Re: [PATCH v4 3/5] locking: Add contended_release tracepoint to sleepable locks

[Paul E. McKenney]

On Thu, Mar 26, 2026 at 03:10:02PM +0000, Dmitry Ilvokhin wrote:
> Add the contended_release trace event. This tracepoint fires on the
> holder side when a contended lock is released, complementing the
> existing contention_begin/contention_end tracepoints which fire on the
> waiter side.
> 
> This enables correlating lock hold time under contention with waiter
> events by lock address.
> 
> Add trace_contended_release() calls to the slowpath unlock paths of
> sleepable locks: mutex, rtmutex, semaphore, rwsem, percpu-rwsem, and
> RT-specific rwbase locks.
> 
> Where possible, trace_contended_release() fires before the lock is
> released and before the waiter is woken. For some lock types, the
> tracepoint fires after the release but before the wake. Making the
> placement consistent across all lock types is not worth the added
> complexity.
> 
> For reader/writer locks, the tracepoint fires for every reader releasing
> while a writer is waiting, not only for the last reader.
> 
> Signed-off-by: Dmitry Ilvokhin <d@xxxxxxxxxxxx>

Looks plausible:

Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> ---
>  include/trace/events/lock.h   | 17 +++++++++++++++++
>  kernel/locking/mutex.c        |  4 ++++
>  kernel/locking/percpu-rwsem.c | 11 +++++++++++
>  kernel/locking/rtmutex.c      |  1 +
>  kernel/locking/rwbase_rt.c    |  6 ++++++
>  kernel/locking/rwsem.c        | 10 ++++++++--
>  kernel/locking/semaphore.c    |  4 ++++
>  7 files changed, 51 insertions(+), 2 deletions(-)
> 
> diff --git a/include/trace/events/lock.h b/include/trace/events/lock.h
> index da978f2afb45..1ded869cd619 100644
> --- a/include/trace/events/lock.h
> +++ b/include/trace/events/lock.h
> @@ -137,6 +137,23 @@ TRACE_EVENT(contention_end,
>  	TP_printk("%p (ret=%d)", __entry->lock_addr, __entry->ret)
>  );
>  
> +TRACE_EVENT(contended_release,
> +
> +	TP_PROTO(void *lock),
> +
> +	TP_ARGS(lock),
> +
> +	TP_STRUCT__entry(
> +		__field(void *, lock_addr)
> +	),
> +
> +	TP_fast_assign(
> +		__entry->lock_addr = lock;
> +	),
> +
> +	TP_printk("%p", __entry->lock_addr)
> +);
> +
>  #endif /* _TRACE_LOCK_H */
>  
>  /* This part must be outside protection */
> diff --git a/kernel/locking/mutex.c b/kernel/locking/mutex.c
> index 427187ff02db..6c2c9312eb8f 100644
> --- a/kernel/locking/mutex.c
> +++ b/kernel/locking/mutex.c
> @@ -997,6 +997,9 @@ static noinline void __sched __mutex_unlock_slowpath(struct mutex *lock, unsigne
>  		wake_q_add(&wake_q, next);
>  	}
>  
> +	if (trace_contended_release_enabled() && waiter)
> +		trace_contended_release(lock);
> +
>  	if (owner & MUTEX_FLAG_HANDOFF)
>  		__mutex_handoff(lock, next);
>  
> @@ -1194,6 +1197,7 @@ EXPORT_SYMBOL(ww_mutex_lock_interruptible);
>  
>  EXPORT_TRACEPOINT_SYMBOL_GPL(contention_begin);
>  EXPORT_TRACEPOINT_SYMBOL_GPL(contention_end);
> +EXPORT_TRACEPOINT_SYMBOL_GPL(contended_release);
>  
>  /**
>   * atomic_dec_and_mutex_lock - return holding mutex if we dec to 0
> diff --git a/kernel/locking/percpu-rwsem.c b/kernel/locking/percpu-rwsem.c
> index f3ee7a0d6047..46b5903989b8 100644
> --- a/kernel/locking/percpu-rwsem.c
> +++ b/kernel/locking/percpu-rwsem.c
> @@ -263,6 +263,9 @@ void percpu_up_write(struct percpu_rw_semaphore *sem)
>  {
>  	rwsem_release(&sem->dep_map, _RET_IP_);
>  
> +	if (trace_contended_release_enabled() && wq_has_sleeper(&sem->waiters))
> +		trace_contended_release(sem);
> +
>  	/*
>  	 * Signal the writer is done, no fast path yet.
>  	 *
> @@ -292,6 +295,14 @@ EXPORT_SYMBOL_GPL(percpu_up_write);
>  void __percpu_up_read(struct percpu_rw_semaphore *sem)
>  {
>  	lockdep_assert_preemption_disabled();
> +	/*
> +	 * After percpu_up_write() completes, rcu_sync_is_idle() can still
> +	 * return false during the grace period, forcing readers into this
> +	 * slowpath. Only trace when a writer is actually waiting for
> +	 * readers to drain.
