Profiling Signal Handling Performance

In this lesson, you will learn how to measure the latency between the moment a signal is sent to a process and the exact moment its signal handler begins execution.

The Signal Delivery Pipeline

When a process sends a signal to another (e.g., via kill()), the kernel must perform several steps: updating the target process's pending signal mask, waking the target if it is sleeping, and eventually forcing the target to jump into the user-space handler. In high-performance systems, this "delivery latency"—the interval between the syscall and the handler entry—can become a bottleneck if signals are used for inter-process coordination.

Measuring the Latency

To profile this, we use ebpf (via bpftrace) to capture the timestamp at the syscall exit and the timestamp at the entry of the signal handler. Because the kernel doesn't provide a direct "signal delivery latency" metric, we must correlate the sys_enter_kill (or tkill) with the uprobe placed on the signal handler function in user-space.

Consider this bpftrace snippet to track the time delta:

# Capture the timestamp when a signal is sent to a specific PID
kprobe:sys_kill {
    @start[tid] = nsecs;
}

# Capture the entry to your signal handler function
uprobe:/path/to/binary:my_signal_handler {
    if (@start[pid]) {
        printf("Signal delivery latency: %d ns\n", nsecs - @start[pid]);
        delete(@start[pid]);
    }
}

Why Latency Spikes

The duration between the signal being queued and being handled is highly dependent on the target process's state. If the target is currently executing inside a long-running critical section or is blocked on a mutex with interrupts disabled, the latency will spike. Furthermore, if the system is experiencing high context-switch overhead, the time taken for the scheduler to preempt the current task and switch to the signal-receiving task becomes part of this measured delay.

By profiling this, you can identify if your application's signal-based IPC is causing "jitter" due to scheduler delays rather than inherent kernel signal processing time.

Key Takeaway: Signal handling performance is a function of both kernel delivery overhead and the target process’s readiness; use uprobe and kprobe correlation to isolate the scheduling lag from the actual signal processing time.