When measuring latency when running a latency critical workload
on mlx5 pmd drivers we noticed high latency can occur due to
delayed doorbell record update flush.
This can be reproduced using the simple program [1]
against testpmd macswap fwd mode. This utility sends
a raw ethernet frame to the dpdk port and measures the
time between send and the received mirrored frame.
This patchset guarantees immediate doorbell updates
visibility by making the doorbell a non-cacheble memory.
In addition, we relax the memory barrier for dma-able
memory.
Without this fix the tsc delta was 3550760-5993019 cycles
(which translates to 2-6 ms on 1.7 GHz processor).
With the fix applied the tsc delta reduced to 17740-29663
(wich translates to 9-17 us).
Shahaf Shuler (2):
net/mlx5: replace memory barrier type
net/mlx5: don't map doorbell register to write combining
drivers/net/mlx5/mlx5.c | 2 ++
drivers/net/mlx5/mlx5_rxtx.h | 2 +-
2 files changed, 3 insertions(+), 1 deletion(-)
[1]:
/*
* compiling: gcc test.c -o test
* run using: ./test <local_iface> <dest_mac>
*/
#include <arpa/inet.h>
#include <linux/if_packet.h>
#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <sys/ioctl.h>
#include <sys/socket.h>
#include <net/if.h>
#include <netinet/ether.h>
#define BUF_SIZ 1024
static inline uint64_t rte_rdtsc(void)
{
union {
uint64_t tsc_64;
struct {
uint32_t lo_32;
uint32_t hi_32;
};
} tsc;
asm volatile("rdtsc" :
"=a" (tsc.lo_32),
"=d" (tsc.hi_32));
return tsc.tsc_64;
}
int main(int argc, char *argv[])
{
int sockfd;
struct ifreq if_idx;
struct ifreq if_mac;
int tx_len = 0;
char sendbuf[BUF_SIZ];
struct ether_header *eh = (struct ether_header *) sendbuf;
struct sockaddr_ll socket_address;
char ifname[IFNAMSIZ];
int values[6];
struct ether_header expected;
uint64_t payload = 0xB16B00B5;
uint8_t buffer[1024];
int result;
uint64_t before_rcv;
uint64_t after_rcv;
uint64_t delta;
int numbytes;
if (argc != 3) {
fprintf(stderr, "device name and dest mac\n");
return -1;
}
strcpy(ifname, argv[1]);
result = sscanf(argv[2], "%x:%x:%x:%x:%x:%x",
&values[0], &values[1], &values[2], &values[3],
&values[4], &values[5]);
if (result != 6) {
fprintf(stderr, "invalid mac\n");
return -1;
}
/* Open RAW socket to send on */
if ((sockfd = socket(AF_PACKET, SOCK_RAW, htons(ETH_P_ALL))) == -1) {
perror("socket");
}
/* Get the index of the interface to send on */
memset(&if_idx, 0, sizeof(struct ifreq));
strncpy(if_idx.ifr_name, ifname, IFNAMSIZ-1);
if (ioctl(sockfd, SIOCGIFINDEX, &if_idx) < 0)
perror("SIOCGIFINDEX");
/* Get the MAC address of the interface to send on */
memset(&if_mac, 0, sizeof(struct ifreq));
strncpy(if_mac.ifr_name, ifname, IFNAMSIZ-1);
if (ioctl(sockfd, SIOCGIFHWADDR, &if_mac) < 0)
perror("SIOCGIFHWADDR");
/* Construct the Ethernet header */
memset(sendbuf, 0, BUF_SIZ);
/* Ethernet header */
eh->ether_shost[0] = ((uint8_t *)&if_mac.ifr_hwaddr.sa_data)[0];
eh->ether_shost[1] = ((uint8_t *)&if_mac.ifr_hwaddr.sa_data)[1];
eh->ether_shost[2] = ((uint8_t *)&if_mac.ifr_hwaddr.sa_data)[2];
eh->ether_shost[3] = ((uint8_t *)&if_mac.ifr_hwaddr.sa_data)[3];
eh->ether_shost[4] = ((uint8_t *)&if_mac.ifr_hwaddr.sa_data)[4];
eh->ether_shost[5] = ((uint8_t *)&if_mac.ifr_hwaddr.sa_data)[5];
eh->ether_dhost[0] = values[0];
eh->ether_dhost[1] = values[1];
eh->ether_dhost[2] = values[2];
eh->ether_dhost[3] = values[3];
eh->ether_dhost[4] = values[4];
eh->ether_dhost[5] = values[5];
/* Ethertype field */
eh->ether_type = htons(ETH_P_IP);
tx_len += sizeof(struct ether_header);
memcpy(&sendbuf[tx_len], &payload, sizeof(payload));
tx_len += sizeof(payload);
/* Index of the network device */
socket_address.sll_ifindex = if_idx.ifr_ifindex;
/* Address length*/
socket_address.sll_halen = ETH_ALEN;
/* Destination MAC */
socket_address.sll_addr[0] = values[0];
socket_address.sll_addr[1] = values[1];
socket_address.sll_addr[2] = values[2];
socket_address.sll_addr[3] = values[3];
socket_address.sll_addr[4] = values[4];
socket_address.sll_addr[5] = values[5];
memcpy(&expected.ether_dhost, &eh->ether_shost, ETH_ALEN);
memcpy(&expected.ether_shost, &eh->ether_dhost, ETH_ALEN);
expected.ether_type = eh->ether_type;
/* Send packet */
if (sendto(sockfd, sendbuf, tx_len, 0, (struct
sockaddr*)&socket_address, sizeof(struct sockaddr_ll)) < 0) {
printf("Send failed\n");
return -2;
}
before_rcv = rte_rdtsc();
while (1) {
numbytes = recvfrom(sockfd, buffer, BUF_SIZ, 0, NULL, NULL);
if (numbytes <= 0)
continue;
after_rcv = rte_rdtsc();
if (memcmp(&expected, buffer, sizeof(expected)) != 0)
continue;
if (memcmp(&payload, &buffer[sizeof(expected)],
sizeof(payload)) == 0) {
break;
}
}
delta = after_rcv - before_rcv;
printf("RTT is %lu tsc \n", delta);
return 0;
}
--
2.7.4
