Re: [PATCH 3/3] rcu: Assume rcu_report_dead() always deals with local CPU

From: Paul E. McKenney
Date: Wed May 19 2021 - 14:51:12 EST

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On Wed, May 19, 2021 at 02:09:30AM +0200, Frederic Weisbecker wrote:
> rcu_report_dead() is always called locally from the idle path. Passing
> a CPU number to it suggests otherwise and is rather error-prone as the
> code inside relies on locality.
>
> Robustify the function prototype and refine the name along the way.
>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

Makes a lot of sense, thank you!

On the function name, here is the list:

int rcutree_prepare_cpu(unsigned int cpu) -- notifier from any CPU.
void rcu_cpu_starting(unsigned int cpu) -- direct call on incoming CPU.
int rcutree_online_cpu(unsigned int cpu) -- notifier from any CPU.

int rcutree_offline_cpu(unsigned int cpu) -- notifier from any CPU.
void rcu_report_dead(unsigned int cpu) -- direct call on outgoing CPU.
void rcutree_migrate_callbacks(int cpu) -- direct call from surviving CPU.
int rcutree_dead_cpu(unsigned int cpu) -- notifier from any CPU.

Note that rcu_report_dead() can also be invoked from cpu_die_early() on
other CPU when onlining a CPU fails. This happens on arm64. Which might
be an arm64 bug, but unless I am missing something it is a case where
rcu_report_dead() is called non-locally.

Which means that I have to hold off on this patch for the moment.

And the naming is currently a bit random, isn't it? :-/

Maybe rcutree_*_cpu() if there is a CPU parameter and rcutree_*_self()
if all calls run on the CPU in question? I cannot immediately think of
a reason to make names reflect whether the corresponding functions are
directly called or are called via notifier. Thoughts?

