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SubjectRe: [PATCH v4 net-next 09/11] skbuff: allow to optionally use NAPI cache from __alloc_skb()
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On Thu, 2021-02-11 at 14:28 +0000, Alexander Lobakin wrote:
> From: Paolo Abeni <pabeni@redhat.com> on Thu, 11 Feb 2021 11:16:40 +0100 wrote:
> > What about changing __napi_alloc_skb() to always use
> > the __napi_build_skb(), for both kmalloc and page backed skbs? That is,
> > always doing the 'data' allocation in __napi_alloc_skb() - either via
> > page_frag or via kmalloc() - and than call __napi_build_skb().
> >
> > I think that should avoid adding more checks in __alloc_skb() and
> > should probably reduce the number of conditional used
> > by __napi_alloc_skb().
>
> I thought of this too. But this will introduce conditional branch
> to set or not skb->head_frag. So one branch less in __alloc_skb(),
> one branch more here, and we also lose the ability to __alloc_skb()
> with decached head.

Just to try to be clear, I mean something alike the following (not even
build tested). In the fast path it has less branches than the current
code - for both kmalloc and page_frag allocation.

---
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index 785daff48030..a242fbe4730e 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -506,23 +506,12 @@ struct sk_buff *__napi_alloc_skb(struct napi_struct *napi, unsigned int len,
gfp_t gfp_mask)
{
struct napi_alloc_cache *nc;
+ bool head_frag, pfmemalloc;
struct sk_buff *skb;
void *data;

len += NET_SKB_PAD + NET_IP_ALIGN;

- /* If requested length is either too small or too big,
- * we use kmalloc() for skb->head allocation.
- */
- if (len <= SKB_WITH_OVERHEAD(1024) ||
- len > SKB_WITH_OVERHEAD(PAGE_SIZE) ||
- (gfp_mask & (__GFP_DIRECT_RECLAIM | GFP_DMA))) {
- skb = __alloc_skb(len, gfp_mask, SKB_ALLOC_RX, NUMA_NO_NODE);
- if (!skb)
- goto skb_fail;
- goto skb_success;
- }
-
nc = this_cpu_ptr(&napi_alloc_cache);
len += SKB_DATA_ALIGN(sizeof(struct skb_shared_info));
len = SKB_DATA_ALIGN(len);
@@ -530,25 +519,34 @@ struct sk_buff *__napi_alloc_skb(struct napi_struct *napi, unsigned int len,
if (sk_memalloc_socks())
gfp_mask |= __GFP_MEMALLOC;

- data = page_frag_alloc(&nc->page, len, gfp_mask);
+ if (len <= SKB_WITH_OVERHEAD(1024) ||
+ len > SKB_WITH_OVERHEAD(PAGE_SIZE) ||
+ (gfp_mask & (__GFP_DIRECT_RECLAIM | GFP_DMA))) {
+ data = kmalloc_reserve(len, gfp_mask, NUMA_NO_NODE, &pfmemalloc);
+ head_frag = 0;
+ len = 0;
+ } else {
+ data = page_frag_alloc(&nc->page, len, gfp_mask);
+ pfmemalloc = nc->page.pfmemalloc;
+ head_frag = 1;
+ }
if (unlikely(!data))
return NULL;

skb = __build_skb(data, len);
if (unlikely(!skb)) {
- skb_free_frag(data);
+ if (head_frag)
+ skb_free_frag(data);
+ else
+ kfree(data);
return NULL;
}

- if (nc->page.pfmemalloc)
- skb->pfmemalloc = 1;
- skb->head_frag = 1;
+ skb->pfmemalloc = pfmemalloc;
+ skb->head_frag = head_frag;

-skb_success:
skb_reserve(skb, NET_SKB_PAD + NET_IP_ALIGN);
skb->dev = napi->dev;
-
-skb_fail:
return skb;
}
EXPORT_SYMBOL(__napi_alloc_skb);
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