Subject: Re: [RFC PATCH 1/3] net: introduce kfree_skb_bulk() user of kmem_cache_free_bulk()

From: Jesper Dangaard Brouer <[email-masked]>
Date: Mon, 7 Sep 2015 10:41:01 +0200

On Fri, 4 Sep 2015 11:47:17 -0700 Tom Herbert <[email-masked]> wrote:

> On Fri, Sep 4, 2015 at 10:00 AM, Jesper Dangaard Brouer <[email-masked]> wrote:
> > Introduce the first user of SLAB bulk free API kmem_cache_free_bulk(),
> > in the network stack in form of function kfree_skb_bulk() which bulk
> > free SKBs (not skb clones or skb->head, yet).
> >
[...]
> > +/**
> > + *     kfree_skb_bulk - bulk free SKBs when refcnt allows to
> > + *     @skbs: array of SKBs to free
> > + *     @size: number of SKBs in array
> > + *
> > + *     If SKB refcnt allows for free, then release any auxiliary data
> > + *     and then bulk free SKBs to the SLAB allocator.
> > + *
> > + *     Note that interrupts must be enabled when calling this function.
> > + */
> > +void kfree_skb_bulk(struct sk_buff **skbs, unsigned int size)
> > +{
>
> What not pass a list of skbs (e.g. using skb->next)?

Because the next layer, the slab API needs an array:
  kmem_cache_free_bulk(struct kmem_cache *s, size_t size, void **p)

Look at the patch:
 [PATCH V2 3/3] slub: build detached freelist with look-ahead
 http://thread.gmane.org/gmane.linux.kernel.mm/137469/focus=137472

Where I use this array to progressively scan for objects belonging to
the same page.  (A subtle detail is I manage to zero out the array,
which is good from a security/error-handling point of view, as pointers
to the objects are not left dangling on the stack).

I cannot argue that, writing skb->next comes as an additional cost,
because the slUb free also writes into this cacheline.  Perhaps the
slAb allocator does not?

[...]
> > +       if (likely(cnt)) {
> > +               kmem_cache_free_bulk(skbuff_head_cache, cnt, (void **) skbs);
> > +       }
> > +}
> > +EXPORT_SYMBOL(kfree_skb_bulk);

-- 
Best regards,
  Jesper Dangaard Brouer
  MSc.CS, Sr. Network Kernel Developer at Red Hat
  Author of http://www.iptv-analyzer.org
  LinkedIn: http://www.linkedin.com/in/brouer