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

On Fri, 4 Sep 2015 11:47:17 -0700 Tom Herbert [off-list ref] wrote:

On Fri, Sep 4, 2015 at 10:00 AM, Jesper Dangaard Brouer [off-list ref] wrote:

quoted

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).

[...]

quoted

+/**
+ *     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?

[...]

quoted

+       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