How do you troubleshoot high CPU usage on a Linux server?

Systematic CPU investigation:

1. top / htop: Identify the process consuming CPU. Note: is it user space or kernel (%us vs %sy)?
2. ps aux –sort=-%cpu: Snapshot of top CPU consumers.
3. perf top: See which kernel functions are hot.
4. strace -p <PID>: Trace system calls to understand what a process is doing.
5. vmstat 1: Observe context switches (cs) and interrupts (in).

Common causes: runaway application bug, CPU-intensive query (full table scan), kernel work from high I/O (softirqs), insufficient CPU for the workload.