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

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Message-ID: <20220318132424.GA1665646@odroid>
Date:   Fri, 18 Mar 2022 13:24:24 +0000
From:   Hyeonggon Yoo <42.hyeyoo@...il.com>
To:     Boqun Feng <boqun.feng@...il.com>
Cc:     Namhyung Kim <namhyung@...nel.org>,
        Peter Zijlstra <peterz@...radead.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 ;-/

I agree this can be simplified a bit.

> 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?
>

But IMO inlining tracepoints is generally not a good idea.
Will increase kernel size a lot.

> Regards,
> Boqun
> 
> > +			tracing = true;
> > +		}
> >  		raw_spin_unlock_irq(&sem->lock);
> >  		timeout = schedule_timeout(timeout);
> >  		raw_spin_lock_irq(&sem->lock);
> > -		if (waiter.up)
> > +		if (waiter.up) {
> > +			trace_contention_end(sem, 0);
> >  			return 0;
> > +		}
> >  	}
> >  
> >   timed_out:
> > +	if (tracing)
> > +		trace_contention_end(sem, -ETIME);
> >  	list_del(&waiter.list);
> >  	return -ETIME;
> >  
> >   interrupted:
> > +	if (tracing)
> > +		trace_contention_end(sem, -EINTR);
> >  	list_del(&waiter.list);
> >  	return -EINTR;
> >  }
> > -- 
> > 2.35.1.894.gb6a874cedc-goog
> >