Re: [PATCH 10/21] context_tracking: Take idle eqs entrypoints over RCU

[Mark Rutland]

Hi Frederic,

On Tue, May 03, 2022 at 12:00:40PM +0200, Frederic Weisbecker wrote:
> The RCU dynticks counter is going to be merged into the context tracking
> subsystem. Start with moving the idle extended quiescent states
> entrypoints to context tracking. For now those are dumb redirections to
> existing RCU calls.

I was a bit confused looking at this, because that redirection only exists for
CONFIG_CONTEXT_TRACKING, and is empty otherwise.

I see this patch makes TREE_RCU select CONTEXT_TRACKING, which means that
works. Since that also means building the rest of the context tracking code, I
think it'd be worth mentioning that in the commit message.

Do all architectures which can use TREE_RCU today already support context
tracking? If not, do those work by default?

Thanks,
Mark.

> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Cc: Neeraj Upadhyay <quic_neeraju@xxxxxxxxxxx>
> Cc: Uladzislau Rezki <uladzislau.rezki@xxxxxxxx>
> Cc: Joel Fernandes <joel@xxxxxxxxxxxxxxxxx>
> Cc: Boqun Feng <boqun.feng@xxxxxxxxx>
> Cc: Nicolas Saenz Julienne <nsaenz@xxxxxxxxxx>
> Cc: Marcelo Tosatti <mtosatti@xxxxxxxxxx>
> Cc: Xiongfeng Wang <wangxiongfeng2@xxxxxxxxxx>
> Cc: Yu Liao<liaoyu15@xxxxxxxxxx>
> Cc: Phil Auld <pauld@xxxxxxxxxx>
> Cc: Paul Gortmaker<paul.gortmaker@xxxxxxxxxxxxx>
> Cc: Alex Belits <abelits@xxxxxxxxxxx>
> ---
>  Documentation/RCU/stallwarn.rst   |  4 ++--
>  arch/arm/mach-imx/cpuidle-imx6q.c |  5 +++--
>  drivers/acpi/processor_idle.c     |  5 +++--
>  drivers/cpuidle/cpuidle.c         |  9 +++++----
>  include/linux/context_tracking.h  |  8 ++++++++
>  include/linux/rcupdate.h          |  2 +-
>  kernel/context_tracking.c         | 12 ++++++++++++
>  kernel/locking/lockdep.c          |  2 +-
>  kernel/rcu/Kconfig                |  2 ++
>  kernel/rcu/tree.c                 |  2 --
>  kernel/rcu/update.c               |  2 +-
>  kernel/sched/idle.c               | 10 +++++-----
>  kernel/sched/sched.h              |  1 +
>  13 files changed, 44 insertions(+), 20 deletions(-)
> 
> diff --git a/Documentation/RCU/stallwarn.rst b/Documentation/RCU/stallwarn.rst
> index 1d863b04727c..bdd52b40f307 100644
> --- a/Documentation/RCU/stallwarn.rst
> +++ b/Documentation/RCU/stallwarn.rst
> @@ -97,8 +97,8 @@ warnings:
>  	which will include additional debugging information.
>  
>  -	A low-level kernel issue that either fails to invoke one of the
> -	variants of rcu_user_enter(), rcu_user_exit(), rcu_idle_enter(),
> -	rcu_idle_exit(), rcu_irq_enter(), or rcu_irq_exit() on the one
> +	variants of rcu_user_enter(), rcu_user_exit(), ct_idle_enter(),
> +	ct_idle_exit(), rcu_irq_enter(), or rcu_irq_exit() on the one
>  	hand, or that invokes one of them too many times on the other.
>  	Historically, the most frequent issue has been an omission
>  	of either irq_enter() or irq_exit(), which in turn invoke
> diff --git a/arch/arm/mach-imx/cpuidle-imx6q.c b/arch/arm/mach-imx/cpuidle-imx6q.c
> index 094337dc1bc7..d086cbae09c3 100644
> --- a/arch/arm/mach-imx/cpuidle-imx6q.c
> +++ b/arch/arm/mach-imx/cpuidle-imx6q.c
> @@ -3,6 +3,7 @@
>   * Copyright (C) 2012 Freescale Semiconductor, Inc.
>   */
>  
> +#include <linux/context_tracking.h>
>  #include <linux/cpuidle.h>
>  #include <linux/module.h>
>  #include <asm/cpuidle.h>
> @@ -24,9 +25,9 @@ static int imx6q_enter_wait(struct cpuidle_device *dev,
>  		imx6_set_lpm(WAIT_UNCLOCKED);
>  	raw_spin_unlock(&cpuidle_lock);
>  
> -	rcu_idle_enter();
> +	ct_idle_enter();
