uprobes, kprobes, uretprobes, kretprobes非常棒，但它们也有自己劣处。当函数参数改变了，或想从函数内部获取信息。
还是接着virtio的话题，在上一篇提到过，内核端的virtio是vhost-net。看一下它在内核代码里linux-uek/drivers/vhost/net.c的接收和发送处理函数
static void handle_rx(struct vhost_net *net) {
...
do {
...
} while (likely(!vhost_exceeds_weight(vq, ++recv_pkts, total_len)));
...
}static void handle_tx(struct vhost_net *net) {
...
do {
...
} while (likely(!vhost_exceeds_weight(vq, ++sent_pkts, total_len)));
...
}
在这里可以继续用kprobes和kretprobes，但由于这两个函数没有任何返回值，所以kretprobes获取不到任何信息，而kprobes也只能获取到vhost_net这个数据结构的数据。虽然这个数据结构可以提供一些信息，但像接收包数量recv_pkts和发送包数量sent_pkts的信息，却没办法获取得了。
如果获取这些信息，其中一个方法是使用kernel tracepoints。kernel tracepoints是静态检测的一种形式，因此需要直接在代码中定义和实现跟踪点函数。
实现kernel tracepoints
比如想跟踪total_len
static void handle_rx(struct vhost_net *net) {
...
do {
...
} while (likely(!vhost_exceeds_weight(vq, ++recv_pkts, total_len))); trace_vhost_net_rx_end(recv_pkts, total_len);
...
}
static void handle_tx(struct vhost_net *net) {
...
do {
...
} while (likely(!vhost_exceeds_weight(vq, ++sent_pkts, total_len)));
trace_vhost_net_tx_end(sent_pkts, total_len);
...
}
这里需要定义和实现了两个kernel tracepoint:trace_vhost_net_rx_end,trace_vhost_net_rx_end 。
那么在哪里和怎样定义和实现它们呢。一般的位置是在linux-uek/include/trace/events。这个位置每个头文件都包含很多kernel tracepoint。
原则上，所有系统或子系统的跟踪点定义在它们相应的头文件中。所以,这里的头文件为 linux-uek/include/trace/events/vhost_net.h
/* Kernel tracepoints for vhost_net virtio functions */
#undef TRACE_SYSTEM
#define TRACE_SYSTEM vhost_net#if !defined(_TRACE_VHOST_NET_H) || defined(TRACE_HEADER_MULTI_READ)
#define _TRACE_VHOST_NET_H
#include <linux/tracepoint.h>
#include <linux/vhost.h>
TRACE_EVENT(vhost_net_rx_end,
TP_PROTO(int recv_pkts, int total_len),
TP_ARGS(recv_pkts, total_len),
TP_STRUCT__entry(
__field( int, recv_pkts )
__field( int, total_len )
),
TP_fast_assign(
__entry->recv_pkts = recv_pkts;
__entry->total_len = total_len;
),
TP_printk("recv_pkts=%d, total_len=%d",
__entry->recv_pkts,
__entry->total_len)
);
TRACE_EVENT(vhost_net_tx_end,
TP_PROTO(int sent_pkts, int total_len),
TP_ARGS(sent_pkts, total_len),
TP_STRUCT__entry(
__field( int, sent_pkts )
__field( int, total_len )
),
TP_fast_assign(
__entry->sent_pkts = sent_pkts;
__entry->total_len = total_len;
),
TP_printk("sent_pkts=%d, total_len=%d",
__entry->sent_pkts,
__entry->total_len)
);
#endif /*_TRACE_VHOST_NET_H */
/* This part must be outside protection */
#include <trace/define_trace.h>
这里的函数都没有
trace_前缀，TRACE_EVENT这个宏会自动把它展开成trace_开头的函数。
在net.c文件上包括上这个头文件
// Other header files...
...
#define CREATE_TRACE_POINTS
#include <trace/events/vhost_net.h>
...
