docs/tracing/design.md - zircon - Git at Google

 # Fuchsia Tracing System Design

 This document describes a mechanism for collecting diagnostic trace information
 from running applications on the Fuchsia operating system.

 ## Overview

 The purpose of Fuchsia tracing is to provide a means to collect, aggregate,
 and visualize diagnostic tracing information from Fuchsia user space
 processes and from the Zircon kernel.

 ## Design Goals

 - Lightweight Instrumentation
   - Enabling tracing should not significantly affect the performance of running
     applications.  Trace providers should not need to acquire locks, make
     syscalls, or perform dynamic memory allocation required between the time
     when tracing is activated and when it is disabled.
 - Compact Memory Footprint
   - Trace records are stored compactly in memory so that buffers can remain
     small but hold many events.
 - Crash-proof
   - It is possible to collect partial traces even if trace providers
     terminate (normally or abnormally) during trace collection.
 - Flexible and Universal
   - Can trace code written in any language given a suitable implementation of
     the tracing library.
   - Trace points can be manually inserted by the developer or generated
     dynamically by tools.
 - General
   - The trace format defines general purpose record types which support a
     wide range of data collection needs.
   - Trace data can be transformed into other formats for visualization using
     tools such as Catapult or TraceViz.
 - Extensible
   - New record types can be added in the future without breaking existing tools.
 - Robust
   - Enabling tracing does not compromise the integrity of running components
     or expose them to manipulation by tracing clients.

 ## Moving Parts

 ### Trace Manager

 The trace manager is a system service which coordinates registration of
 trace providers.  It ensures that tracing proceeds in an orderly manner
 and isolates components which offer trace providers from trace clients.

 The trace manager implements two FIDL interfaces:

 - `TraceController`: Provides trace clients with the ability to enumerate
   trace providers and collect trace data.
 - `TraceRegistry`: Provides trace providers with the ability to register
   themselves at runtime so that they can be discovered by the tracing system.

 TODO: The `TraceRegistry` should be replaced by a `Namespace` based approach
 to publish trace providers from components.

 ### Trace Providers

 Components which can be traced or offer tracing information to the system
 implement the `TraceProvider` FIDL interface and register it with the
 `TraceRegistry`.  Once registered, they will receive messages whenever
 tracing is started or stopped and will have the opportunity to provide
 trace data encoded in the [Fuchsia Trace Format](trace_format.md).

 #### Kernel Trace Provider

 The `ktrace_provider` program ingests kernel trace events and publishes
 trace records.  This allows kernel trace data to be captured and visualized
 together with userspace trace data.

 ### Trace Client

 The `trace` program offers command-line access to tracing functionality
 for developers.  It also supports converting Fuchsia trace archives into
 other formats, such as Catapult JSON records which can be visualized
 using Catapult (aka. chrome:://tracing).

 Trace information can also be collected programmatically by using the
 `TraceController` FIDL interface directly.

 ## Libraries

 ### libtrace: The C and C++ Trace Event Library

 Provides macros and inline functions for instrumenting C and C++ programs
 with trace points for capturing trace data during trace execution.

 See `<trace/event.h>`.

 #### C++ Example

 This example records trace events marking the beginning and end of the
 execution of the "DoSomething" function together with its parameters.

 ```c++
 #include <trace/event.h>

 void DoSomething(int a, std::string b) {
   TRACE_DURATION("example", "DoSomething", "a", a, "b", b);

   // Do something
 }
 ```

 #### C Example

 This example records trace events marking the beginning and end of the
 execution of the "DoSomething" function together with its parameters.