Thanx, Paul

> ---
> .../Expedited-Grace-Periods/Expedited-Grace-Periods.rst | 2 +-
> Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-fqs.svg | 2 +-
> Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp.svg | 2 +-
> .../RCU/Design/Memory-Ordering/TreeRCU-hotplug.svg | 2 +-
> Documentation/RCU/Design/Requirements/Requirements.rst | 2 +-
> arch/arm64/kernel/smp.c | 2 +-
> include/linux/rcupdate.h | 2 +-
> kernel/cpu.c | 4 ++--
> kernel/rcu/tree.c | 6 +++---
> 9 files changed, 12 insertions(+), 12 deletions(-)
>
> diff --git a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst
> index 6f89cf1e567d..eddf5eea94f1 100644
> --- a/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst
> +++ b/Documentation/RCU/Design/Expedited-Grace-Periods/Expedited-Grace-Periods.rst
> @@ -181,7 +181,7 @@ operations is carried out at several levels:
> of this wait (or series of waits, as the case may be) is to permit a
> concurrent CPU-hotplug operation to complete.
> #. In the case of RCU-sched, one of the last acts of an outgoing CPU is
> - to invoke ``rcu_report_dead()``, which reports a quiescent state for
> + to invoke ``rcu_report_dead_cpu()``, which reports a quiescent state for
> that CPU. However, this is likely paranoia-induced redundancy.
>
> +-----------------------------------------------------------------------+
> diff --git a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-fqs.svg b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-fqs.svg
> index 7ddc094d7f28..6e7b05ff00d3 100644
> --- a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-fqs.svg
> +++ b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp-fqs.svg
> @@ -1135,7 +1135,7 @@
> font-weight="bold"
> font-size="192"
> id="text202-7-5-3-27-6-5"
> - style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcu_report_dead()</text>
> + style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcu_report_dead_cpu()</text>
> <text
> xml:space="preserve"
> x="3745.7725"
> diff --git a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp.svg b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp.svg
> index 069f6f8371c2..ede6ae66d286 100644
> --- a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp.svg
> +++ b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-gp.svg
> @@ -3274,7 +3274,7 @@
> font-weight="bold"
> font-size="192"
> id="text202-7-5-3-27-6-5"
> - style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcu_report_dead()</text>
> + style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcu_report_dead_cpu()</text>
> <text
> xml:space="preserve"
> x="3745.7725"
> diff --git a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-hotplug.svg b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-hotplug.svg
> index 2c9310ba29ba..e0ab8cd8db2d 100644
> --- a/Documentation/RCU/Design/Memory-Ordering/TreeRCU-hotplug.svg
> +++ b/Documentation/RCU/Design/Memory-Ordering/TreeRCU-hotplug.svg
> @@ -607,7 +607,7 @@
> font-weight="bold"
> font-size="192"
> id="text202-7-5-3-27-6"
> - style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcu_report_dead()</text>
> + style="font-size:192px;font-style:normal;font-weight:bold;text-anchor:start;fill:#000000;stroke-width:0.025in;font-family:Courier">rcu_report_dead_cpu()</text>
> <text
> xml:space="preserve"
> x="3745.7725"
> diff --git a/Documentation/RCU/Design/Requirements/Requirements.rst b/Documentation/RCU/Design/Requirements/Requirements.rst
> index 38a39476fc24..ede6a2bbf69d 100644
> --- a/Documentation/RCU/Design/Requirements/Requirements.rst
> +++ b/Documentation/RCU/Design/Requirements/Requirements.rst
> @@ -1953,7 +1953,7 @@ if offline CPUs block an RCU grace period for too long.
>
> An offline CPU's quiescent state will be reported either:
>
> -1. As the CPU goes offline using RCU's hotplug notifier (rcu_report_dead()).
> +1. As the CPU goes offline using RCU's hotplug notifier (rcu_report_dead_cpu()).
> 2. When grace period initialization (rcu_gp_init()) detects a
> race either with CPU offlining or with a task unblocking on a leaf
> ``rcu_node`` structure whose CPUs are all offline.
> diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
> index dcd7041b2b07..74c0ae3d6c93 100644
> --- a/arch/arm64/kernel/smp.c
> +++ b/arch/arm64/kernel/smp.c
> @@ -414,7 +414,7 @@ void cpu_die_early(void)
>
> /* Mark this CPU absent */
> set_cpu_present(cpu, 0);
> - rcu_report_dead(cpu);
> + rcu_report_dead_cpu();
>
> if (IS_ENABLED(CONFIG_HOTPLUG_CPU)) {
> update_cpu_boot_status(CPU_KILL_ME);
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index d9680b798b21..fda1dc9b4e03 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -85,7 +85,7 @@ static inline int rcu_preempt_depth(void)
> void rcu_init(void);
> extern int rcu_scheduler_active __read_mostly;
> void rcu_sched_clock_irq(int user);
> -void rcu_report_dead(unsigned int cpu);
> +void rcu_report_dead_cpu(void);
> void rcutree_migrate_callbacks(int cpu);
>
> #ifdef CONFIG_TASKS_RCU_GENERIC
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index e538518556f4..852fcbda1fd8 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1016,10 +1016,10 @@ void cpuhp_report_idle_dead(void)
> struct cpuhp_cpu_state *st = this_cpu_ptr(&cpuhp_state);
>
> BUG_ON(st->state != CPUHP_AP_OFFLINE);
> - rcu_report_dead(smp_processor_id());
> + rcu_report_dead_cpu();
> st->state = CPUHP_AP_IDLE_DEAD;
> /*
> - * We cannot call complete after rcu_report_dead() so we delegate it
> + * We cannot call complete after rcu_report_dead_cpu() so we delegate it
> * to an online cpu.
> */
> smp_call_function_single(cpumask_first(cpu_online_mask),
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index a6b448e6e059..824a3b367797 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -4322,11 +4322,11 @@ void rcu_cpu_starting(unsigned int cpu)
> * from the outgoing CPU rather than from the cpuhp_step mechanism.
> * This is because this function must be invoked at a precise location.
> */
> -void rcu_report_dead(unsigned int cpu)
> +void rcu_report_dead_cpu(void)
> {
> unsigned long flags;
> unsigned long mask;
> - struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
> + struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
> struct rcu_node *rnp = rdp->mynode; /* Outgoing CPU's rdp & rnp. */
>
> // Do any dangling deferred wakeups.
> @@ -4334,7 +4334,7 @@ void rcu_report_dead(unsigned int cpu)
>
> /* QS for any half-done expedited grace period. */
> preempt_disable();
> - rcu_report_exp_rdp(this_cpu_ptr(&rcu_data));
> + rcu_report_exp_rdp(rdp);
> preempt_enable();
> rcu_preempt_deferred_qs(current);
>
> --
> 2.25.1
>

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