>  	cpu_do_idle();
> -	rcu_idle_exit();
> +	ct_idle_exit();
>  
>  	raw_spin_lock(&cpuidle_lock);
>  	if (num_idle_cpus-- == num_online_cpus())
> diff --git a/drivers/acpi/processor_idle.c b/drivers/acpi/processor_idle.c
> index 32b20efff5f8..935f4113d5f6 100644
> --- a/drivers/acpi/processor_idle.c
> +++ b/drivers/acpi/processor_idle.c
> @@ -22,6 +22,7 @@
>  #include <linux/cpu.h>
>  #include <linux/minmax.h>
>  #include <acpi/processor.h>
> +#include <linux/context_tracking.h>
>  
>  /*
>   * Include the apic definitions for x86 to have the APIC timer related defines
> @@ -648,11 +649,11 @@ static int acpi_idle_enter_bm(struct cpuidle_driver *drv,
>  		raw_spin_unlock(&c3_lock);
>  	}
>  
> -	rcu_idle_enter();
> +	ct_idle_enter();
>  
>  	acpi_idle_do_entry(cx);
>  
> -	rcu_idle_exit();
> +	ct_idle_exit();
>  
>  	/* Re-enable bus master arbitration */
>  	if (dis_bm) {
> diff --git a/drivers/cpuidle/cpuidle.c b/drivers/cpuidle/cpuidle.c
> index ef2ea1b12cd8..62dd956025f3 100644
> --- a/drivers/cpuidle/cpuidle.c
> +++ b/drivers/cpuidle/cpuidle.c
> @@ -23,6 +23,7 @@
>  #include <linux/suspend.h>
>  #include <linux/tick.h>
>  #include <linux/mmu_context.h>
> +#include <linux/context_tracking.h>
>  #include <trace/events/power.h>
>  
>  #include "cpuidle.h"
> @@ -150,12 +151,12 @@ static void enter_s2idle_proper(struct cpuidle_driver *drv,
>  	 */
>  	stop_critical_timings();
>  	if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
> -		rcu_idle_enter();
> +		ct_idle_enter();
>  	target_state->enter_s2idle(dev, drv, index);
>  	if (WARN_ON_ONCE(!irqs_disabled()))
>  		local_irq_disable();
>  	if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
> -		rcu_idle_exit();
> +		ct_idle_exit();
>  	tick_unfreeze();
>  	start_critical_timings();
>  
> @@ -233,10 +234,10 @@ int cpuidle_enter_state(struct cpuidle_device *dev, struct cpuidle_driver *drv,
>  
>  	stop_critical_timings();
>  	if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
> -		rcu_idle_enter();
> +		ct_idle_enter();
>  	entered_state = target_state->enter(dev, drv, index);
>  	if (!(target_state->flags & CPUIDLE_FLAG_RCU_IDLE))
> -		rcu_idle_exit();
> +		ct_idle_exit();
>  	start_critical_timings();
>  
>  	sched_clock_idle_wakeup_event();
> diff --git a/include/linux/context_tracking.h b/include/linux/context_tracking.h
> index e35ae66b4794..27afb75f2650 100644
> --- a/include/linux/context_tracking.h
> +++ b/include/linux/context_tracking.h
> @@ -119,4 +119,12 @@ extern void context_tracking_init(void);
>  static inline void context_tracking_init(void) { }
>  #endif /* CONFIG_CONTEXT_TRACKING_USER_FORCE */
>  
> +#ifdef CONFIG_CONTEXT_TRACKING
> +extern void ct_idle_enter(void);
> +extern void ct_idle_exit(void);
> +#else
> +static inline void ct_idle_enter(void) { }
> +static inline void ct_idle_exit(void) { }
> +#endif /* !CONFIG_CONTEXT_TRACKING */
> +
>  #endif
> diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
> index 7f12daa4708b..d3b25e65f144 100644
> --- a/include/linux/rcupdate.h
> +++ b/include/linux/rcupdate.h
> @@ -129,7 +129,7 @@ static inline void rcu_nocb_flush_deferred_wakeup(void) { }
>   * @a: Code that RCU needs to pay attention to.
>   *
>   * RCU read-side critical sections are forbidden in the inner idle loop,
> - * that is, between the rcu_idle_enter() and the rcu_idle_exit() -- RCU
> + * that is, between the ct_idle_enter() and the ct_idle_exit() -- RCU
>   * will happily ignore any such read-side critical sections.  However,
>   * things like powertop need tracepoints in the inner idle loop.
>   *


> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> index da067a36b326..987f0b2a3962 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -251,3 +251,15 @@ void __init context_tracking_init(void)
>  #endif
>  
>  #endif /* #ifdef CONFIG_CONTEXT_TRACKING_USER */
> +
> +noinstr void ct_idle_enter(void)
> +{
> +	rcu_idle_enter();
> +}
> +EXPORT_SYMBOL_GPL(ct_idle_enter);
> +
> +void ct_idle_exit(void)
> +{
> +	rcu_idle_exit();
> +}
> +EXPORT_SYMBOL_GPL(ct_idle_exit);
> diff --git a/kernel/locking/lockdep.c b/kernel/locking/lockdep.c
> index c06cab6546ed..5f0dfe37234b 100644
> --- a/kernel/locking/lockdep.c
> +++ b/kernel/locking/lockdep.c
> @@ -6546,7 +6546,7 @@ void lockdep_rcu_suspicious(const char *file, const int line, const char *s)
>  
>  	/*
>  	 * If a CPU is in the RCU-free window in idle (ie: in the section
> -	 * between rcu_idle_enter() and rcu_idle_exit(), then RCU
> +	 * between ct_idle_enter() and ct_idle_exit(), then RCU
>  	 * considers that CPU to be in an "extended quiescent state",
>  	 * which means that RCU will be completely ignoring that CPU.
>  	 * Therefore, rcu_read_lock() and friends have absolutely no
> diff --git a/kernel/rcu/Kconfig b/kernel/rcu/Kconfig
> index 27aab870ae4c..85f3b884fa09 100644
> --- a/kernel/rcu/Kconfig
> +++ b/kernel/rcu/Kconfig
> @@ -8,6 +8,8 @@ menu "RCU Subsystem"
>  config TREE_RCU
>  	bool
>  	default y if SMP
> +	# Dynticks-idle tracking
> +	select CONTEXT_TRACKING
>  	help
>  	  This option selects the RCU implementation that is
>  	  designed for very large SMP system with hundreds or
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 2a6e7723fc4d..92a288668e7c 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -664,7 +664,6 @@ noinstr void rcu_idle_enter(void)
>  	lockdep_assert_irqs_disabled();
>  	rcu_eqs_enter(false);
>  }
> -EXPORT_SYMBOL_GPL(rcu_idle_enter);
>  
>  #ifdef CONFIG_NO_HZ_FULL
>  
> @@ -912,7 +911,6 @@ void rcu_idle_exit(void)
>  	rcu_eqs_exit(false);
>  	local_irq_restore(flags);
>  }
> -EXPORT_SYMBOL_GPL(rcu_idle_exit);
>  
>  #ifdef CONFIG_NO_HZ_FULL
>  /**
> diff --git a/kernel/rcu/update.c b/kernel/rcu/update.c
> index 2e93acad1e31..738842c4886b 100644
> --- a/kernel/rcu/update.c
> +++ b/kernel/rcu/update.c
> @@ -85,7 +85,7 @@ module_param(rcu_normal_after_boot, int, 0444);
>   * and while lockdep is disabled.
>   *
>   * Note that if the CPU is in the idle loop from an RCU point of view (ie:
> - * that we are in the section between rcu_idle_enter() and rcu_idle_exit())
> + * that we are in the section between ct_idle_enter() and ct_idle_exit())
>   * then rcu_read_lock_held() sets ``*ret`` to false even if the CPU did an
>   * rcu_read_lock().  The reason for this is that RCU ignores CPUs that are
>   * in such a section, considering these as in extended quiescent state,
> diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
> index 8f8b5020e76a..6de222b23b49 100644
> --- a/kernel/sched/idle.c
> +++ b/kernel/sched/idle.c
> @@ -53,14 +53,14 @@ static noinline int __cpuidle cpu_idle_poll(void)
>  {
>  	trace_cpu_idle(0, smp_processor_id());
>  	stop_critical_timings();
> -	rcu_idle_enter();
> +	ct_idle_enter();
>  	local_irq_enable();
>  
>  	while (!tif_need_resched() &&
>  	       (cpu_idle_force_poll || tick_check_broadcast_expired()))
>  		cpu_relax();
>  
> -	rcu_idle_exit();
> +	ct_idle_exit();
>  	start_critical_timings();
>  	trace_cpu_idle(PWR_EVENT_EXIT, smp_processor_id());
>  
> @@ -98,12 +98,12 @@ void __cpuidle default_idle_call(void)
>  		 *
>  		 * Trace IRQs enable here, then switch off RCU, and have
>  		 * arch_cpu_idle() use raw_local_irq_enable(). Note that
> -		 * rcu_idle_enter() relies on lockdep IRQ state, so switch that
> +		 * ct_idle_enter() relies on lockdep IRQ state, so switch that
>  		 * last -- this is very similar to the entry code.
>  		 */
>  		trace_hardirqs_on_prepare();
>  		lockdep_hardirqs_on_prepare(_THIS_IP_);
> -		rcu_idle_enter();
> +		ct_idle_enter();
>  		lockdep_hardirqs_on(_THIS_IP_);
>  
>  		arch_cpu_idle();
> @@ -116,7 +116,7 @@ void __cpuidle default_idle_call(void)
>  		 */
>  		raw_local_irq_disable();
>  		lockdep_hardirqs_off(_THIS_IP_);
> -		rcu_idle_exit();
> +		ct_idle_exit();
>  		lockdep_hardirqs_on(_THIS_IP_);
>  		raw_local_irq_enable();
>  
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 58263f90c559..f398f2bf05f4 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -27,6 +27,7 @@
>  #include <linux/capability.h>
>  #include <linux/cgroup_api.h>
>  #include <linux/cgroup.h>
> +#include <linux/context_tracking.h>
>  #include <linux/cpufreq.h>
>  #include <linux/cpumask_api.h>
>  #include <linux/ctype.h>
> -- 
> 2.25.1
>