12 KiB
User-space, Statically Defined Tracing (USDT) for Dash Core
Dash Core includes statically defined tracepoints to allow for more observability during development, debugging, code review, and production usage. These tracepoints make it possible to keep track of custom statistics and enable detailed monitoring of otherwise hidden internals. They have little to no performance impact when unused.
eBPF and USDT Overview
======================
┌──────────────────┐ ┌──────────────┐
│ tracing script │ │ dashd │
│==================│ 2. │==============│
│ eBPF │ tracing │ hooks │ │
│ code │ logic │ into┌─┤►tracepoint 1─┼───┐ 3.
└────┬───┴──▲──────┘ ├─┤►tracepoint 2 │ │ pass args
1. │ │ 4. │ │ ... │ │ to eBPF
User compiles │ │ pass data to │ └──────────────┘ │ program
Space & loads │ │ tracing script │ │
─────────────────┼──────┼─────────────────┼────────────────────┼───
Kernel │ │ │ │
Space ┌──┬─▼──────┴─────────────────┴────────────┐ │
│ │ eBPF program │◄──────┘
│ └───────────────────────────────────────┤
│ eBPF kernel Virtual Machine (sandboxed) │
└──────────────────────────────────────────┘
1. The tracing script compiles the eBPF code and loads the eBPF program into a kernel VM
2. The eBPF program hooks into one or more tracepoints
3. When the tracepoint is called, the arguments are passed to the eBPF program
4. The eBPF program processes the arguments and returns data to the tracing script
The Linux kernel can hook into the tracepoints during runtime and pass data to sandboxed eBPF programs running in the kernel. These eBPF programs can, for example, collect statistics or pass data back to user-space scripts for further processing.
The two main eBPF front-ends with support for USDT are bpftrace and
BPF Compiler Collection (BCC). BCC is used for complex tools and daemons and
bpftrace is preferred for one-liners and shorter scripts. Examples for both can
be found in contrib/tracing.
Tracepoint documentation
The currently available tracepoints are listed here.
Context net
Tracepoint net:inbound_message
Is called when a message is received from a peer over the P2P network. Passes information about our peer, the connection and the message as arguments.
Arguments passed:
- Peer ID as
int64 - Peer Address and Port (IPv4, IPv6, Tor v3, I2P, ...) as
pointer to C-style String(max. length 68 characters) - Connection Type (inbound, feeler, outbound-full-relay, ...) as
pointer to C-style String(max. length 20 characters) - Message Type (inv, ping, getdata, addrv2, ...) as
pointer to C-style String(max. length 20 characters) - Message Size in bytes as
uint64 - Message Bytes as
pointer to unsigned chars(i.e. bytes)
Note: The message is passed to the tracepoint in full, however, due to space limitations in the eBPF kernel VM it might not be possible to pass the message to user-space in full. Messages longer than a 32kb might be cut off. This can be detected in tracing scripts by comparing the message size to the length of the passed message.
Tracepoint net:outbound_message
Is called when a message is send to a peer over the P2P network. Passes information about our peer, the connection and the message as arguments.
Arguments passed:
- Peer ID as
int64 - Peer Address and Port (IPv4, IPv6, Tor v3, I2P, ...) as
pointer to C-style String(max. length 68 characters) - Connection Type (inbound, feeler, outbound-full-relay, ...) as
pointer to C-style String(max. length 20 characters) - Message Type (inv, ping, getdata, addrv2, ...) as
pointer to C-style String(max. length 20 characters) - Message Size in bytes as
uint64 - Message Bytes as
pointer to unsigned chars(i.e. bytes)
Note: The message is passed to the tracepoint in full, however, due to space limitations in the eBPF kernel VM it might not be possible to pass the message to user-space in full. Messages longer than a 32kb might be cut off. This can be detected in tracing scripts by comparing the message size to the length of the passed message.
Context validation
Tracepoint validation:block_connected
Is called after a block is connected to the chain. Can, for example, be used
to benchmark block connections together with -reindex.
Arguments passed:
- Block Header Hash as
pointer to C-style String(64 characters) - Block Height as
int32 - Transactions in the Block as
uint64 - Inputs spend in the Block as
int32 - SigOps in the Block (excluding coinbase SigOps)
uint64 - Time it took to connect the Block in microseconds (µs) as
uint64 - Block Header Hash as
pointer to unsigned chars(i.e. 32 bytes in little-endian)
Note: The 7th argument can't be accessed by bpftrace and is purposefully chosen to be the block header hash as bytes. See bpftrace argument limit for more details.
