Re: [PATCHv2 3/3] rcu: coordinate tick dependency during concurrent offlining

[Frederic Weisbecker <frederic@kernel.org>]

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@kernel.org>
> Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
> Cc: "Paul E. McKenney" <paulmck@kernel.org>
> Cc: David Woodhouse <dwmw@amazon.co.uk>
> Cc: Frederic Weisbecker <frederic@kernel.org>
> Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
> Cc: Josh Triplett <josh@joshtriplett.org>
> Cc: Steven Rostedt <rostedt@goodmis.org>
> Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
> Cc: Lai Jiangshan <jiangshanlai@gmail.com>
> Cc: Joel Fernandes <joel@joelfernandes.org>
> Cc: "Jason A. Donenfeld" <Jason@zx2c4.com>
> To: rcu@vger.kernel.org
> ---
>  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.

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.

Thanks.