Re: [PATCH v4 0/5] mm: zone lock tracepoint instrumentation

[Dmitry Ilvokhin]

Thanks for the discussion and for the feedback on generic lock
instrumentation.

Following up on the earlier points about lightweight LOCK_STAT and
tracepoints.

While the thread has focused on spin_lock() calls, the real gap is on
the unlock path: there is currently no release-side tracepoint to
correlate holders and waiters or measure contended hold times.

A possible generic solution is a trace_contended_release() for spin
locks, for example:

    if (trace_contended_release_enabled() &&
        atomic_read(&lock->val) & ~_Q_LOCKED_MASK)
        trace_contended_release(lock);

This might work on x86, but could increase code size and regress
performance on arches where spin_unlock() is inlined, such as arm64
under !PREEMPTION.

So even a generic release-side tracepoint has nontrivial downsides (not
to mention lightweight LOCK_STAT, since a disabled tracepoint is about
as lightweight as it gets, the more stats we add, the less lightweight
the mechanism becomes).

The zone lock wrappers provide a practical, production-safe solution to
observe this specific high-contention lock, if the generic approach
proves impractical.