 Unlike in C++, it is necessary to specify the type of each trace argument.
 In C++ such annotations are supported but are optional since the compiler
 can infer the type itself.

 ```c
 #include <trace/event.h>

 void DoSomething(int a, const char* b) {
   TRACE_DURATION("example", "DoSomething", "a", TA_INT32(a), "b", TA_STRING(b));

   // Do something
 }
 ```

 #### Suppressing Tracing Within a Compilation Unit

 To completely suppress tracing within a compilation unit, define the NTRACE
 macro prior to including the trace headers.  This causes the macros to
 behave as if tracing is always disabled so they will not produce trace
 records and they will have zero runtime overhead.

 ```c
 #define NTRACE
 #include <trace/event.h>

 void DoSomething(void) {
   // This will never produce trace records because the NTRACE macro was
   // defined above.
   TRACE_DURATION("example", "DoSomething");
 }
 ```

 ### libtrace-provider: Trace Provider Library

 This library provides C and C++ functions to register a process's trace
 engine with the Fuchsia tracing system.  For tracing to work in your process,
 you must initialize the trace provider at some point during its execution
 (or implement your own trace handler to register the trace engine some
 other way).

 The trace provider requires an asynchronous dispatcher to operate.

 #### C++ Example

 ```c++
 #include <lib/async-loop/cpp/loop.h>
 #include <trace-provider/provider.h>

 int main(int argc, char** argv) {
   // Create a message loop.
    async::Loop loop(&kAsyncLoopConfigNoAttachToThread);

   // Start a thread for the loop to run on.
   // We could instead use async_loop_run() to run on the current thread.
   zx_status_t status = loop.StartThread();
   if (status != ZX_OK) exit(1);

   // Create the trace provider.
   trace::TraceProvider trace_provider(loop.dispatcher());

   // Do something...

   // The loop and trace provider will shut down once the scope exits.
   return 0;
 }
 ```

 #### C Example

 ```c
 #include <lib/async-loop/cpp/loop.h>
 #include <trace-provider/provider.h>

 int main(int argc, char** argv) {
   zx_status_t status;
   async_loop_t* loop;
   trace_provider_t* trace_provider;

   // Create a message loop.
   status = async_loop_create(&kAsyncLoopConfigNoAttachToThread, &loop);
   if (status != ZX_OK) exit(1);

   // Start a thread for the loop to run on.
   // We could instead use async_loop_run() to run on the current thread.
   status = async_loop_start_thread(loop, "loop", NULL);
   if (status != ZX_OK) exit(1);

   // Create the trace provider.
   async_dispatcher_t* dispatcher = async_loop_get_dispatcher(loop);
   trace_provider = trace_provider_create(dispatcher);
   if (!trace_provider) exit(1);

   // Do something...

   // Tear down.
   trace_provider_destroy(trace_provider);
   async_loop_shutdown(loop);
   return 0;
 }
 ```

 ### libtrace-reader: Trace Reader Library

 Provides C++ types and functions for reading trace archives.

 See `<trace-reader/reader.h>`.

 ## Transport Protocol

 When the developer initiates tracing, the trace manager asks all relevant
 trace providers to start tracing and provides each one with a trace buffer
 VMO into which they should write their trace records.

 While a trace is running, the trace manager continues watching for newly
 registered trace providers and activates them if needed.

 What happens when a trace provider's trace buffer becomes full while a trace
 is running depends on the buffering mode.
 See [Buffering Modes](#Buffering Modes) below.

 When tracing finishes, the trace manager asks all of the active trace providers
 to stop tracing then waits a short time for them to acknowledge that they
 have finished writing out their trace events.

 The trace manager then reads and validates trace data written into the trace
 buffer VMOs by trace providers and creates a trace archive.  The trace manager
 can often recover partial data even when trace providers terminate abnormally
 as long as they managed to store some data into their trace buffers.
 Note that in streaming mode the trace manager only needs to save the
 currently active rolling buffer.
 See [Buffering Modes](#Buffering Modes) below.

 The trace manager delivers the resulting trace archive to its client through
 a socket.  This data is guaranteed to be well-formed according to the
 Fuchsia trace format (but it may be nonsensical if trace providers
 deliberately emit garbage data).

 These are some important invariants of the transport protocol:
 - There are no synchronization points between the trace manager and trace
   providers other than starting or stopping collection.
 - Trace providers (components being traced) only ever write to trace buffers;
   they never read from them.
 - The trace manager only ever reads from trace buffers; it never writes to them.
 - Trace clients never see the original trace buffers; they receive trace
   archives over a socket from the trace manager.  This protects trace providers
   from manipulation by trace clients.

