Re: [PATCH 13/19] rcu/context-tracking: Move deferred nocb resched to context tracking

[Paul E. McKenney]

On Wed, Mar 02, 2022 at 04:48:04PM +0100, Frederic Weisbecker wrote:
> To prepare for migrating the RCU eqs accounting code to context tracking,
> split the last-resort deferred nocb resched from rcu_user_enter() and
> move it into a separate call from context tracking.
> 
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

Acked-by: Paul E. McKenney <paulmck@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>
> ---
>  include/linux/rcutree.h   |  6 ++++++
>  kernel/context_tracking.c |  8 ++++++++
>  kernel/rcu/tree.c         | 15 ++-------------
>  3 files changed, 16 insertions(+), 13 deletions(-)
> 
> diff --git a/include/linux/rcutree.h b/include/linux/rcutree.h
> index e05334c4c3d1..6d111a3c0cc0 100644
> --- a/include/linux/rcutree.h
> +++ b/include/linux/rcutree.h
> @@ -78,4 +78,10 @@ int rcutree_dead_cpu(unsigned int cpu);
>  int rcutree_dying_cpu(unsigned int cpu);
>  void rcu_cpu_starting(unsigned int cpu);
>  
> +#if defined(CONFIG_NO_HZ_FULL) && (!defined(CONFIG_GENERIC_ENTRY) || !defined(CONFIG_KVM_XFER_TO_GUEST_WORK))
> +void rcu_irq_work_resched(void);
> +#else
> +static inline void rcu_irq_work_resched(void) { }
> +#endif
> +
>  #endif /* __LINUX_RCUTREE_H */
> diff --git a/kernel/context_tracking.c b/kernel/context_tracking.c
> index 155534c409fc..7be7a2044d3a 100644
> --- a/kernel/context_tracking.c
> +++ b/kernel/context_tracking.c
> @@ -60,6 +60,8 @@ static __always_inline void context_tracking_recursion_exit(void)
>   */
>  void noinstr __ct_user_enter(enum ctx_state state)
>  {
> +	lockdep_assert_irqs_disabled();
> +
>  	/* Kernel threads aren't supposed to go to userspace */
>  	WARN_ON_ONCE(!current->mm);
>  
> @@ -81,6 +83,12 @@ void noinstr __ct_user_enter(enum ctx_state state)
>  				vtime_user_enter(current);
>  				instrumentation_end();
>  			}
> +			/*
> +			 * Other than generic entry implementation, we may be past the last
> +			 * rescheduling opportunity in the entry code. Trigger a self IPI
> +			 * that will fire and reschedule once we resume in user/guest mode.
> +			 */
> +			rcu_irq_work_resched();
>  			rcu_user_enter();
>  		}
>  		/*
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index c2528e65de0c..938537958c27 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -686,7 +686,7 @@ static DEFINE_PER_CPU(struct irq_work, late_wakeup_work) =
>   * last resort is to fire a local irq_work that will trigger a reschedule once IRQs
>   * get re-enabled again.
>   */
> -noinstr static void rcu_irq_work_resched(void)
> +noinstr void rcu_irq_work_resched(void)
>  {
>  	struct rcu_data *rdp = this_cpu_ptr(&rcu_data);
>  
> @@ -702,10 +702,7 @@ noinstr static void rcu_irq_work_resched(void)
>  	}
>  	instrumentation_end();
>  }
> -
> -#else
> -static inline void rcu_irq_work_resched(void) { }
> -#endif
> +#endif /* #if !defined(CONFIG_GENERIC_ENTRY) || !defined(CONFIG_KVM_XFER_TO_GUEST_WORK) */
>  
>  /**
>   * rcu_user_enter - inform RCU that we are resuming userspace.
> @@ -720,14 +717,6 @@ static inline void rcu_irq_work_resched(void) { }
>   */
>  noinstr void rcu_user_enter(void)
>  {
> -	lockdep_assert_irqs_disabled();
> -
> -	/*
> -	 * Other than generic entry implementation, we may be past the last
> -	 * rescheduling opportunity in the entry code. Trigger a self IPI
> -	 * that will fire and reschedule once we resume in user/guest mode.
> -	 */
> -	rcu_irq_work_resched();
>  	rcu_eqs_enter(true);
>  }
>  
> -- 
> 2.25.1
>