On 2021/9/2 9:13, Eric Dumazet wrote:
On Wed, Sep 1, 2021 at 5:47 PM Yunsheng Lin linyunsheng@huawei.com wrote:
On 2021/9/1 18:39, Yunsheng Lin wrote:
Since tcp_tx_skb_cache is disabled by default in: commit 0b7d7f6b2208 ("tcp: add tcp_tx_skb_cache sysctl")
Add tcp_tx_skb_cache_key checking in sk_stream_alloc_skb() to avoid possible branch-misses.
Signed-off-by: Yunsheng Lin linyunsheng@huawei.com
Also, the sk->sk_tx_skb_cache may be both changed by allocation and freeing side, I assume there may be some implicit protection here too, such as the NAPI protection for rx?
Hi, Eric Is there any implicit protection for sk->sk_tx_skb_cache? As my understanding, sk_stream_alloc_skb() seems to be protected by lock_sock(), and the sk_wmem_free_skb() seems to be mostly happening in NAPI polling for TCP(when ack packet is received) without lock_sock(), so it seems there is no protection here?
Please look again. This is protected by socket lock of course. Otherwise sk_mem_uncharge() would be very broken, sk->sk_forward_alloc is not an atomic field.
Thanks for clarifying. I have been looking for a point to implement the socket'pp_alloc_cache for page pool, and sk_wmem_free_skb() seems like the place to avoid the scalablity problem of ptr_ring in page pool.
The protection for sk_wmem_free_skb() is in tcp_v4_rcv(), right? https://elixir.bootlin.com/linux/latest/source/net/ipv4/tcp_ipv4.c#L2081
TCP stack has no direct relation with NAPI. It can run over loopback interface, no NAPI there. .