linux-kernel - Re: [PATCH v2 18/27] rcu: Rename rcu_dynticks_in_eqs_since() into rcu_watching_changed

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Date: Tue, 14 May 2024 12:42:08 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Valentin Schneider <vschneid@...hat.com>
Cc: "Paul E. McKenney" <paulmck@...nel.org>, rcu@...r.kernel.org,
	linux-kernel@...r.kernel.org, Peter Zijlstra <peterz@...radead.org>,
	Neeraj Upadhyay <quic_neeraju@...cinc.com>,
	Joel Fernandes <joel@...lfernandes.org>,
	Josh Triplett <josh@...htriplett.org>,
	Boqun Feng <boqun.feng@...il.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
	Lai Jiangshan <jiangshanlai@...il.com>,
	Zqiang <qiang.zhang1211@...il.com>
Subject: Re: [PATCH v2 18/27] rcu: Rename rcu_dynticks_in_eqs_since() into
 rcu_watching_changed_since()

Le Mon, May 13, 2024 at 08:40:09PM +0200, Valentin Schneider a écrit :
> On 08/05/24 12:59, Frederic Weisbecker wrote:
> > Le Tue, May 07, 2024 at 10:14:08AM -0700, Paul E. McKenney a écrit :
> >> On Tue, May 07, 2024 at 03:48:18PM +0200, Frederic Weisbecker wrote:
> >> > Indeed in practice the function only checks a change. But semantically it really
> >> > checks a trip to eqs because this function is only ever called after a failing
> >> > call to rcu_dynticks_in_eqs().
> >> >
> >> > So not sure about that one rename. Paul?
> >>
> >> As you say, Valentin is technically correct.  Me, I am having a hard
> >> time getting too excited one way or the other.  ;-)
> >>
> >> I suggest thinking in terms of rate-bounding the change.  If you do
> >> change it, don't change it again for a few years.
> >
> > Makes sense!
> >
> >>
> >> Either way, should comments be changed or added?
> >>
> >> Of course, the scientific way to evaluate this is to whose a couple
> >> dozen people the old code and a couple dozen other people the new code,
> >> and see if one group or the other has statistically significantly lower
> >> levels of confusion.  I don't see how this is feasible, but it is the
> >> (painfully) correct way.  On the other hand, it would have the beneficial
> >> side effect of getting more people exposed to Linux-kernel-RCU internals.
> >> Unfortunately, it might also have the additional side effect of making
> >> them (more) annoyed at RCU.  ;-)
> >
> > Sounds good!
> >
> > I divided myself in two blank RCU subjects for a double blind study
> > and locked those people up overnight with a paper containing both proposals.
> >
> > I opened the door five minutes ago and they both elected by mutual agreement
> > rcu_watching_changed_since()! Also they are thirsty.
> >
> > Congratulations Valentin! :-)
> 
> :-)
> 
> Now, not that I like wasting everyone's time, but... I hadn't taken a step
> back to realize the calling context implied this would always be used to
> check an entry into EQS, per the waiting loop structures. With this in
> mind, how about the following? 
> 
> 
> /**
>  * rcu_watching_stopped_since() - Has RCU stopped watching a given CPU since
>  * the specified @snap?
>  *
>  * @rdp: The rcu_data corresponding to the CPU for which to check EQS.
>  * @snap: rcu_watching snapshot taken when the CPU wasn't in an EQS.
>  *
>  * Returns true if the CPU corresponding to @rcu_data has spent some time in an

@rdp

>  * extended quiescent state since @snap. Note that this doesn't check if it
>  * /still/ is in an EQS, just that it went through one since @snap.
>  *
>  * This is meant to be used in a loop waiting for a CPU to go through an EQS.
>  */
> static bool rcu_watching_stopped_since(struct rcu_data *rdp, int snap)
> {
> 	if (WARN_ON_ONCE(rcu_watching_in_eqs(snap)))
> 		return true;
> 
> 	return snap != rcu_dynticks_snap(rdp->cpu);
> }
> 
>

Yep, looks good to me!

Thanks.