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Message-ID: <e649212fde9f0fdee23909ca0d14158d32bb7425.1738877290.git.pabeni@redhat.com>
Date: Thu, 6 Feb 2025 22:28:48 +0100
From: Paolo Abeni <pabeni@...hat.com>
To: netdev@...r.kernel.org
Cc: "David S. Miller" <davem@...emloft.net>,
Eric Dumazet <edumazet@...gle.com>,
Jakub Kicinski <kuba@...nel.org>,
Simon Horman <horms@...nel.org>,
Andrew Lunn <andrew+netdev@...n.ch>,
Alexander Duyck <alexanderduyck@...com>
Subject: [PATCH net] Revert "net: skb: introduce and use a single page frag cache"
This reverts commit dbae2b062824 ("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 3226b158e67c ("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 bf9f1baa279f ("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@...gle.com>
Fixes: dbae2b062824 ("net: skb: introduce and use a single page frag cache")
Signed-off-by: Paolo Abeni <pabeni@...hat.com>
---
@Eric: IIRC, once upon time, you reported a similar regression, which
was root-caused to the SLOB allocator. Please let me know if you prefer
your Reported-by tag here.
---
include/linux/netdevice.h | 1 -
net/core/dev.c | 17 +++++++
net/core/skbuff.c | 103 ++------------------------------------
3 files changed, 22 insertions(+), 99 deletions(-)
diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
index 03bb584c62cf..8c91cdf06afd 100644
--- a/include/linux/netdevice.h
+++ b/include/linux/netdevice.h
@@ -4109,7 +4109,6 @@ 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)
diff --git a/net/core/dev.c b/net/core/dev.c
index b91658e8aedb..55e356a68db6 100644
--- a/net/core/dev.c
+++ b/net/core/dev.c
@@ -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),
diff --git a/net/core/skbuff.c b/net/core/skbuff.c
index a441613a1e6c..6a99c453397f 100644
--- a/net/core/skbuff.c
+++ b/net/core/skbuff.c
@@ -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))
--
2.48.1
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