检索 UDP 套接字的 SO_INCOMING_CPU？

Question

套接字 API 提供了一个SO_INCOMING_CPU选项，其详细信息在手册页:

   SO_INCOMING_CPU (gettable since Linux 3.19, settable since Linux 4.4)
         Sets or gets the CPU affinity of a socket.  Expects an integer
         flag.

             int cpu = 1;
             setsockopt(fd, SOL_SOCKET, SO_INCOMING_CPU, &cpu, sizeof(cpu));

         Because all of the packets for a single stream (i.e., all
         packets for the same 4-tuple) arrive on the single RX queue
         that is associated with a particular CPU, the typical use case
         is to employ one listening process per RX queue, with the
         incoming flow being handled by a listener on the same CPU that
         is handling the RX queue.  This provides optimal NUMA behavior
         and keeps CPU caches hot.

我有一个服务器，它创建 UDP 套接字、绑定到接口和recv()数据。我想得到SO_INCOMING_CPU这个套接字的。那可能吗？

我写了一个测试程序，测试程序始终返回-1。

我做了一些printk()调试，看起来问题是没有inet_daddr与套接字关联：https://github.com/torvalds/linux/blob/v4.20-rc2/net/ipv4/udp.c#L1870

我认为这是有道理的：UDP 是无连接的，因此套接字可以从任何地址接收数据包。我想我在某处读到套接字的 CPU 亲和力是 4 元组的函数，因此，套接字不一定具有固定的 CPU 亲和力。

然而，考虑到 udp.c 中有初始化 CPU 亲和力的代码，我想我可能会遗漏一些东西。

有什么方法可以SO_INCOMING_CPU使用服务器端 UDP 套接字吗？

这是我提到的（稍微草率的）测试程序：

#include <netinet/in.h>
#include <stdio.h>
#include <string.h>
#include <sys/socket.h>
#include <unistd.h>

int main()
{
    int sockfd = socket(AF_INET, SOCK_DGRAM, 0);

    if (sockfd < 0)
    {  
        perror("Error opening socket");
        return 0;
    }

    struct sockaddr_in myaddr;

    memset((char *)&myaddr, 0, sizeof(myaddr));
    myaddr.sin_family = AF_INET;
    myaddr.sin_addr.s_addr = htonl(INADDR_ANY);
    myaddr.sin_port = htons(4242);

    if (bind(sockfd, (struct sockaddr *)&myaddr, sizeof(myaddr)) < 0) {
        perror("bind failed");
        return 0;
    }

    #define BUFSIZE 20
    unsigned char buf[BUFSIZE];     /* receive buffer */
    struct sockaddr_in remaddr;     /* remote address */
    socklen_t addrlen = sizeof(remaddr);            /* length of addresses */
    int recvlen = recvfrom(sockfd, buf, BUFSIZE, 0, (struct sockaddr *)&remaddr, &addrlen);

    sleep(1);

    int cpu = 0;
    socklen_t len = sizeof(cpu);
    int ret = getsockopt(sockfd, SOL_SOCKET, SO_INCOMING_CPU, &cpu, &len);

    // Sample Output: Incoming CPU is -1 (ret = 0, recvlen = 5)
    printf("Incoming CPU is %d (ret = %d, recvlen = %d)\n", cpu, ret, recvlen);
}