 ## Buffering Modes

 There are three buffering modes: oneshot, circular, and streaming.
 They specify different behaviors when the trace buffer fills.

 Note that in all cases trace provider behavior is independent of each other.
 Other trace providers can continue to record trace events into their own
 buffers as usual until the trace stops, even as one provider's buffer fills.
 This may result in a partially incomplete trace.

 ### Oneshot

 If the buffer becomes full then that trace provider will stop recording events.

 ### Circular

 The trace buffer is effectively divided into three pieces: the "durable" buffer
 and two "rolling" buffers. The durable buffer is for records important enough
 that we don't want to risk dropping them. These include records for thread and
 string references.

 Tracing begins by writing to the first rolling buffer. Once one rolling buffer
 fills tracing continues by writing to the other one.

 If the durable buffer fills then tracing for the provider stops. Tracing in
 other providers continues as usual.

 ### Streaming

 The trace buffer is effectively divided into three pieces: the "durable" buffer
 and two "rolling" buffers. The durable buffer is for records important enough
 that we don't want to risk dropping them. These include records for thread and
 string references.

 Tracing begins by writing to the first rolling buffer. Once one rolling buffer
 fills tracing continues by writing to the other one, if it is available, and
 notifying the trace manager that the buffer is full. If the other rolling
 buffer is not available, then records are dropped until it becomes available.
 The other rolling buffer is unavailable between the point when it filled and
 when the manager reports back that the buffer's contents have been saved.

 Whether records get dropped depends on the rate at which records are created
 vs the rate at which the trace manager can save the buffers. This can result
 in a partially incomplete trace, but is less important than perturbing program
 performance by waiting for a buffer to be saved.

 If the durable buffer fills then tracing for the provider stops. Tracing in
 other providers continues as usual.

 ## Trace Manager/Provider FIFO Protocol

 Notification of trace provider startup and shutdown is done via a FIFO,
 the handle of which is passed from the trace manager to each trace provider
 as part of the initial "start tracing" request. The form of each message is
 defined in `<trace-provider/provider.h>`. Packets are fixed size with the
 following format:

 ```cpp
 typedef struct trace_provider_packet {
     // One of TRACE_PROVIDER_*.
     uint16_t request;

     // For alignment and future concerns, must be zero.
     uint16_t reserved;

     // Optional data for the request.
     // The contents depend on the request.
     // If unused they must be passed as zero.
     uint32_t data32;
     uint64_t data64;
 } trace_provider_packet_t;
 ```

 ### FIFO Packets

 The following packets are defined:

 **TRACE_PROVIDER_STARTED**

 Sent from trace providers to the trace manager.
 Notify the trace manager that the provider has received the "start tracing"
 request and is starting to collect trace data.
 The `data32` field of the packet contains the version number of the FIFO
 protocol that the provider is using. The value is specified by
 **TRACE_PROVIDER_FIFO_PROTOCOL_VERSION** in `<trace-provider/provider.h>`.
 If the trace manager sees a protocol it doesn't understand it will close
 its side of the FIFO and ignore all trace data from the provider.

 **TRACE_PROVIDER_SAVE_BUFFER**

 Sent from trace providers to the trace manager in streaming mode.
 Notify the trace manager that a buffer is full and needs saving.
 This request is only used in streaming mode.
 The `data32` field contains the "wrap count" which is the number of times
 writing has switched from one buffer to the next. The buffer that needs saving
 is `(data32 & 1)`.
 The `data64` field contains the offset of the end of data written to the
 "durable" buffer.

 Only one buffer save request may be sent at a time. The next one cannot be
 sent until **TRACE_PROVIDER_BUFFER_SAVED** is received acknowledging the
 previous request.

