100% CPU usage while idle awaiting user input

Description

Claude CLI consumes 100%+ CPU while idle waiting for user input at the prompt. The process should be sleeping/blocked on stdin read, but instead is actively running.

System Information

• Platform: Raspberry Pi 5 (ARM64)
• OS: Linux (Debian-based)
• Claude CLI Version: Latest from npm

Diagnostic Output

top output

PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
8175 jferraro  20   0   71.6g 959712  52480 R 114.0   5.9  22:28 claude

Note: Process is in R (running) state, not S (sleeping).

strace analysis

Running strace -p <pid> -f reveals three issues:

Issue 1: Busy-polling /proc stats

[pid  8175] pread64(10, "18762362 239712 13120 13620 0 16"..., 256, 0) = 41
[pid  8175] pread64(10, "18762362 239712 13120 13620 0 16"..., 256, 0) = 41
[pid  8175] pread64(10, "18762362 239712 13120 13620 0 16"..., 256, 0) = 41

The main thread is reading /proc/self/stat (process statistics) in a tight loop with no sleep between reads. This appears to be resource monitoring code (likely for status bar memory display) without any rate limiting.

Issue 2: Spinlock contention

[pid  8550] sched_yield()
[pid  8551] sched_yield()
[pid  8552] sched_yield()

Three worker threads are in tight sched_yield() loops - classic spinlock starvation pattern. They yield CPU but immediately retry, burning cycles.

Issue 3: 16ms timer (60 Hz)

timerfd_settime(7, ..., {it_value={tv_sec=0, tv_nsec=16000000}}, ...)

A 60Hz timer is running (reasonable for UI), but combined with busy polling creates excessive load.

Expected Behavior

When idle at the input prompt, the process should:

1. Block on stdin read (sleeping state)
2. Poll resource stats at most once per second, not continuously
3. Worker threads should be sleeping on condition variables, not spinning

Impact

• Drains battery on laptops
• Causes thermal throttling on low-power devices (Pi)
• Wastes significant CPU resources
• Makes the system less responsive for other tasks

Suggested Fix

Replace the busy-poll resource monitoring with a timer-based approach (e.g., poll /proc/self/stat once per second). Review worker thread synchronization to use proper blocking primitives instead of spinlocks.

$ echo $TERM
xterm-256color

$ echo $SHELL
/bin/bash

$ claude --version
2.1.20 (Claude Code)