Profile and fix Linux performance problems using `perf`. Workflows: (A) hardware counters -- IPC, cache-miss, branch mispredictions; (B) hotspot profiling -- which functions and source lines consume CPU, with SIMD and accumulator detection; (C) cache-line contention -- false sharing, HITM, `perf c2c`; (D) core-count scaling -- dual-profile comparison, bottleneck categorization; (E) structured hotspot report with annotated source and pattern observations. Resolution strategies: TTAS spinlock, SIMD upconversion, parallel accumulator, structured false-sharing fix, per-CPU stats. Trigger on: perf, profiling, profile, hotspot, hotspots, cache miss, IPC, false sharing, HITM, sc…
