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Revert "net: skb: introduce and use a single page frag cache"

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This reverts commit dbae2b0628 ("net: skb: introduce and use a single
page frag cache"). The intended goal of such change was to counter a
performance regression introduced by commit 3226b158e6 ("net: avoid
32 x truesize under-estimation for tiny skbs").

Unfortunately, the blamed commit introduces another regression for the
virtio_net driver. Such a driver calls napi_alloc_skb() with a tiny
size, so that the whole head frag could fit a 512-byte block.

The single page frag cache uses a 1K fragment for such allocation, and
the additional overhead, under small UDP packets flood, makes the page
allocator a bottleneck.

Thanks to commit bf9f1baa27 ("net: add dedicated kmem_cache for
typical/small skb->head"), this revert does not re-introduce the
original regression. Actually, in the relevant test on top of this
revert, I measure a small but noticeable positive delta, just above
noise level.

The revert itself required some additional mangling due to the
introduction of the SKB_HEAD_ALIGN() helper and local lock infra in the
affected code.

Suggested-by: Eric Dumazet <edumazet@google.com>
Fixes: dbae2b0628 ("net: skb: introduce and use a single page frag cache")
Signed-off-by: Paolo Abeni <pabeni@redhat.com>
Link: https://patch.msgid.link/e649212fde9f0fdee23909ca0d14158d32bb7425.1738877290.git.pabeni@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>
This commit is contained in:
Paolo Abeni 2025-02-06 22:28:48 +01:00 • committed by Jakub Kicinski
parent cb827db50a
commit 011b033590
3 changed files with 22 additions and 99 deletions
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1
include/linux/netdevice.h
View file

@ -4115,7 +4115,6 @@ void netif_receive_skb_list(struct list_head *head);
gro_result_t napi_gro_receive(struct napi_struct *napi, struct sk_buff *skb);
void napi_gro_flush(struct napi_struct *napi, bool flush_old);
struct sk_buff *napi_get_frags(struct napi_struct *napi);
void napi_get_frags_check(struct napi_struct *napi);
gro_result_t napi_gro_frags(struct napi_struct *napi);
static inline void napi_free_frags(struct napi_struct *napi)

17
net/core/dev.c
View file

@ -6920,6 +6920,23 @@ netif_napi_dev_list_add(struct net_device *dev, struct napi_struct *napi)
list_add_rcu(&napi->dev_list, higher); /* adds after higher */
}
/* Double check that napi_get_frags() allocates skbs with
* skb->head being backed by slab, not a page fragment.
* This is to make sure bug fixed in 3226b158e67c
* ("net: avoid 32 x truesize under-estimation for tiny skbs")
* does not accidentally come back.
*/
static void napi_get_frags_check(struct napi_struct *napi)
{
struct sk_buff *skb;
local_bh_disable();
skb = napi_get_frags(napi);
WARN_ON_ONCE(skb && skb->head_frag);
napi_free_frags(napi);
local_bh_enable();
}
void netif_napi_add_weight_locked(struct net_device *dev,
struct napi_struct *napi,
int (*poll)(struct napi_struct *, int),

