Re: [PATCH v4 0/5] locking: contended_release tracepoint instrumentation

[Matthew Wilcox <willy@infradead.org>]

On Thu, Mar 26, 2026 at 03:09:59PM +0000, Dmitry Ilvokhin wrote:
> The existing contention_begin/contention_end tracepoints fire on the
> waiter side. The lock holder's identity and stack can be captured at
> contention_begin time (e.g. perf lock contention --lock-owner), but
> this reflects the holder's state when a waiter arrives, not when the
> lock is actually released.
> 
> This series adds a contended_release tracepoint that fires on the
> holder side when a lock with waiters is released. This provides:
> 
> - Hold time estimation: when the holder's own acquisition was
>   contended, its contention_end (acquisition) and contended_release
>   can be correlated to measure how long the lock was held under
>   contention.
> 
> - The holder's stack at release time, which may differ from what perf lock
>   contention --lock-owner captures if the holder does significant work between
>   the waiter's arrival and the unlock.

As someone who's not an expert in this area (so please use short words
to explain it to me), why do we want to know how long this holder took
to release the lock from when it became contended?

I understand why we want to know how long any given waiter had to wait
to gain the lock (but we already have tracepoints which show that).

I also don't understand why we want to know the holder's stack at
release time.  The stack at contention-begin time will include
the point at which the lock was acquired which should be correlated
with where the lock was released.

Perhaps examples might help me understand why we want this?