RFC-0013: Named Trace Buffers

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📄 RFC Doc: 0013-named-trace-buffers.md

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Named Trace Buffers

Authors: @primiano

Status: Implemented
PR: #4112

Problem

Currently, when configuring a Perfetto trace, data sources reference buffers
using a 0-based index (target_buffer: N). This index corresponds to the
position of the buffer in the TraceConfig.buffers array.

This addressing scheme is error-prone for humans:

• Easy to make off-by-one errors
• When adding or removing buffers, all indices may need to be updated
• Hard to understand which buffer a data source targets without counting
• Config files become harder to maintain in a centralized manner. Internally we generate configs from a higher level language (GCL) and the translation is harder with indexes.

Decision

Add support for named buffer addressing while maintaining full backwards
compatibility with index-based addressing.

Design

Proto Changes

TraceConfig.BufferConfig gets a new optional name field:

message BufferConfig {
  optional uint32 size_kb = 1;
  optional FillPolicy fill_policy = 4;
  // ... existing fields ...
  optional string name = 7;  // New field
}

DataSourceConfig gets a new optional target_buffer_name field:

message DataSourceConfig {
  optional string name = 1;
  optional uint32 target_buffer = 2;
  // ... existing fields ...
  optional string target_buffer_name = 11;  // New field
}

Resolution Logic

When setting up a tracing session, TracingServiceImpl will:

1. Build a map of buffer names to indices for buffers that have names
2. Validate that buffer names are unique within the session
3. For each data source:
   • If only target_buffer is set: use it (current behavior)
   • If only target_buffer_name is set: look up the index from the map
   • If both are set: verify they resolve to the same buffer index
   • If target_buffer_name is set but not found: return an error

Backwards Compatibility

For deploying configs that work on both old and new versions of Perfetto:

• Old Perfetto + new config with both fields: Old Perfetto ignores
  target_buffer_name and uses target_buffer. Works correctly.
• New Perfetto + old config with only index: Works as before.
• New Perfetto + config with both fields: Validates consistency and uses
  the resolved index.

This is why supporting both target_buffer and target_buffer_name
simultaneously is important: it allows a single config to work across Perfetto
versions during a transition period.

Error Cases

The service will reject configs with:

• Duplicate buffer names within the same session
• target_buffer_name that doesn't match any buffer name
• Both target_buffer and target_buffer_name set but resolving to different
  buffers

Example

Before (index-based):

buffers { size_kb: 1024 }  # index 0
buffers { size_kb: 4096 }  # index 1
data_sources {
  config {
    name: "linux.ftrace"
    target_buffer: 1
  }
}

After (name-based):

buffers { size_kb: 1024 name: "small" }
buffers { size_kb: 4096 name: "ftrace" }
data_sources {
  config {
    name: "linux.ftrace"
    target_buffer_name: "ftrace"
  }
}

With backwards compatibility:

buffers { size_kb: 1024 name: "small" }
buffers { size_kb: 4096 name: "ftrace" }
data_sources {
  config {
    name: "linux.ftrace"
    target_buffer: 1
    target_buffer_name: "ftrace"
  }
}

Alternatives considered

Using only names (no index fallback)

Pro:

• Simpler implementation
• Forces cleaner configs

Con:

• Breaking change for existing configs
• No backwards compatibility path

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💬 Discussion Guidelines:

• This discussion is automatically synced with the RFC document
• Please provide constructive feedback and suggestions

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📝 RFC Document Updated

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