linux-kernel - Re: [PATCH 2/2] locking: Apply contention tracepoints in the slow path

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Date:   Fri, 18 Mar 2022 17:43:26 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Namhyung Kim <namhyung@...nel.org>, Ingo Molnar <mingo@...nel.org>,
        Will Deacon <will@...nel.org>,
        Waiman Long <longman@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Byungchul Park <byungchul.park@....com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Arnd Bergmann <arnd@...db.de>,
        Radoslaw Burny <rburny@...gle.com>, linux-arch@...r.kernel.org,
        bpf@...r.kernel.org
Subject: Re: [PATCH 2/2] locking: Apply contention tracepoints in the slow
 path

On Fri, Mar 18, 2022 at 08:55:32PM +0800, Boqun Feng wrote:
> On Wed, Mar 16, 2022 at 03:45:48PM -0700, Namhyung Kim wrote:
> [...]
> > @@ -209,6 +210,7 @@ static inline int __sched __down_common(struct semaphore *sem, long state,
> >  								long timeout)
> >  {
> >  	struct semaphore_waiter waiter;
> > +	bool tracing = false;
> >  
> >  	list_add_tail(&waiter.list, &sem->wait_list);
> >  	waiter.task = current;
> > @@ -220,18 +222,28 @@ static inline int __sched __down_common(struct semaphore *sem, long state,
> >  		if (unlikely(timeout <= 0))
> >  			goto timed_out;
> >  		__set_current_state(state);
> > +		if (!tracing) {
> > +			trace_contention_begin(sem, 0);
> 
> This looks a littl ugly ;-/ Maybe we can rename __down_common() to
> ___down_common() and implement __down_common() as:
> 
> 	static inline int __sched __down_common(...)
> 	{
> 		int ret;
> 		trace_contention_begin(sem, 0);
> 		ret = ___down_common(...);
> 		trace_contention_end(sem, ret);
> 		return ret;
> 	}
> 
> Thoughts?

Yeah, that works, except I think he wants a few extra
__set_current_state()'s like so:

diff --git a/kernel/locking/semaphore.c b/kernel/locking/semaphore.c
index 9ee381e4d2a4..e2049a7e0ea4 100644
--- a/kernel/locking/semaphore.c
+++ b/kernel/locking/semaphore.c
@@ -205,8 +205,7 @@ struct semaphore_waiter {
  * constant, and thus optimised away by the compiler.  Likewise the
  * 'timeout' parameter for the cases without timeouts.
  */
-static inline int __sched __down_common(struct semaphore *sem, long state,
-								long timeout)
+static __always_inline int ___down_common(struct semaphore *sem, long state, long timeout)
 {
 	struct semaphore_waiter waiter;
 
@@ -227,15 +226,28 @@ static inline int __sched __down_common(struct semaphore *sem, long state,
 			return 0;
 	}
 
- timed_out:
+timed_out:
 	list_del(&waiter.list);
 	return -ETIME;
 
- interrupted:
+interrupted:
 	list_del(&waiter.list);
 	return -EINTR;
 }
 
+static __always_inline int __down_common(struct semaphore *sem, long state, long timeout)
+{
+	int ret;
+
+	__set_current_state(state);
+	trace_contention_begin(sem, 0);
+	ret = ___down_common(sem, state, timeout);
+	__set_current_state(TASK_RUNNING);
+	trace_contention_end(sem, ret);
+
+	return ret;
+}
+
 static noinline void __sched __down(struct semaphore *sem)
 {
 	__down_common(sem, TASK_UNINTERRUPTIBLE, MAX_SCHEDULE_TIMEOUT);