 **TRACE_PROVIDER_BUFFER_SAVED**

 Sent from the trace manager to trace providers in streaming mode.
 Notify the trace provider that the requested buffer has been saved.
 The `data32` and `data64` fields must have the same values from the
 originating **TRACE_PROVIDER_SAVE_BUFFER** request.
	# Fuchsia Tracing System Design

	This document describes a mechanism for collecting diagnostic trace information
	from running applications on the Fuchsia operating system.

	## Overview

	The purpose of Fuchsia tracing is to provide a means to collect, aggregate,
	and visualize diagnostic tracing information from Fuchsia user space
	processes and from the Zircon kernel.

	## Design Goals

	- Lightweight Instrumentation
	- Enabling tracing should not significantly affect the performance of running
	applications. Trace providers should not need to acquire locks, make
	syscalls, or perform dynamic memory allocation required between the time
	when tracing is activated and when it is disabled.
	- Compact Memory Footprint
	- Trace records are stored compactly in memory so that buffers can remain
	small but hold many events.
	- Crash-proof
	- It is possible to collect partial traces even if trace providers
	terminate (normally or abnormally) during trace collection.
	- Flexible and Universal
	- Can trace code written in any language given a suitable implementation of
	the tracing library.
	- Trace points can be manually inserted by the developer or generated
	dynamically by tools.
	- General
	- The trace format defines general purpose record types which support a
	wide range of data collection needs.
	- Trace data can be transformed into other formats for visualization using
	tools such as Catapult or TraceViz.
	- Extensible
	- New record types can be added in the future without breaking existing tools.
	- Robust
	- Enabling tracing does not compromise the integrity of running components
	or expose them to manipulation by tracing clients.

	## Moving Parts

	### Trace Manager

	The trace manager is a system service which coordinates registration of
	trace providers. It ensures that tracing proceeds in an orderly manner
	and isolates components which offer trace providers from trace clients.

	The trace manager implements two FIDL interfaces:

	- `TraceController`: Provides trace clients with the ability to enumerate
	trace providers and collect trace data.
	- `TraceRegistry`: Provides trace providers with the ability to register
	themselves at runtime so that they can be discovered by the tracing system.

	TODO: The `TraceRegistry` should be replaced by a `Namespace` based approach
	to publish trace providers from components.

	### Trace Providers

	Components which can be traced or offer tracing information to the system
	implement the `TraceProvider` FIDL interface and register it with the
	`TraceRegistry`. Once registered, they will receive messages whenever
	tracing is started or stopped and will have the opportunity to provide
	trace data encoded in the [Fuchsia Trace Format](trace_format.md).

	#### Kernel Trace Provider

	The `ktrace_provider` program ingests kernel trace events and publishes
	trace records. This allows kernel trace data to be captured and visualized
	together with userspace trace data.

	### Trace Client

	The `trace` program offers command-line access to tracing functionality
	for developers. It also supports converting Fuchsia trace archives into
	other formats, such as Catapult JSON records which can be visualized
	using Catapult (aka. chrome:://tracing).

	Trace information can also be collected programmatically by using the
	`TraceController` FIDL interface directly.

	## Libraries

	### libtrace: The C and C++ Trace Event Library

	Provides macros and inline functions for instrumenting C and C++ programs
	with trace points for capturing trace data during trace execution.

	See `<trace/event.h>`.

	#### C++ Example

	This example records trace events marking the beginning and end of the
	execution of the "DoSomething" function together with its parameters.

	```c++
	#include <trace/event.h>

	void DoSomething(int a, std::string b) {
	TRACE_DURATION("example", "DoSomething", "a", a, "b", b);

	// Do something
	}
	```

	#### C Example

	This example records trace events marking the beginning and end of the
	execution of the "DoSomething" function together with its parameters.

