[Talk Proposal] ftrace: An instruction one too many

[Naveen N. Rao]

Title:
ftrace: An instruction one too many

Abstract:
ftrace, short for the function tracer, is the defacto Linux kernel tracing technology enabling advanced observability on Linux. At its core, ftrace works by hooking into the entry of kernel functions. It does so by having the compiler generate a suitable instruction sequence at the beginning of each kernel function. On powerpc, this translates to a two instruction sequence:
mflr r0; /* save return address */
bl _mcount; /* call into ftrace */

For such a simple two instruction sequence though, there were a surprising number of performance issues reported. In this talk, I intend to describe the issues identified and how we went about solving them. While the session is specific to powerpc architecture, the challenges discussed apply to most RISC architectures.

Outline:
- Intro to ftrace
- Short primer on powerpc ISA/ABI
- Challenge #1: Races with runtime instruction patching
- Challenge #2: Aligning with the branch predictor
- Challenge #3: Minimizing impact on icache

Talk preference:
Lightning talk


Regards,
Naveen