linux-kernel - Re: [PATCH v4 2/5] rcu/tree: Clarify comments about FQS loop reporting quiescent states

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Date:   Mon, 10 Aug 2020 15:22:34 -0400
From:   Joel Fernandes <joel@...lfernandes.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     linux-kernel@...r.kernel.org, Davidlohr Bueso <dave@...olabs.net>,
        Jonathan Corbet <corbet@....net>,
        Josh Triplett <josh@...htriplett.org>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        linux-doc@...r.kernel.org,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Mauro Carvalho Chehab <mchehab+samsung@...nel.org>,
        neeraju@...eaurora.org, peterz@...radead.org,
        Randy Dunlap <rdunlap@...radead.org>, rcu@...r.kernel.org,
        Steven Rostedt <rostedt@...dmis.org>, tglx@...utronix.de,
        vineethrp@...il.com
Subject: Re: [PATCH v4 2/5] rcu/tree: Clarify comments about FQS loop
 reporting quiescent states

On Mon, Aug 10, 2020 at 11:06:47AM -0700, Paul E. McKenney wrote:
> On Fri, Aug 07, 2020 at 01:07:19PM -0400, Joel Fernandes (Google) wrote:
> > At least since v4.19, the FQS loop no longer reports quiescent states
> > for offline CPUs unless it is an emergency.
> > 
> > This commit therefore fixes the comment in rcu_gp_init() to match the
> > current code.
> > 
> > Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
> > ---
> >  kernel/rcu/tree.c | 8 +++++---
> >  1 file changed, 5 insertions(+), 3 deletions(-)
> > 
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index a49fa3b60faa..2fb66cdbfa25 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -1701,9 +1701,11 @@ static bool rcu_gp_init(void)
> >  
> >  	/*
> >  	 * Apply per-leaf buffered online and offline operations to the
> > -	 * rcu_node tree.  Note that this new grace period need not wait
> > -	 * for subsequent online CPUs, and that quiescent-state forcing
> > -	 * will handle subsequent offline CPUs.
> > +	 * rcu_node tree. Note that this new grace period need not wait for
> > +	 * subsequent online CPUs, and that RCU hooks in the CPU offlining
> > +	 * path, when combined with checks in this function, will handle CPUs
> > +	 * that are currently going offline or that go offline later. Refer to
> > +	 * RCU's Requirements documentation about hotplug requirements as well.
> >  	 */
> >  	rcu_state.gp_state = RCU_GP_ONOFF;
> >  	rcu_for_each_leaf_node(rnp) {
> 
> Very good!  I pulled this in with light edits as shown below.
> Please let me know if I messed something up.

Yes, looks good, thanks!

 - Joel


> 
> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> commit a6117399840b963f90cc5322ef9ea7c52de639b2
> Author: Joel Fernandes (Google) <joel@...lfernandes.org>
> Date:   Fri Aug 7 13:07:19 2020 -0400
> 
>     rcu/tree: Clarify comments about FQS loop reporting quiescent states
>     
>     Since at least v4.19, the FQS loop no longer reports quiescent states
>     for offline CPUs except in emergency situations.
>     
>     This commit therefore fixes the comment in rcu_gp_init() to match the
>     current code.
>     
>     Signed-off-by: Joel Fernandes (Google) <joel@...lfernandes.org>
>     Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index bcc6160..59e1943 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -1730,10 +1730,13 @@ static bool rcu_gp_init(void)
>  	raw_spin_unlock_irq_rcu_node(rnp);
>  
>  	/*
> -	 * Apply per-leaf buffered online and offline operations to the
> -	 * rcu_node tree.  Note that this new grace period need not wait
> -	 * for subsequent online CPUs, and that quiescent-state forcing
> -	 * will handle subsequent offline CPUs.
> +	 * Apply per-leaf buffered online and offline operations to
> +	 * the rcu_node tree. Note that this new grace period need not
> +	 * wait for subsequent online CPUs, and that RCU hooks in the CPU
> +	 * offlining path, when combined with checks in this function,
> +	 * will handle CPUs that are currently going offline or that will
> +	 * go offline later.  Please also refer to "Hotplug CPU" section
> +	 * of RCU's Requirements documentation.
>  	 */
>  	rcu_state.gp_state = RCU_GP_ONOFF;
>  	rcu_for_each_leaf_node(rnp) {