	Unlike in C++, it is necessary to specify the type of each trace argument.
	In C++ such annotations are supported but are optional since the compiler
	can infer the type itself.

	```c
	#include <trace/event.h>

	void DoSomething(int a, const char* b) {
	TRACE_DURATION("example", "DoSomething", "a", TA_INT32(a), "b", TA_STRING(b));

	// Do something
	}
	```

	#### Suppressing Tracing Within a Compilation Unit

	To completely suppress tracing within a compilation unit, define the NTRACE
	macro prior to including the trace headers. This causes the macros to
	behave as if tracing is always disabled so they will not produce trace
	records and they will have zero runtime overhead.

	```c
	#define NTRACE
	#include <trace/event.h>

	void DoSomething(void) {
	// This will never produce trace records because the NTRACE macro was
	// defined above.
	TRACE_DURATION("example", "DoSomething");
	}
	```

	### libtrace-provider: Trace Provider Library

	This library provides C and C++ functions to register a process's trace
	engine with the Fuchsia tracing system. For tracing to work in your process,
	you must initialize the trace provider at some point during its execution
	(or implement your own trace handler to register the trace engine some
	other way).

	The trace provider requires an asynchronous dispatcher to operate.

	#### C++ Example

	```c++
	#include <lib/async-loop/cpp/loop.h>
	#include <trace-provider/provider.h>

	int main(int argc, char** argv) {
	// Create a message loop.
	async::Loop loop(&kAsyncLoopConfigNoAttachToThread);

	// Start a thread for the loop to run on.
	// We could instead use async_loop_run() to run on the current thread.
	zx_status_t status = loop.StartThread();
	if (status != ZX_OK) exit(1);

	// Create the trace provider.
	trace::TraceProvider trace_provider(loop.dispatcher());

	// Do something...

	// The loop and trace provider will shut down once the scope exits.
	return 0;
	}
	```

	#### C Example

	```c
	#include <lib/async-loop/cpp/loop.h>
	#include <trace-provider/provider.h>

	int main(int argc, char** argv) {
	zx_status_t status;
	async_loop_t* loop;
	trace_provider_t* trace_provider;

	// Create a message loop.
	status = async_loop_create(&kAsyncLoopConfigNoAttachToThread, &loop);
	if (status != ZX_OK) exit(1);

	// Start a thread for the loop to run on.
	// We could instead use async_loop_run() to run on the current thread.
	status = async_loop_start_thread(loop, "loop", NULL);
	if (status != ZX_OK) exit(1);

	// Create the trace provider.
	async_dispatcher_t* dispatcher = async_loop_get_dispatcher(loop);
	trace_provider = trace_provider_create(dispatcher);
	if (!trace_provider) exit(1);

	// Do something...

	// Tear down.
	trace_provider_destroy(trace_provider);
	async_loop_shutdown(loop);
	return 0;
	}
	```

	### libtrace-reader: Trace Reader Library

	Provides C++ types and functions for reading trace archives.

	See `<trace-reader/reader.h>`.

	## Transport Protocol

	When the developer initiates tracing, the trace manager asks all relevant
	trace providers to start tracing and provides each one with a trace buffer
	VMO into which they should write their trace records.

	While a trace is running, the trace manager continues watching for newly
	registered trace providers and activates them if needed.

	What happens when a trace provider's trace buffer becomes full while a trace
	is running depends on the buffering mode.
	See [Buffering Modes](#Buffering Modes) below.

	When tracing finishes, the trace manager asks all of the active trace providers
	to stop tracing then waits a short time for them to acknowledge that they
	have finished writing out their trace events.

	The trace manager then reads and validates trace data written into the trace
	buffer VMOs by trace providers and creates a trace archive. The trace manager
	can often recover partial data even when trace providers terminate abnormally
	as long as they managed to store some data into their trace buffers.
	Note that in streaming mode the trace manager only needs to save the
	currently active rolling buffer.
	See [Buffering Modes](#Buffering Modes) below.

	The trace manager delivers the resulting trace archive to its client through
	a socket. This data is guaranteed to be well-formed according to the
	Fuchsia trace format (but it may be nonsensical if trace providers
	deliberately emit garbage data).

	These are some important invariants of the transport protocol:
	- There are no synchronization points between the trace manager and trace
	providers other than starting or stopping collection.
	- Trace providers (components being traced) only ever write to trace buffers;
	they never read from them.
	- The trace manager only ever reads from trace buffers; it never writes to them.
	- Trace clients never see the original trace buffers; they receive trace
	archives over a socket from the trace manager. This protects trace providers
	from manipulation by trace clients.

