+++ /dev/null
-
-RFC - SIDE ABI
-
-[ This document is under heavy construction. Please beware of the
- potholes as you wander through it. ]
-
-* Introduction
-
-The purpose of the SIDE ABI is to allow a kernel tracer and many
-user-space tracers to attach to static and dynamic instrumentation of
-user-space applications.
-
-The SIDE ABI expresses the instrumentation description as data (no
-generated code). Instrumentation arguments are passed on the stack as an
-array of typed items, along with a reference to the instrumentation
-description.
-
-The following ABIs are introduced to let applications declare their
-instrumentation and insert instrumentation calls:
-
-- an event description ABI,
-- a type description ABI,
-- an event and type attribute ABI, which allows associating key-value
- tuples to events and types,
-- an ABI defining how applications provide arguments to instrumentation
- calls.
-
-The combination of the type description and type argument ABIs is later
-refered to as the SIDE type system.
-
-The ABI exposed to kernel and user-space tracers allow them to list
-and connect to the instrumentation, and conditionally enables
-instrumentation when at least one tracer is using it.
-
-The type description and type argument ABIs include support for
-statically known types and dynamic types. Nested structures, arrays, and
-variable-length arrays are supported.
-
-The libside C API is a reference implementation of the SIDE ABI for
-instrumentation of C/C++ applications by the Linux kernel through the
-User Events ABI and by user-space tracers following the default calling
-convention (System V ELF ABI on Linux, MS ABI on Windows).
-
-A set of macros is provided with the libside C API for convenience of
-C/C++ application instrumentation.
-
-
-* Genesis
-
-The SIDE ABI and libside library learn from the user feedback about
-experience with LTTng-UST and Linux kernel tracepoints, and therefore
-they introduce significant changes (and vast simplifications) to the way
-instrumentation is done compared to LTTng-UST and Linux kernel
-tracepoints.
-
-- Linux kernel User Events ABI
- - Exposes a stable ABI allowing applications to register their event
- names/field types to the kernel,
- - Can be expected to have a large effect on application instrumentation,
- - My concerns:
- – Should be co-designed with a userspace instrumentation API/ABI rather than only
- focusing on the kernel ABI,
- – Should allow purely userspace tracers to use the same instrumentation as userspace
- tracers implemented within the Linux kernel,
- – Tracers can target their specific use-cases, but infrastructure should be shared,
- – Limit fragmentation of the instrumentation ecosystem.
-
-- Improvements over tracepoints:
- - Improve compiler error reporting vs tracepoints
- - API uses standard header inclusion practices
- - share ABI across runtimes (no need to reimplement tracepoints for
- each language, or to use string only payloads)
-
-- Improvements over SDT: allow expressing additional event semantic
- (e.g. user attributes, versioning, nested and compound data types)
- - libside has less impact on control flow when disabled (no stack setup)
- - SDT ABI is focused on architecture calling conventions, libside ABI
- is easier to use from runtime environments which have an ABI
- different from the native architecture (golang, rust, python, java).
- libside instrumentation ABI calls a small fixed set of functions.
-
-- Comparison with ETW
- - similar to libside in terms of array of arguments,
- - does not support pre-registration of events (static typing)
- - type information received at runtime from the instrumentation
- callsite.
-
-
-* Desiderata
-
-- Common instrumentation for kernel and purely userspace tracers,
- - Instrumentation is self-described,
- - Support compound and nested types,
- - Support pre-registration of events,
- - Do not rely on compiled event-specific code,
- - Independent from ELF,
- - Simple ABI for instrumented code, kernel, and user-space tracers,
- - Support concurrent tracers,
- - Expose API to allow dynamic instrumentation libraries to register
- their events/payloads.
-
-- Support statically typed instrumentation
-
-- Support dynamically typed instrumentation
- - Natively cover dynamically-typed languages
- - The support for events with dynamic fields allows lessening the number
- of statically declared events in situation where an application
- possesses seldom-used events with a large variety of parameter types.
- - The support for mixed static and dynamic event fields allows
- implementation of post-processing string formatting along with a
- variadic payload, while keeping trace data in a structured format.
-
-- Performance considerations for userspace tracers.
- - Maintain performance characteristics comparable to existing
- userspace tracers.
- - Low overhead, good scalability when used by userspace tracers.
-
-- Allows tracing user-space through a kernel tracer. Even through it is
- an approach that adds more overhead, it has the benefit of not
- requiring agent threads to be deployed into applications, which is
- useful to trace locked-down processes.
-
-- Instrumentation registration APIs
- - Instrumentation can be generated at runtime
- - dynamic patching,
- - JIT
- - Instrumentation can be declared statically (static instrumentation)
- - Instrumentation can be enabled dynamically.
- - Very low overhead when not in use.
-
-- libside must be extensible in the future.
- - Extension scheme should allow adding new types in the future without
- requiring complex logic to future-proof tracers.
- - Exposed types are invariant,
- - libside ABI and API can be extended by adding new types.
-
-- the side ABI should allow multiple instances and versions within
- a process (e.g. libside for C/C++, Java side ABI, Python side ABI...).
-
-- Both event description and payload are data (no generated text).
- - It allows tracers to directly interpret the event payload from their
- description, removing the need for code generation. This lessens the
- instruction cache pollution compared to code generation approaches.
- - Tracer interpreter for filtering and field capture can directly use
- the instrumentation data, without need for setting up a structured
- argument layout on the stack within the tracer.
-
-- Validation of argument vs event description coherence.
-
-- Passing arguments to events should be:
- - Conveniently express application data structures to be expected as
- instrumentation input.
- - Flexible,
- - Efficient,
- - If all are not possible combined, specialize types for each purpose.
-
-- Allow tracers to passively collect application state transitions.
-
-- Allow tracers to actively sample the current state of an application.
-
-- Error messages generated when misusing the API should be easy to
- comprehend and resolve.
-
-- Allow expressing additional custom semantic augmenting events and
- types.
-
-
-* Design / Architecture
-
-- Compiler error messages are easy to understand because it is a simple
- header file without any repeated inclusion tricks.
-
-- Variadic events.
-
-- Instrumentation API/ABI:
- – Type system,
- - Type visitor callbacks
- - (perfetto)
- - Stack-copy types
- - Data-gathering types
- - Dynamic types.
- – Helper macros for C/C++,
- – Express instrumentation description as data,
- – Instrumentation arguments are passed on the stack as a data array
- (similar to iovec) along with a reference to instrumentation
- description,
- – Instrumentation is conditionally enabled when at least one tracer is
- registered to it.
-
-- Tracer-agnostic API/ABI:
- – Available events notifications,
- – Conditionally enabling instrumentation,
- – Synchronize registered user-space tracer callbacks with RCU,
- – Co-designed to interact with User Events.
-
-- Application state dump
- - How are applications/libraries meant to provide state information ?
- - How are tracers meant to interact with state dump ?
- - statedump mode polling
- - statedump mode agent thread
-
-- RCU to synchronize userspace tracers registration vs invocation
-
-- How tracers are meant to interact with libside ?
-
-- How is C/C++ language instrumentation is meant to be used ?
-
-- How are dynamic instrumentation facilities meant to interact with
- libside ?
-
-- How is a kernel tracer meant to interact with libside ?
-
-- How is gdb (ptrace) meant to interact with libside ?
-
-- Validation that instrumentation arguments match event description
- fields cannot be done by the compiler, requires either:
- - run time check,
- - static checker (only for static instrumentation).
-
-- Event attributes.
-
-- Type attributes.
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