On Wed, Jul 28, 2021 at 04:03:02PM -0400, Mathieu Desnoyers wrote:
> ----- On Jul 28, 2021, at 3:45 PM, paulmck paulmck@xxxxxxxxxx wrote:
> [...]
> >
> > And how about like this?
> >
> > Thanx, Paul
> >
> > ------------------------------------------------------------------------
> >
> > commit cb8914dcc6443cca15ce48d937a93c0dfdb114d3
> > Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
> > Date: Wed Jul 28 12:38:42 2021 -0700
> >
> > rcu: Move rcu_dynticks_eqs_online() to rcu_cpu_starting()
> >
> > The purpose of rcu_dynticks_eqs_online() is to adjust the ->dynticks
> > counter of an incoming CPU if required. It is currently is invoked
>
> "is currently is" -> "is currently"
Good catch, fixed!
> > from rcutree_prepare_cpu(), which runs before the incoming CPU is
> > running, and thus on some other CPU. This makes the per-CPU accesses in
> > rcu_dynticks_eqs_online() iffy at best, and it all "works" only because
> > the running CPU cannot possibly be in dyntick-idle mode, which means
> > that rcu_dynticks_eqs_online() never has any effect. One could argue
> > that this means that rcu_dynticks_eqs_online() is unnecessary, however,
> > removing it makes the CPU-online process vulnerable to slight changes
> > in the CPU-offline process.
>
> Why favor moving this from the prepare_cpu to the cpu_starting hotplug step,
> rather than using the target cpu's rdp from rcutree_prepare_cpu ? Maybe there
> was a good reason for having this very early in the prepare_cpu step ?
Some years back, there was a good reason. This reason was that
rcutree_prepare_cpu() marked the CPU as being online from an RCU
viewpoint. But now rcu_cpu_starting() is the one that marks the CPU as
being online, so the ->dynticks check can be deferred to this function.
> Also, the commit message refers to this bug as having no effect because the
> running CPU cannot possibly be in dyntick-idle mode. I understand that calling
> this function was indeed effect-less, but then why is it OK for the CPU coming
> online to skip this call in the first place ? This commit message hints at
> "slight changes in the CPU-offline process" which could break it, but therer is
> no explanation of what makes this not an actual bug fix.
Because rcutorture would not have suffered in silence had this
situation ever arisen.
I have updated the commit log to answer these questions as shown
below. Thoughts?
Thanx, Paul
------------------------------------------------------------------------
commit 516c8c4cc6fce62539f7e0182739812db4591c1d
Author: Paul E. McKenney <paulmck@xxxxxxxxxx>
Date: Wed Jul 28 12:38:42 2021 -0700
rcu: Move rcu_dynticks_eqs_online() to rcu_cpu_starting()
The purpose of rcu_dynticks_eqs_online() is to adjust the ->dynticks
counter of an incoming CPU when required. It is currently invoked
from rcutree_prepare_cpu(), which runs before the incoming CPU is
running, and thus on some other CPU. This makes the per-CPU accesses in
rcu_dynticks_eqs_online() iffy at best, and it all "works" only because
the running CPU cannot possibly be in dyntick-idle mode, which means
that rcu_dynticks_eqs_online() never has any effect.
It is currently OK for rcu_dynticks_eqs_online() to have no effect, but
only because the CPU-offline process just happens to leave ->dynticks in
the correct state. After all, if ->dynticks were in the wrong state on a
just-onlined CPU, rcutorture would complain bitterly the next time that
CPU went idle, at least in kernels built with CONFIG_RCU_EQS_DEBUG=y,
for example, those built by rcutorture scenario TREE04. One could
argue that this means that rcu_dynticks_eqs_online() is unnecessary,
however, removing it would make the CPU-online process vulnerable to
slight changes in the CPU-offline process.
One could also ask why it is safe to move the rcu_dynticks_eqs_online()
call so late in the CPU-online process. Indeed, there was a time when it
would not have been safe, which does much to explain its current location.
However, the marking of a CPU as online from an RCU perspective has long
since moved from rcutree_prepare_cpu() to rcu_cpu_starting(), and all
that is required is that ->dynticks be set correctly by the time that
the CPU is marked as online from an RCU perspective. After all, the RCU
grace-period kthread does not check to see if offline CPUs are also idle.
(In case you were curious, this is one reason why there is quiescent-state
reporting as part of the offlining process.)
This commit therefore moves the call to rcu_dynticks_eqs_online() from
rcutree_prepare_cpu() to rcu_cpu_starting(), this latter being guaranteed
to be running on the incoming CPU. The call to this function must of
course be placed before this rcu_cpu_starting() announces this CPU's
presence to RCU.
Reported-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxx>
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0172a5fd6d8de..aa00babdaf544 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -4129,7 +4129,6 @@ int rcutree_prepare_cpu(unsigned int cpu)
rdp->n_force_qs_snap = READ_ONCE(rcu_state.n_force_qs);
rdp->blimit = blimit;
rdp->dynticks_nesting = 1; /* CPU not up, no tearing. */
- rcu_dynticks_eqs_online();
raw_spin_unlock_rcu_node(rnp); /* irqs remain disabled. */
/*
@@ -4249,6 +4248,7 @@ void rcu_cpu_starting(unsigned int cpu)
mask = rdp->grpmask;
WRITE_ONCE(rnp->ofl_seq, rnp->ofl_seq + 1);
WARN_ON_ONCE(!(rnp->ofl_seq & 0x1));
+ rcu_dynticks_eqs_online();
smp_mb(); // Pair with rcu_gp_cleanup()'s ->ofl_seq barrier().
raw_spin_lock_irqsave_rcu_node(rnp, flags);
WRITE_ONCE(rnp->qsmaskinitnext, rnp->qsmaskinitnext | mask);