	## Buffering Modes

	There are three buffering modes: oneshot, circular, and streaming.
	They specify different behaviors when the trace buffer fills.

	Note that in all cases trace provider behavior is independent of each other.
	Other trace providers can continue to record trace events into their own
	buffers as usual until the trace stops, even as one provider's buffer fills.
	This may result in a partially incomplete trace.

	### Oneshot

	If the buffer becomes full then that trace provider will stop recording events.

	### Circular

	The trace buffer is effectively divided into three pieces: the "durable" buffer
	and two "rolling" buffers. The durable buffer is for records important enough
	that we don't want to risk dropping them. These include records for thread and
	string references.

	Tracing begins by writing to the first rolling buffer. Once one rolling buffer
	fills tracing continues by writing to the other one.

	If the durable buffer fills then tracing for the provider stops. Tracing in
	other providers continues as usual.

	### Streaming

	The trace buffer is effectively divided into three pieces: the "durable" buffer
	and two "rolling" buffers. The durable buffer is for records important enough
	that we don't want to risk dropping them. These include records for thread and
	string references.

	Tracing begins by writing to the first rolling buffer. Once one rolling buffer
	fills tracing continues by writing to the other one, if it is available, and
	notifying the trace manager that the buffer is full. If the other rolling
	buffer is not available, then records are dropped until it becomes available.
	The other rolling buffer is unavailable between the point when it filled and
	when the manager reports back that the buffer's contents have been saved.

	Whether records get dropped depends on the rate at which records are created
	vs the rate at which the trace manager can save the buffers. This can result
	in a partially incomplete trace, but is less important than perturbing program
	performance by waiting for a buffer to be saved.

	If the durable buffer fills then tracing for the provider stops. Tracing in
	other providers continues as usual.

	## Trace Manager/Provider FIFO Protocol

	Notification of trace provider startup and shutdown is done via a FIFO,
	the handle of which is passed from the trace manager to each trace provider
	as part of the initial "start tracing" request. The form of each message is
	defined in `<trace-provider/provider.h>`. Packets are fixed size with the
	following format:

	```cpp
	typedef struct trace_provider_packet {
	// One of TRACE_PROVIDER_*.
	uint16_t request;

	// For alignment and future concerns, must be zero.
	uint16_t reserved;

	// Optional data for the request.
	// The contents depend on the request.
	// If unused they must be passed as zero.
	uint32_t data32;
	uint64_t data64;
	} trace_provider_packet_t;
	```

	### FIFO Packets

	The following packets are defined:

	TRACE_PROVIDER_STARTED

	Sent from trace providers to the trace manager.
	Notify the trace manager that the provider has received the "start tracing"
	request and is starting to collect trace data.
	The `data32` field of the packet contains the version number of the FIFO
	protocol that the provider is using. The value is specified by
	TRACE_PROVIDER_FIFO_PROTOCOL_VERSION in `<trace-provider/provider.h>`.
	If the trace manager sees a protocol it doesn't understand it will close
	its side of the FIFO and ignore all trace data from the provider.

	TRACE_PROVIDER_SAVE_BUFFER

	Sent from trace providers to the trace manager in streaming mode.
	Notify the trace manager that a buffer is full and needs saving.
	This request is only used in streaming mode.
	The `data32` field contains the "wrap count" which is the number of times
	writing has switched from one buffer to the next. The buffer that needs saving
	is `(data32 & 1)`.
	The `data64` field contains the offset of the end of data written to the
	"durable" buffer.

	Only one buffer save request may be sent at a time. The next one cannot be
	sent until TRACE_PROVIDER_BUFFER_SAVED is received acknowledging the
	previous request.

	TRACE_PROVIDER_BUFFER_SAVED

	Sent from the trace manager to trace providers in streaming mode.
	Notify the trace provider that the requested buffer has been saved.
	The `data32` and `data64` fields must have the same values from the
	originating TRACE_PROVIDER_SAVE_BUFFER request.