Adding tracepoints to Dash Core
To add a new tracepoint, #include <util/trace.h> in the compilation unit where
the tracepoint is inserted. Use one of the TRACEx macros listed below
depending on the number of arguments passed to the tracepoint. Up to 12
arguments can be provided. The context and event specify the names by which
the tracepoint is referred to. Please use snake_case and try to make sure that
the tracepoint names make sense even without detailed knowledge of the
implementation details. Do not forget to update the tracepoint list in this
document.
#define TRACE(context, event)
#define TRACE1(context, event, a)
#define TRACE2(context, event, a, b)
#define TRACE3(context, event, a, b, c)
#define TRACE4(context, event, a, b, c, d)
#define TRACE5(context, event, a, b, c, d, e)
#define TRACE6(context, event, a, b, c, d, e, f)
#define TRACE7(context, event, a, b, c, d, e, f, g)
#define TRACE8(context, event, a, b, c, d, e, f, g, h)
#define TRACE9(context, event, a, b, c, d, e, f, g, h, i)
#define TRACE10(context, event, a, b, c, d, e, f, g, h, i, j)
#define TRACE11(context, event, a, b, c, d, e, f, g, h, i, j, k)
#define TRACE12(context, event, a, b, c, d, e, f, g, h, i, j, k, l)
For example:
TRACE6(net, inbound_message,
pnode->GetId(),
pnode->m_addr_name.c_str(),
pnode->ConnectionTypeAsString().c_str(),
sanitizedType.c_str(),
msg.data.size(),
msg.data.data()
);
Guidelines and best practices
Clear motivation and use-case
Tracepoints need a clear motivation and use-case. The motivation should outweigh the impact on, for example, code readability. There is no point in adding tracepoints that don't end up being used.
Provide an example
When adding a new tracepoint, provide an example. Examples can show the use case and help reviewers testing that the tracepoint works as intended. The examples can be kept simple but should give others a starting point when working with the tracepoint. See existing examples in contrib/tracing/.
No expensive computations for tracepoints
Data passed to the tracepoint should be inexpensive to compute. Although the tracepoint itself only has overhead when enabled, the code to compute arguments is always run - even if the tracepoint is not used. For example, avoid serialization and parsing.
Semi-stable API
Tracepoints should have a semi-stable API. Users should be able to rely on the tracepoints for scripting. This means tracepoints need to be documented, and the argument order ideally should not change. If there is an important reason to change argument order, make sure to document the change and update the examples using the tracepoint.
eBPF Virtual Machine limits
Keep the eBPF Virtual Machine limits in mind. eBPF programs receiving data from the tracepoints run in a sandboxed Linux kernel VM. This VM has a limited stack size of 512 bytes. Check if it makes sense to pass larger amounts of data, for example, with a tracing script that can handle the passed data.
bpftrace argument limit
While tracepoints can have up to 12 arguments, bpftrace scripts currently only
support reading from the first six arguments (arg0 till arg5) on x86_64.
bpftrace currently lacks real support for handling and printing binary data,
like block header hashes and txids. When a tracepoint passes more than six
arguments, then string and integer arguments should preferably be placed in the
first six argument fields. Binary data can be placed in later arguments. The BCC
supports reading from all 12 arguments.
Strings as C-style String
Generally, strings should be passed into the TRACEx macros as pointers to
C-style strings (a null-terminated sequence of characters). For C++
std::strings, c_str() can be used. It's recommended to document the
maximum expected string size if known.
Listing available tracepoints
Multiple tools can list the available tracepoints in a dashd binary with
USDT support.
GDB - GNU Project Debugger
To list probes in Dash Core, use info probes in gdb:
$ gdb ./src/dashd
…
(gdb) info probes
Type Provider Name Where Semaphore Object
stap net inbound_message 0x000000000014419e /src/dashd
stap net outbound_message 0x0000000000107c05 /src/dashd
stap validation block_connected 0x00000000002fb10c /src/dashd
…
With readelf
The readelf tool can be used to display the USDT tracepoints in Dash Core.
Look for the notes with the description NT_STAPSDT.
$ readelf -n ./src/dashd | grep NT_STAPSDT -A 4 -B 2
Displaying notes found in: .note.stapsdt
Owner Data size Description
stapsdt 0x0000005d NT_STAPSDT (SystemTap probe descriptors)
Provider: net
Name: outbound_message
Location: 0x0000000000107c05, Base: 0x0000000000579c90, Semaphore: 0x0000000000000000
Arguments: -8@%r12 8@%rbx 8@%rdi 8@192(%rsp) 8@%rax 8@%rdx
…
With tplist
The tplist tool is provided by BCC (see Installing BCC). It displays kernel
tracepoints or USDT probes and their formats (for more information, see the
tplist usage demonstration). There are slight binary naming differences
between distributions. For example, on
Ubuntu the binary is called tplist-bpfcc.
$ tplist -l ./src/dashd -v
b'net':b'outbound_message' [sema 0x0]
1 location(s)
6 argument(s)
…