On Fri, Sep 16, 2022 at 03:42:58PM +0200, Frederic Weisbecker wrote:
> On Thu, Sep 15, 2022 at 01:58:25PM +0800, Pingfan Liu wrote:
> > As Paul pointed out "The tick_dep_clear() is SMP-safe because it uses
> > atomic operations, but the problem is that if there are multiple
> > nohz_full CPUs going offline concurrently, the first CPU to invoke
> > rcutree_dead_cpu() will turn the tick off. This might require an
> > atomically manipulated counter to mediate the calls to
> > rcutree_dead_cpu(). "
> >
> > This patch introduces a new member ->dying to rcu_node, which reflects
> > the number of concurrent offlining cpu. TICK_DEP_BIT_RCU is set by
> > the first entrance and cleared by the last.
> >
> > Note: now, tick_dep_set() is put under the rnp->lock, but since it takes
> > no lock, no extra locking order is introduced.
> >
> > Suggested-by: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > Signed-off-by: Pingfan Liu <kernelfans@xxxxxxxxx>
> > Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxx>
> > Cc: David Woodhouse <dwmw@xxxxxxxxxxxx>
> > Cc: Frederic Weisbecker <frederic@xxxxxxxxxx>
> > Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> > Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
> > Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
> > Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
> > Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
> > Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> > Cc: "Jason A. Donenfeld" <Jason@xxxxxxxxx>
> > To: rcu@xxxxxxxxxxxxxxx
> > ---
> > kernel/rcu/tree.c | 19 ++++++++++++++-----
> > kernel/rcu/tree.h | 1 +
> > 2 files changed, 15 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > index 8a829b64f5b2..f8bd0fc5fd2f 100644
> > --- a/kernel/rcu/tree.c
> > +++ b/kernel/rcu/tree.c
> > @@ -2164,13 +2164,19 @@ int rcutree_dead_cpu(unsigned int cpu)
> > {
> > struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> > struct rcu_node *rnp = rdp->mynode; /* Outgoing CPU's rdp & rnp. */
> > + unsigned long flags;
> > + u8 dying;
> >
> > if (!IS_ENABLED(CONFIG_HOTPLUG_CPU))
> > return 0;
> >
> > WRITE_ONCE(rcu_state.n_online_cpus, rcu_state.n_online_cpus - 1);
> > - // Stop-machine done, so allow nohz_full to disable tick.
> > - tick_dep_clear(TICK_DEP_BIT_RCU);
> > + raw_spin_lock_irqsave_rcu_node(rnp, flags);
> > + dying = --rnp->dying;
> > + if (!dying)
> > + // Stop-machine done, so allow nohz_full to disable tick.
> > + tick_dep_clear(TICK_DEP_BIT_RCU);
> > + raw_spin_lock_irqsave_rcu_node(rnp, flags);
>
> Note this is only locking the rdp's node, not the root node.
> Therefore if CPU 0 and CPU 256 are going off at the same time and they
> don't belong to the same node, the above won't protect against concurrent
> TICK_DEP_BIT_RCU set/clear.
>
Nice, thanks for the careful thoughts. How about moving the counting
place to the root node?
> My suspicion is that we don't need this TICK_DEP_BIT_RCU tick dependency
> anymore. I believe it was there because of issues that were fixed with:
>
> 53e87e3cdc15 (timers/nohz: Last resort update jiffies on nohz_full IRQ entry)
> and:
>
> a1ff03cd6fb9 (tick: Detect and fix jiffies update stall)
>
> It's unfortunately just suspicion because the reason for that tick dependency
> is unclear but I believe it should be safe to remove now.
>
I have gone through this tick dependency again, but got less.
I think at least from the RCU's viewpoint, it is useless since
multi_cpu_stop()->rcu_momentary_dyntick_idle() has eliminate the
requirement for tick interrupt.
Is there a way to have a convincing test so that these code can be removed?
Or this code will be got along with?
Thanks,
Pingfan
