linux-kernel - Re: [RFC v2 4/5] rcu: Use for_each_leaf_node_cpu() in force_qs

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Date:   Wed, 21 Dec 2016 12:18:08 +0800
From:   Boqun Feng <boqun.feng@...il.com>
To:     "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>
Cc:     Colin Ian King <colin.king@...onical.com>,
        Mark Rutland <mark.rutland@....com>,
        linux-kernel@...r.kernel.org,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>
Subject: Re: [RFC v2 4/5] rcu: Use for_each_leaf_node_cpu() in force_qs_rnp()

On Tue, Dec 20, 2016 at 07:40:24PM -0800, Paul E. McKenney wrote:
[...]
> > 
> > Agreed, my intent is to keep this overcare check for couples of releases
> > and if no one shoots his/her foot, we can remove it, if not, it
> > definitely means this part is subtle, and we need to pay more attention
> > to it, maybe write some regression tests for this particular problem to
> > help developers avoid it.
> > 
> > This check is supposed to be removed, so I'm not stick to keeping it.
> 
> I suggest keeping through validation.  If it triggers during that time,
> consider keeping it longer.  If it does not trigger, remove it before
> it goes upstream.
> 

Good point ;-)

[...]
> > > > 
> > > > But this brings a side question, is the callsite of rcu_cpu_starting()
> > > > is correct? Given rcu_cpu_starting() ignores the @cpu parameter and only
> > > > set _this_ cpu's bit in a leaf node?
> > > 
> > > The calls from notify_cpu_starting() are called from the various
> > > start_kernel_secondary(), secondary_start_kernel(), and similarly
> > > named functions.  These are called on the incoming CPU early in that
> > > CPU's execution.  The call from rcu_init() is correct until such time
> > > as more than one CPU can be running at rcu_init() time.  And that
> > > day might be coming, so please see the untested patch below.
> > 
> > Looks better than mine ;-)
> > 
> > But do we need to worry that we start rcu on each CPU twice, which may
> > slow down the boot?
> 
> We only start a given CPU once.  The boot CPU at rcu_init() time, and
> the rest at CPU-hotplug time.  Unless of course a CPU is later taken

Confused... we call rcu_cpu_starting() in a for_each_online_cpu() loop
in rcu_init(), so we basically start all online CPUs there after
applying your patch. And all the rest CPUs will get themselves start
again at CPU-hotplug time, right?

Besides, without your patch, we started the boot CPU many times in the
for_each_online_cpu() loop.

Am I missing something subtle?

Regards,
Boqun

> offline, in which case we start it again when it comes back online.
> 
> 							Thanx, Paul
> 
> > Regards,
> > Boqun
> > 
> > > 							Thanx, Paul
> > > 
> > > ------------------------------------------------------------------------
> > > 
> > > commit 1e84402587173d6d4da8645689f0e24c877b3269
> > > Author: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > Date:   Tue Dec 20 07:17:58 2016 -0800
> > > 
> > >     rcu: Make rcu_cpu_starting() use its "cpu" argument
> > >     
> > >     The rcu_cpu_starting() function uses this_cpu_ptr() to locate the
> > >     incoming CPU's rcu_data structure.  This works for the boot CPU and for
> > >     all CPUs onlined after rcu_init() executes (during very early boot).
> > >     Currently, this is the full set of CPUs, so all is well.  But if
> > >     anyone ever parallelizes boot before rcu_init() time, it will fail.
> > >     This commit therefore substitutes the rcu_cpu_starting() function's
> > >     this_cpu_pointer() for per_cpu_ptr(), future-proofing the code and
> > >     (arguably) improving readability.
> > >     
> > >     Reported-by: Boqun Feng <boqun.feng@...il.com>
> > >     Signed-off-by: Paul E. McKenney <paulmck@...ux.vnet.ibm.com>
> > > 
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index b9d3c0e30935..083cb8a6299c 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -4017,7 +4017,7 @@ void rcu_cpu_starting(unsigned int cpu)
> > >  	struct rcu_state *rsp;
> > >  
> > >  	for_each_rcu_flavor(rsp) {
> > > -		rdp = this_cpu_ptr(rsp->rda);
> > > +		rdp = per_cpu_ptr(rsp->rda, cpu);
> > >  		rnp = rdp->mynode;
> > >  		mask = rdp->grpmask;
> > >  		raw_spin_lock_irqsave_rcu_node(rnp, flags);
> > > 
> 
> 

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