Re: [PATCH] tick/nohz: Only check for RCU deferred wakeup on user/guest entry when needed

[Paul E. McKenney]

On Thu, May 27, 2021 at 01:34:41PM +0200, Frederic Weisbecker wrote:
> Checking for and processing RCU-nocb deferred wakeup upon user/guest
> entry is only relevant when nohz_full runs on the local CPU, otherwise
> the periodic tick should take care of it.

The above initially confused me into thinking that we were dealing
with non-local CPUs, and the patch itself straightened me out.
How about something like this?

	Because the periodic tick is active during user/guest execution
	on non-nohz_full CPUs, only nohz_full CPUs need to check for
	RCU-nocb deferred wakeup on user/guest transitions.

> Make sure we don't needlessly pollute these fast-paths as a -3%
> performance regression on a will-it-scale.per_process_ops has been
> reported so far.
> 
> Reported-by: kernel test robot <oliver.sang@xxxxxxxxx>
> Fixes: 47b8ff194c1f (entry: Explicitly flush pending rcuog wakeup before last rescheduling point)
> Fixes: 4ae7dc97f726 (entry/kvm: Explicitly flush pending rcuog wakeup before last rescheduling point)
> Cc: Ingo Molnar <mingo@xxxxxxxxxx>
> Cc: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: stable@xxxxxxxxxxxxxxx
> Cc: Paul E. McKenney <paulmck@xxxxxxxxxx>
> Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
> Signed-off-by: Frederic Weisbecker <frederic@xxxxxxxxxx>

Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxx>

> ---
>  include/linux/entry-kvm.h | 3 ++-
>  include/linux/tick.h      | 7 +++++++
>  kernel/entry/common.c     | 5 +++--
>  kernel/time/tick-sched.c  | 1 +
>  4 files changed, 13 insertions(+), 3 deletions(-)
> 
> diff --git a/include/linux/entry-kvm.h b/include/linux/entry-kvm.h
> index 8b2b1d68b954..136b8d97d8c0 100644
> --- a/include/linux/entry-kvm.h
> +++ b/include/linux/entry-kvm.h
> @@ -3,6 +3,7 @@
>  #define __LINUX_ENTRYKVM_H
>  
>  #include <linux/entry-common.h>
> +#include <linux/tick.h>
>  
>  /* Transfer to guest mode work */
>  #ifdef CONFIG_KVM_XFER_TO_GUEST_WORK
> @@ -57,7 +58,7 @@ int xfer_to_guest_mode_handle_work(struct kvm_vcpu *vcpu);
>  static inline void xfer_to_guest_mode_prepare(void)
>  {
>  	lockdep_assert_irqs_disabled();
> -	rcu_nocb_flush_deferred_wakeup();
> +	tick_nohz_user_enter_prepare();
>  }
>  
>  /**
> diff --git a/include/linux/tick.h b/include/linux/tick.h
> index 7340613c7eff..1a0ff88fa107 100644
> --- a/include/linux/tick.h
> +++ b/include/linux/tick.h
> @@ -11,6 +11,7 @@
>  #include <linux/context_tracking_state.h>
>  #include <linux/cpumask.h>
>  #include <linux/sched.h>
> +#include <linux/rcupdate.h>
>  
>  #ifdef CONFIG_GENERIC_CLOCKEVENTS
>  extern void __init tick_init(void);
> @@ -300,4 +301,10 @@ static inline void tick_nohz_task_switch(void)
>  		__tick_nohz_task_switch();
>  }
>  
> +static inline void tick_nohz_user_enter_prepare(void)
> +{
> +	if (tick_nohz_full_cpu(smp_processor_id()))
> +		rcu_nocb_flush_deferred_wakeup();
> +}
> +
>  #endif
> diff --git a/kernel/entry/common.c b/kernel/entry/common.c
> index a0b3b04fb596..bf16395b9e13 100644
> --- a/kernel/entry/common.c
> +++ b/kernel/entry/common.c
> @@ -5,6 +5,7 @@
>  #include <linux/highmem.h>
>  #include <linux/livepatch.h>
>  #include <linux/audit.h>
> +#include <linux/tick.h>
>  
>  #include "common.h"
>  
> @@ -186,7 +187,7 @@ static unsigned long exit_to_user_mode_loop(struct pt_regs *regs,
>  		local_irq_disable_exit_to_user();
>  
>  		/* Check if any of the above work has queued a deferred wakeup */
> -		rcu_nocb_flush_deferred_wakeup();
> +		tick_nohz_user_enter_prepare();
>  
>  		ti_work = READ_ONCE(current_thread_info()->flags);
>  	}
> @@ -202,7 +203,7 @@ static void exit_to_user_mode_prepare(struct pt_regs *regs)
>  	lockdep_assert_irqs_disabled();
>  
>  	/* Flush pending rcuog wakeup before the last need_resched() check */
> -	rcu_nocb_flush_deferred_wakeup();
> +	tick_nohz_user_enter_prepare();
>  
>  	if (unlikely(ti_work & EXIT_TO_USER_MODE_WORK))
>  		ti_work = exit_to_user_mode_loop(regs, ti_work);
> diff --git a/kernel/time/tick-sched.c b/kernel/time/tick-sched.c
> index 828b091501ca..6784f27a3099 100644
> --- a/kernel/time/tick-sched.c
> +++ b/kernel/time/tick-sched.c
> @@ -230,6 +230,7 @@ static void tick_sched_handle(struct tick_sched *ts, struct pt_regs *regs)
>  
>  #ifdef CONFIG_NO_HZ_FULL
>  cpumask_var_t tick_nohz_full_mask;
> +EXPORT_SYMBOL_GPL(tick_nohz_full_mask);
>  bool tick_nohz_full_running;
>  EXPORT_SYMBOL_GPL(tick_nohz_full_running);
>  static atomic_t tick_dep_mask;
> -- 
> 2.25.1
>