DRAFT: tracing: Classify probe misses

[dwindsor]

Currently, the kernel accounts for missed probes via a counter nmissed in struct kprobe and struct kretprobe. To better profile eBPF performance regarding missed probes, it would be useful to know why each miss occurred.

Add additional members to struct kprobe and struct kretprobe to tally individual misses by cause (e.g. nmissed_ss for drops due to single-stepping, etc).

There will be a companion patch to bpftool to extract these statistics in userspace.

[admsev]

@gooroodev please review

[gooroodev]

1. Summary of Changes:

The pull request introduces the classification of missed probes in the kprobes subsystem. The key changes include:

• Modifying the kprobes_inc_nmissed_count function to accept a new flags parameter that indicates the type of miss.
• Adding new constants for different types of misses (KPROBE_MISSED_SS, KPROBE_MISSED_OBJPOOL, KPROBE_MISSED_RETHOOK, KPROBE_MISSED_CALLBACK, KPROBE_MISSED_FTRACE_CALLBACK).
• Updating the kprobe and kretprobe structures to include counters for each type of miss.
• Adjusting various functions to pass the appropriate miss type flag when calling kprobes_inc_nmissed_count.
• Adding debug print statements to log the counts of different types of misses.

2. Issues, Bugs, or Typos:

Issue 1:

The flags check in kprobes_inc_nmissed_count uses else if statements, which means only one type of miss will be counted even if multiple flags are set. This might not be the intended behavior if multiple miss types can be flagged simultaneously.

Improvement:
Replace else if with if to ensure all relevant miss types are counted.

if (flags & KPROBE_MISSED_SS) {
    p->nmissed_ss++;
}
if (flags & KPROBE_MISSED_OBJPOOL) {
    p->nmissed_objpool++;
}
if (flags & KPROBE_MISSED_RETHOOK) {
    p->nmissed_rethook++;
}
if (flags & KPROBE_MISSED_CALLBACK) {
    p->nmissed_callback++;
}
if (flags & KPROBE_MISSED_FTRACE_CALLBACK) {
    p->nmissed_ftrace_callback++;
}

Issue 2:

The debug print statement in kprobes_inc_nmissed_count might be too verbose for production code.

Improvement:
Wrap the debug print statement with a conditional compilation flag to enable it only in debug builds.

#ifdef DEBUG
printk("ss=%lu:objpool=%lu;rethook=%lu;callback=%lu;ftrace=%lu\n", p->nmissed_ss, p->nmissed_objpool, p->nmissed_rethook, p->nmissed_callback, p->nmissed_ftrace_callback);
#endif

3. General Code Quality and Style Review:

• Code Readability: The code is generally readable, but the addition of comments explaining the purpose of each miss type and the conditions under which they are incremented would improve understandability.
• Consistency: The code maintains consistent formatting and naming conventions, adhering to common kernel coding practices.
• Functionality: The changes seem to be logically correct, assuming the intended behavior is to classify and count different types of probe misses.
• Performance: The added counters and conditional checks should have minimal performance impact, but the debug print statement could slow down execution if left in production.

Final Remarks:

The pull request effectively introduces a useful feature for classifying and counting different types of missed probes. With the proposed improvements, the code will be more robust and maintainable.

Yours, Gooroo.dev. To receive reviews automatically, install Github App

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