> +	 */
> +	if (trace_contended_release_enabled() && rcuwait_active(&sem->writer))
> +		trace_contended_release(sem);
>  	/*
>  	 * slowpath; reader will only ever wake a single blocked
>  	 * writer.
> diff --git a/kernel/locking/rtmutex.c b/kernel/locking/rtmutex.c
> index ccaba6148b61..3db8a840b4e8 100644
> --- a/kernel/locking/rtmutex.c
> +++ b/kernel/locking/rtmutex.c
> @@ -1466,6 +1466,7 @@ static void __sched rt_mutex_slowunlock(struct rt_mutex_base *lock)
>  		raw_spin_lock_irqsave(&lock->wait_lock, flags);
>  	}
>  
> +	trace_contended_release(lock);
>  	/*
>  	 * The wakeup next waiter path does not suffer from the above
>  	 * race. See the comments there.
> diff --git a/kernel/locking/rwbase_rt.c b/kernel/locking/rwbase_rt.c
> index 82e078c0665a..74da5601018f 100644
> --- a/kernel/locking/rwbase_rt.c
> +++ b/kernel/locking/rwbase_rt.c
> @@ -174,6 +174,8 @@ static void __sched __rwbase_read_unlock(struct rwbase_rt *rwb,
>  static __always_inline void rwbase_read_unlock(struct rwbase_rt *rwb,
>  					       unsigned int state)
>  {
> +	if (trace_contended_release_enabled() && rt_mutex_owner(&rwb->rtmutex))
> +		trace_contended_release(rwb);
>  	/*
>  	 * rwb->readers can only hit 0 when a writer is waiting for the
>  	 * active readers to leave the critical section.
> @@ -205,6 +207,8 @@ static inline void rwbase_write_unlock(struct rwbase_rt *rwb)
>  	unsigned long flags;
>  
>  	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
> +	if (trace_contended_release_enabled() && rt_mutex_has_waiters(rtm))
> +		trace_contended_release(rwb);
>  	__rwbase_write_unlock(rwb, WRITER_BIAS, flags);
>  }
>  
> @@ -214,6 +218,8 @@ static inline void rwbase_write_downgrade(struct rwbase_rt *rwb)
>  	unsigned long flags;
>  
>  	raw_spin_lock_irqsave(&rtm->wait_lock, flags);
> +	if (trace_contended_release_enabled() && rt_mutex_has_waiters(rtm))
> +		trace_contended_release(rwb);
>  	/* Release it and account current as reader */
>  	__rwbase_write_unlock(rwb, WRITER_BIAS - 1, flags);
>  }
> diff --git a/kernel/locking/rwsem.c b/kernel/locking/rwsem.c
> index bf647097369c..602d5fd3c91a 100644
> --- a/kernel/locking/rwsem.c
> +++ b/kernel/locking/rwsem.c
> @@ -1387,6 +1387,8 @@ static inline void __up_read(struct rw_semaphore *sem)
>  	rwsem_clear_reader_owned(sem);
>  	tmp = atomic_long_add_return_release(-RWSEM_READER_BIAS, &sem->count);
>  	DEBUG_RWSEMS_WARN_ON(tmp < 0, sem);
> +	if (trace_contended_release_enabled() && (tmp & RWSEM_FLAG_WAITERS))
> +		trace_contended_release(sem);
>  	if (unlikely((tmp & (RWSEM_LOCK_MASK|RWSEM_FLAG_WAITERS)) ==
>  		      RWSEM_FLAG_WAITERS)) {
>  		clear_nonspinnable(sem);
> @@ -1413,8 +1415,10 @@ static inline void __up_write(struct rw_semaphore *sem)
>  	preempt_disable();
>  	rwsem_clear_owner(sem);
>  	tmp = atomic_long_fetch_add_release(-RWSEM_WRITER_LOCKED, &sem->count);
> -	if (unlikely(tmp & RWSEM_FLAG_WAITERS))
> +	if (unlikely(tmp & RWSEM_FLAG_WAITERS)) {
> +		trace_contended_release(sem);
>  		rwsem_wake(sem);
> +	}
>  	preempt_enable();
>  }
>  
> @@ -1437,8 +1441,10 @@ static inline void __downgrade_write(struct rw_semaphore *sem)
>  	tmp = atomic_long_fetch_add_release(
>  		-RWSEM_WRITER_LOCKED+RWSEM_READER_BIAS, &sem->count);
>  	rwsem_set_reader_owned(sem);
> -	if (tmp & RWSEM_FLAG_WAITERS)
> +	if (tmp & RWSEM_FLAG_WAITERS) {
> +		trace_contended_release(sem);
>  		rwsem_downgrade_wake(sem);
> +	}
>  	preempt_enable();
>  }
>  
> diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
> index 74d41433ba13..35ac3498dca5 100644
> --- a/kernel/locking/semaphore.c
> +++ b/kernel/locking/semaphore.c
> @@ -230,6 +230,10 @@ void __sched up(struct semaphore *sem)
>  		sem->count++;
>  	else
>  		__up(sem, &wake_q);
> +
> +	if (trace_contended_release_enabled() && !wake_q_empty(&wake_q))
> +		trace_contended_release(sem);
> +
>  	raw_spin_unlock_irqrestore(&sem->lock, flags);
>  	if (!wake_q_empty(&wake_q))
>  		wake_up_q(&wake_q);
> -- 
> 2.52.0
>