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

[Andrew Morton]

On Thu, 19 Mar 2026 13:22:54 +0000 Dmitry Ilvokhin <d@xxxxxxxxxxxx> wrote:

> On Mon, Mar 16, 2026 at 05:40:50PM +0000, Dmitry Ilvokhin wrote:
> 
> [...]
> 
> > 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.
> 
> I took a stab at this idea and submitted an RFC [1].
> 
> The implementation builds on your earlier observation from Matthew that
> _raw_spin_unlock() is not inlined in most configurations. In those
> cases, when the tracepoint is disabled, this adds a single NOP on the
> fast path, with the conditional check staying out of line. The measured
> text size increase in this configuration is +983 bytes.
> 
> For configurations where _raw_spin_unlock() is inlined, the
> instrumentation does increase code size more noticeably
> (+71 KB in my measurements), since the check and out of line call is
> replicated at each call site.
> 
> This provides a generic release-side signal for contended locks,
> allowing: correlation of lock holders with waiters and measurement of
> contended hold times
> 
> This RFC addressing the same visibility gap without introducing per-lock
> instrumentation.
> 
> If this tradeoff is acceptable, this could be a generic alternative to
> lock-specific tracepoints.
> 
> [1]: https://lore.kernel.org/all/51aad0415b78c5a39f2029722118fa01eac77538.1773858853.git.d@xxxxxxxxxxxx 

That submission has met a disappointing response.

How should I proceed with this series "mm: zone lock tracepoint
instrumentation"?  It's not urgent so I'm inclined to put this on hold
while you pursue "locking: Add contended_release tracepoint to spinning
locks"?

Please send that v2 sometime and hopefully Steven can help push it along?