Fast and flexible tracepoints in x86
Christian Harper‐Cyr, Michel Dagenais, Ahmad Shahnejat Bushehri · Software Practice and Experience · 2019
Summary Tracing is often the most effective technique for analyzing the performance of complex multithreaded applications. This paper presents an improvement on existing techniques for dynamic tracepoint insertion. To add a tracepoint, the technique inserts a jump at the tracing point, possibly replacing several shorter instructions. This jump embeds trap instructions inside its offset at the address of every replaced instruction. This makes the jump thread safe if any thread is about to execute a replaced instruction. It also makes it jump safe if a jump landing pad is at one of the replaced instructions. In both cases, a trap will be raised, and the thread can be redirected to the out‐of‐line equivalent instruction. The use of a jump instead of a trap to execute the tracepoint improves the performance of the execution. It also adds the flexibility to place the tracepoint at almost any instruction, since multiple instructions can be replaced atomically and safely. The downside of this technique is the increased memory usage, since it requires unaligned allocations with high external fragmentation.