由于kernel tracepoint是静态的，需要重新编译内核，在内核代码目录下执行：
make -j 144
make modules_install
make install
然后重启机器，选择新内核启动。
最好在
/etc/default/grub里菜单加多一项，指向新内核，如果新内核有问题，可以使用之前正常的内核进入系统
使用以下参数启动虚拟机
-device virtio-net-pci,netdev=tapnet
-netdev tap,id=tapnet,vhost=on,ifname=tap0,script=/etc/qemu-ifup,downscript=no
使用BCC工具看一下新的kernel tracepoint是否加载
python tplist.py | grep vhost
开始实现eBPF程序：
#!/usr/bin/pythonfrom bcc import BPF
from time import sleep
bpf_txt = """BPF_HISTOGRAM(rx_pkts_hist);
BPF_HISTOGRAM(rx_len_hist);
BPF_HISTOGRAM(tx_pkts_hist);
BPF_HISTOGRAM(tx_len_hist);
struct our_ktp_args {
u64 __unused__;
int packets;
int total_length;
};
int rx_end(struct our_ktp_args *args) {
int recv_pkts = args->packets;
int len = args->total_length;
rx_pkts_hist.increment(bpf_log2l(recv_pkts));
rx_len_hist.increment(bpf_log2l(len));
return 0;
}
int tx_end(struct our_ktp_args *args) {
int sent_pkts = args->packets;
int len = args->total_length;
tx_pkts_hist.increment(bpf_log2l(sent_pkts));
tx_len_hist.increment(bpf_log2l(len));
return 0;
}
"""
# Load BPF program
bpf_ctx = BPF(text=bpf_txt)
bpf_ctx.attach_tracepoint(tp="vhost_net:vhost_net_rx_end",
fn_name="rx_end")
bpf_ctx.attach_tracepoint(tp="vhost_net:vhost_net_tx_end",
fn_name="tx_end")
# Print header
print("Aggregating data from vhost-net RX & TX virtqueues... "
"Hit Ctrl-C to end.")
# Format output
while 1:
try:
sleep(1)
except KeyboardInterrupt:
print("\n")
break
# Output
print("Vhost-net RX log2 received packets histogram")
print("--------------------------------------------")
bpf_ctx["rx_pkts_hist"].print_log2_hist("recv pkts")
print("\n")
print("Vhost-net RX log2 total length histogram")
print("----------------------------------------")
bpf_ctx["rx_len_hist"].print_log2_hist("total len")
print("\n")
print("-------------------------------------------------")
print("\n")
print("Vhost-net TX log2 sent packets histogram")
print("----------------------------------------")
bpf_ctx["tx_pkts_hist"].print_log2_hist("sent pkts")
print("\n")
print("Vhost-net TX log2 total length histogram")
print("----------------------------------------")
bpf_ctx["tx_len_hist"].print_log2_hist("total len")
print("\n")
运行一把
./kernel_tracepts.py
Aggregating data from vhost-net RX & TX virtqueues... Hit Ctrl-C to end.
^CVhost-net RX log2 received packets histogram
--------------------------------------------
recv pkts : count distribution
0 -> 1 : 482291 |****************************************|
2 -> 3 : 40457 |*** |
4 -> 7 : 534 | |
8 -> 15 : 4 | |
16 -> 31 : 2 | |
Vhost-net RX log2 total length histogram
----------------------------------------
total len : count distribution
0 -> 1 : 330 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 0 | |
32 -> 63 : 8 | |
64 -> 127 : 32 | |
128 -> 255 : 9 | |
256 -> 511 : 8 | |
512 -> 1023 : 30 | |
1024 -> 2047 : 15275 |* |
2048 -> 4095 : 15516 |* |
4096 -> 8191 : 57986 |***** |
8192 -> 16383 : 433015 |****************************************|
16384 -> 32767 : 1060 | |
32768 -> 65535 : 14 | |
65536 -> 131071 : 4 | |
131072 -> 262143 : 1 | |
-------------------------------------------------
Vhost-net TX log2 sent packets histogram
----------------------------------------
sent pkts : count distribution
0 -> 1 : 264948 |****************************************|
2 -> 3 : 8358 |* |
4 -> 7 : 199 | |
8 -> 15 : 1 | |
Vhost-net TX log2 total length histogram
----------------------------------------
total len : count distribution
0 -> 1 : 18 | |
2 -> 3 : 0 | |
4 -> 7 : 0 | |
8 -> 15 : 0 | |
16 -> 31 : 0 | |
32 -> 63 : 8 | |
64 -> 127 : 264919 |****************************************|
128 -> 255 : 7998 |* |
256 -> 511 : 558 | |
512 -> 1023 : 5 | |
struct our_ktp_args：保存内核跟踪点参数的自定义数据结构
vhost_net_tx_end 的格式可以从/sys/kernel/debug/tracing/events/vhost_net/vhost_net_tx_end/format找到
attach_tracepoint(tp='...', fn_name='...'):检测内核跟踪点 tp，并在命中时运行 BPF 函数 fn_name.
tp格式：header-name:tracepoint-name. 由于头文件名是vhost_net.h，所以，header-name是vhost_net而头文件里定义了vhost_net_rx_end和vhost_net_tx_end函数，这两个就是tracepoint-name
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