21:10 +0800, Wei Yongjun wrote:
> > > > From: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
> > > > 
> > > > Remove pointless conditional before kfree_skb().
> > > > 
> > > > Signed-off-by: Wei Yongjun <yongjun_wei@trendmicro.com.cn>
> > > > ---
> > > >  include/linux/skbuff.h | 3 +--
> > > >  1 file changed, 1 insertion(+), 2 deletions(-)
> > > > 
> > > > diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
> > > > index 7632c87..0b846d9 100644
> > > > --- a/include/linux/skbuff.h
> > > > +++ b/include/linux/skbuff.h
> > > > @@ -2464,8 +2464,7 @@ static inline void nf_conntrack_get_reasm(struct sk_buff *skb)
> > > >  }
> > > >  static inline void nf_conntrack_put_reasm(struct sk_buff *skb)
> > > >  {
> > > > -	if (skb)
> > > > -		kfree_skb(skb);
> > > > +	kfree_skb(skb);
> > > >  }
> > > >  #endif
> > > >  #ifdef CONFIG_BRIDGE_NETFILTER
> > > > 
> > > 
> > > 
> > > Its not exactly pointless.
> > > 
> > > Its a tradeoff between kernel code size, and ability for cpu to predict
> > > a branch in kfree_skb()
> > > 
> > > This test is in hot path, and therefore this patch can potentially have
> > > a performance impact.
> > > 
> > > I really think most kfree_skb() calls are done with a non NULL param,
> > > so the branch prediction is good.
> > > 
> > > But after this patch, things are totally different.
> > >
> > 
> > But then the kfree_skb() is somewhat misleading because it does
> > check for NULL argument. One would have to remember if it's in
> > hot path or not. So, what about a new macro to pair with
> > kfree_skb()? That would document the code and would also
> > make easier to remember about the performance issue.
> > 
> > For instance:
> > /* kfree_skb() version to be used in hot code path
> >  * as the branch prediction can improve performance
> >  */
> > #define kfree_skb_hot(skb)	\
> > 	if (skb)		\
> > 		kfree_skb(skb)	\
> 
> Really kfree_skb() is not misleading at all :
> 
> if (unlikely(!skb))
> 	return;
> 
> So while its _perfectly_ valid to call kfree_skb(NULL), this code
> expect callers to not abuse this facility.

Well, I don't think that is obvious. Neither the patch's author.

> And nf_conntrack_put_reasm() is called from skb_release_head_state()
> 
> We know in this code that most of the time, skb will be NULL.

yeah, but it looks pointless to check the same thing twice.

> I dont think we need to add another API for this case and see one
> hundred patches coming _trying_ to use this new API.

Ok, and what if kfree_skb() becomes a macro that first checks
if the skb is NULL and if not, call the _kfree_skb() to
continue as before?

#define kfree_skb(skb)		\
        if (skb)		\
		_kfree_skb(skb)	\


void _kfree_skb(struct sk_buff *skb)
{                       
        if (likely(atomic_read(&skb->users) == 1))
                smp_rmb();      
        else if (likely(!atomic_dec_and_test(&skb->users)))
                return;
        trace_kfree_skb(skb, __builtin_return_address(0));
        __kfree_skb(skb);
}

Same API which would work for either use-cases. At the cost of
additional size in the binary.

> Just review patches and shout if something bad happens.

I hope we always have you around to catch these cases :)

fbl
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Eric Dumazet - Aug. 29, 2012, 3:38 a.m.
On Tue, 2012-08-28 at 17:39 -0300, Flavio Leitner wrote:

> Ok, and what if kfree_skb() becomes a macro that first checks
> if the skb is NULL and if not, call the _kfree_skb() to
> continue as before?
> 
> #define kfree_skb(skb)		\
>         if (skb)		\
> 		_kfree_skb(skb)	\

Then its adding a conditional test on each call site and increase
kernel code size.

So if you plan submitting such patch, please keep the whole thing out of
line.



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David Miller - Aug. 30, 2012, 5:39 p.m.
From: Eric Dumazet <eric.dumazet@gmail.com>
Date: Tue, 28 Aug 2012 20:38:48 -0700

> On Tue, 2012-08-28 at 17:39 -0300, Flavio Leitner wrote:
> 
>> Ok, and what if kfree_skb() becomes a macro that first checks
>> if the skb is NULL and if not, call the _kfree_skb() to
>> continue as before?
>> 
>> #define kfree_skb(skb)		\
>>         if (skb)		\
>> 		_kfree_skb(skb)	\
> 
> Then its adding a conditional test on each call site and increase
> kernel code size.
> 
> So if you plan submitting such patch, please keep the whole thing out of
> line.

I'm tossing this entire series.

Each and every case must be investigated individually and:

1) If the check is kept, a big comment explaining why is added
   to the code.

2) If the check is removed, a big piece of explanatory text is
   added to the commit log message explaining everything in
   full detail.
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Patch

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 7632c87..0b846d9 100644
--- a/include/linux/skbuff.h
+++ b/include/linux/skbuff.h
@@ -2464,8 +2464,7 @@  static inline void nf_conntrack_get_reasm(struct sk_buff *skb)
 }
 static inline void nf_conntrack_put_reasm(struct sk_buff *skb)
 {
-	if (skb)
-		kfree_skb(skb);
+	kfree_skb(skb);
 }
 #endif
 #ifdef CONFIG_BRIDGE_NETFILTER