103
net/core/skbuff.c
View file

@ -220,67 +220,9 @@ static void skb_under_panic(struct sk_buff *skb, unsigned int sz, void *addr)
#define NAPI_SKB_CACHE_BULK 16
#define NAPI_SKB_CACHE_HALF (NAPI_SKB_CACHE_SIZE / 2)
#if PAGE_SIZE == SZ_4K
#define NAPI_HAS_SMALL_PAGE_FRAG 1
#define NAPI_SMALL_PAGE_PFMEMALLOC(nc) ((nc).pfmemalloc)
/* specialized page frag allocator using a single order 0 page
* and slicing it into 1K sized fragment. Constrained to systems
* with a very limited amount of 1K fragments fitting a single
* page - to avoid excessive truesize underestimation
*/
struct page_frag_1k {
void *va;
u16 offset;
bool pfmemalloc;
};
static void *page_frag_alloc_1k(struct page_frag_1k *nc, gfp_t gfp)
{
struct page *page;
int offset;
offset = nc->offset - SZ_1K;
if (likely(offset >= 0))
goto use_frag;
page = alloc_pages_node(NUMA_NO_NODE, gfp, 0);
if (!page)
return NULL;
nc->va = page_address(page);
nc->pfmemalloc = page_is_pfmemalloc(page);
offset = PAGE_SIZE - SZ_1K;
page_ref_add(page, offset / SZ_1K);
use_frag:
nc->offset = offset;
return nc->va + offset;
}
#else
/* the small page is actually unused in this build; add dummy helpers
* to please the compiler and avoid later preprocessor's conditionals
*/
#define NAPI_HAS_SMALL_PAGE_FRAG 0
#define NAPI_SMALL_PAGE_PFMEMALLOC(nc) false
struct page_frag_1k {
};
static void *page_frag_alloc_1k(struct page_frag_1k *nc, gfp_t gfp_mask)
{
return NULL;
}
#endif
struct napi_alloc_cache {
local_lock_t bh_lock;
struct page_frag_cache page;
struct page_frag_1k page_small;
unsigned int skb_count;
void *skb_cache[NAPI_SKB_CACHE_SIZE];
};
@ -290,23 +232,6 @@ static DEFINE_PER_CPU(struct napi_alloc_cache, napi_alloc_cache) = {
.bh_lock = INIT_LOCAL_LOCK(bh_lock),
};
/* Double check that napi_get_frags() allocates skbs with
* skb->head being backed by slab, not a page fragment.
* This is to make sure bug fixed in 3226b158e67c
* ("net: avoid 32 x truesize under-estimation for tiny skbs")
* does not accidentally come back.
*/
void napi_get_frags_check(struct napi_struct *napi)
{
struct sk_buff *skb;
local_bh_disable();
skb = napi_get_frags(napi);
WARN_ON_ONCE(!NAPI_HAS_SMALL_PAGE_FRAG && skb && skb->head_frag);
napi_free_frags(napi);
local_bh_enable();
}
void *__napi_alloc_frag_align(unsigned int fragsz, unsigned int align_mask)
{
struct napi_alloc_cache *nc = this_cpu_ptr(&napi_alloc_cache);
@ -813,10 +738,8 @@ struct sk_buff *napi_alloc_skb(struct napi_struct *napi, unsigned int len)
/* If requested length is either too small or too big,
* we use kmalloc() for skb->head allocation.
* When the small frag allocator is available, prefer it over kmalloc
* for small fragments
*/
if ((!NAPI_HAS_SMALL_PAGE_FRAG && len <= SKB_WITH_OVERHEAD(1024)) ||
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 | SKB_ALLOC_NAPI,
@ -826,32 +749,16 @@ struct sk_buff *napi_alloc_skb(struct napi_struct *napi, unsigned int len)
goto skb_success;
}
len = SKB_HEAD_ALIGN(len);
if (sk_memalloc_socks())
gfp_mask |= __GFP_MEMALLOC;
local_lock_nested_bh(&napi_alloc_cache.bh_lock);
nc = this_cpu_ptr(&napi_alloc_cache);
if (NAPI_HAS_SMALL_PAGE_FRAG && len <= SKB_WITH_OVERHEAD(1024)) {
/* we are artificially inflating the allocation size, but
* that is not as bad as it may look like, as:
* - 'len' less than GRO_MAX_HEAD makes little sense
* - On most systems, larger 'len' values lead to fragment
* size above 512 bytes
* - kmalloc would use the kmalloc-1k slab for such values
* - Builds with smaller GRO_MAX_HEAD will very likely do
* little networking, as that implies no WiFi and no
* tunnels support, and 32 bits arches.
*/
len = SZ_1K;
data = page_frag_alloc_1k(&nc->page_small, gfp_mask);
pfmemalloc = NAPI_SMALL_PAGE_PFMEMALLOC(nc->page_small);
} else {
len = SKB_HEAD_ALIGN(len);
data = page_frag_alloc(&nc->page, len, gfp_mask);
pfmemalloc = page_frag_cache_is_pfmemalloc(&nc->page);
}
data = page_frag_alloc(&nc->page, len, gfp_mask);
pfmemalloc = page_frag_cache_is_pfmemalloc(&nc->page);
local_unlock_nested_bh(&napi_alloc_cache.bh_lock);
if (unlikely(!data))