Re: [PATCH v2 1/8] rcu: Introduce call_rcu_lazy() API implementation

[Joel Fernandes <joel@joelfernandes.org>]

On Wed, Jun 22, 2022 at 10:50:55PM +0000, Joel Fernandes (Google) wrote:
[..]
> diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
> index 2ccf5845957d..fec4fad6654b 100644
> --- a/kernel/rcu/tree.h
> +++ b/kernel/rcu/tree.h
> @@ -267,8 +267,9 @@ struct rcu_data {
>  /* Values for nocb_defer_wakeup field in struct rcu_data. */
>  #define RCU_NOCB_WAKE_NOT	0
>  #define RCU_NOCB_WAKE_BYPASS	1
> -#define RCU_NOCB_WAKE		2
> -#define RCU_NOCB_WAKE_FORCE	3
> +#define RCU_NOCB_WAKE_LAZY	2
> +#define RCU_NOCB_WAKE		3
> +#define RCU_NOCB_WAKE_FORCE	4
>  
>  #define RCU_JIFFIES_TILL_FORCE_QS (1 + (HZ > 250) + (HZ > 500))
>  					/* For jiffies_till_first_fqs and */
> @@ -436,9 +437,10 @@ static struct swait_queue_head *rcu_nocb_gp_get(struct rcu_node *rnp);
>  static void rcu_nocb_gp_cleanup(struct swait_queue_head *sq);
>  static void rcu_init_one_nocb(struct rcu_node *rnp);
>  static bool rcu_nocb_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> -				  unsigned long j);
> +				  unsigned long j, bool lazy);
>  static bool rcu_nocb_try_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> -				bool *was_alldone, unsigned long flags);
> +				bool *was_alldone, unsigned long flags,
> +				bool lazy);
>  static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_empty,
>  				 unsigned long flags);
>  static int rcu_nocb_need_deferred_wakeup(struct rcu_data *rdp, int level);
> diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
> index e369efe94fda..b9244f22e102 100644
> --- a/kernel/rcu/tree_nocb.h
> +++ b/kernel/rcu/tree_nocb.h
> @@ -256,6 +256,8 @@ static bool wake_nocb_gp(struct rcu_data *rdp, bool force)
>  	return __wake_nocb_gp(rdp_gp, rdp, force, flags);
>  }
>  
> +#define LAZY_FLUSH_JIFFIES (10 * HZ)
> +
>  /*
>   * Arrange to wake the GP kthread for this NOCB group at some future
>   * time when it is safe to do so.
> @@ -272,7 +274,10 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
>  	 * Bypass wakeup overrides previous deferments. In case
>  	 * of callback storm, no need to wake up too early.
>  	 */
> -	if (waketype == RCU_NOCB_WAKE_BYPASS) {
> +	if (waketype == RCU_NOCB_WAKE_LAZY) {
> +		mod_timer(&rdp_gp->nocb_timer, jiffies + LAZY_FLUSH_JIFFIES);
> +		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
> +	} else if (waketype == RCU_NOCB_WAKE_BYPASS) {
>  		mod_timer(&rdp_gp->nocb_timer, jiffies + 2);
>  		WRITE_ONCE(rdp_gp->nocb_defer_wakeup, waketype);
>  	} else {
> @@ -296,7 +301,7 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
>   * Note that this function always returns true if rhp is NULL.
>   */
>  static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
> -				     unsigned long j)
> +				     unsigned long j, bool lazy)
>  {
>  	struct rcu_cblist rcl;
>  
> @@ -310,7 +315,13 @@ static bool rcu_nocb_do_flush_bypass(struct rcu_data *rdp, struct rcu_head *rhp,
>  	/* Note: ->cblist.len already accounts for ->nocb_bypass contents. */
>  	if (rhp)
>  		rcu_segcblist_inc_len(&rdp->cblist); /* Must precede enqueue. */
> -	rcu_cblist_flush_enqueue(&rcl, &rdp->nocb_bypass, rhp);
> +
> +	trace_printk("call_rcu_lazy callbacks = %ld\n", READ_ONCE(rdp->nocb_bypass.lazy_len));

Before anyone yells at me, that trace_printk() has been politely asked to take
a walk :-). It got mad at me, but on the next iteration, it wont be there.

thanks,

 - Joel