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tutorials: end-user, and python developer
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brendangregg committed Jul 25, 2016
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# BPF Compiler Collection (BCC)

BCC is a toolkit for creating efficient kernel tracing and manipulation
programs, and includes several useful tools and examples. It makes use of eBPF
(Extended Berkeley Packet Filters), a new feature that was first added to
Linux 3.15. Much of what BCC uses requires Linux 4.1 and above.
programs, and includes several useful tools and examples. It makes use of
extended BPF (Berkeley Packet Filters), formally known as eBPF, a new feature
that was first added to Linux 3.15. Much of what BCC uses requires Linux 4.1
and above.

eBPF was [described by](https://lkml.org/lkml/2015/4/14/232) Ingo Molnár as:

> One of the more interesting features in this cycle is the ability to attach eBPF programs (user-defined, sandboxed bytecode executed by the kernel) to kprobes. This allows user-defined instrumentation on a live kernel image that can never crash, hang or interfere with the kernel negatively.
BCC makes eBPF programs easier to write, with kernel instrumentation in C
and a front-end in Python. It is suited for many tasks, including performance
analysis and network traffic control.
BCC makes BPF programs easier to write, with kernel instrumentation in C
(and includes a C wrapper around LLVM), and front-ends in Python and lua.
It is suited for many tasks, including performance analysis and network
traffic control.

## Screenshot

Expand Down Expand Up @@ -170,46 +172,10 @@ The features of this toolkit include:
In the future, more bindings besides python will likely be supported. Feel free
to add support for the language of your choice and send a pull request!

## Tutorial
## Tutorials

The BCC toolchain is currently composed of two parts: a C wrapper around LLVM,
and a Python API to interact with the running program. Later, we will go into
more detail of how this all works.

### Hello, World

First, we should include the BPF class from the bpf module:
```python
from bcc import BPF
```

Since the C code is so short, we will embed it inside the python script.

The BPF program always takes at least one argument, which is a pointer to the
context for this type of program. Different program types have different calling
conventions, but for this one we don't care so `void *` is fine.
```python
BPF(text='int kprobe__sys_clone(void *ctx) { bpf_trace_printk("Hello, World!\\n"); return 0; }').trace_print()
```

For this example, we will call the program every time `fork()` is called by a
userspace process. Underneath the hood, fork translates to the `clone` syscall.
BCC recognizes prefix `kprobe__`, and will auto attach our program to the kernel symbol `sys_clone`.

The python process will then print the trace printk circular buffer until ctrl-c
is pressed. The BPF program is removed from the kernel when the userspace
process that loaded it closes the fd (or exits).

Output:
```
bcc/examples$ sudo python hello_world.py
python-7282 [002] d... 3757.488508: : Hello, World!
```

For an explanation of the meaning of the printed fields, see the trace_pipe
section of the [kernel ftrace doc](https://www.kernel.org/doc/Documentation/trace/ftrace.txt).

[Source code listing](examples/hello_world.py)
- [docs/tutorial.md](docs/tutorial.md): Using bcc tools to solve performance, troubleshooting, and networking issues.
- [docs/tutorial_bcc_python_developer.md](docs/tutorial_bcc_python_developer.md): Developing new bcc programs using the Python interface.

### Networking

Expand All @@ -222,75 +188,6 @@ multiple granularities. See the code [here](examples/networking/tunnel_monitor).

[![Screenshot](http://img.youtube.com/vi/yYy3Cwce02k/0.jpg)](https://youtu.be/yYy3Cwce02k)

### Tracing

Here is a slightly more complex tracing example than Hello World. This program
will be invoked for every task change in the kernel, and record in a BPF map
the new and old pids.

The C program below introduces two new concepts.
The first is the macro `BPF_TABLE`. This defines a table (type="hash"), with key
type `key_t` and leaf type `u64` (a single counter). The table name is `stats`,
containing 1024 entries maximum. One can `lookup`, `lookup_or_init`, `update`,
and `delete` entries from the table.
The second concept is the prev argument. This argument is treated specially by
the BCC frontend, such that accesses to this variable are read from the saved
context that is passed by the kprobe infrastructure. The prototype of the args
starting from position 1 should match the prototype of the kernel function being
kprobed. If done so, the program will have seamless access to the function
parameters.
```c
#include <uapi/linux/ptrace.h>
#include <linux/sched.h>

struct key_t {
u32 prev_pid;
u32 curr_pid;
};
// map_type, key_type, leaf_type, table_name, num_entry
BPF_TABLE("hash", struct key_t, u64, stats, 1024);
// attach to finish_task_switch in kernel/sched/core.c, which has the following
// prototype:
// struct rq *finish_task_switch(struct task_struct *prev)
int count_sched(struct pt_regs *ctx, struct task_struct *prev) {
struct key_t key = {};
u64 zero = 0, *val;

key.curr_pid = bpf_get_current_pid_tgid();
key.prev_pid = prev->pid;

val = stats.lookup_or_init(&key, &zero);
(*val)++;
return 0;
}
```
[Source code listing](examples/tracing/task_switch.c)
The userspace component loads the file shown above, and attaches it to the
`finish_task_switch` kernel function.
The [] operator of the BPF object gives access to each BPF_TABLE in the
program, allowing pass-through access to the values residing in the kernel. Use
the object as you would any other python dict object: read, update, and deletes
are all allowed.
```python
from bcc import BPF
from time import sleep
b = BPF(src_file="task_switch.c")
b.attach_kprobe(event="finish_task_switch", fn_name="count_sched")
# generate many schedule events
for i in range(0, 100): sleep(0.01)
for k, v in b["stats"].items():
print("task_switch[%5d->%5d]=%u" % (k.prev_pid, k.curr_pid, v.value))
```
[Source code listing](examples/tracing/task_switch.py)

## Getting started

See [INSTALL.md](INSTALL.md) for installation steps on your platform.

## Contributing

Already pumped up to commit some code? Here are some resources to join the
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