Re: [PATCH] skbuff: remove pointless conditional before kfree_skb()

Eric Dumazet
2012-08-28T20:09:59Z

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.