Re: [PATCH v6 1/4] rcu: Make call_rcu() lazy to save power

[Frederic Weisbecker]

On Thu, Sep 22, 2022 at 10:01:01PM +0000, Joel Fernandes (Google) wrote:
> @@ -3902,7 +3939,11 @@ static void rcu_barrier_entrain(struct rcu_data *rdp)
>  	rdp->barrier_head.func = rcu_barrier_callback;
>  	debug_rcu_head_queue(&rdp->barrier_head);
>  	rcu_nocb_lock(rdp);
> -	WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies));
> +	/*
> +	 * Flush the bypass list, but also wake up the GP thread as otherwise
> +	 * bypass/lazy CBs maynot be noticed, and can cause real long delays!
> +	 */
> +	WARN_ON_ONCE(!rcu_nocb_flush_bypass(rdp, NULL, jiffies, FLUSH_BP_WAKE));

This fixes an issue that goes beyond lazy implementation. It should be done
in a separate patch, handling rcu_segcblist_entrain() as well, with "Fixes: " tag.

And then FLUSH_BP_WAKE is probably not needed anymore. 

>  	if (rcu_segcblist_entrain(&rdp->cblist, &rdp->barrier_head)) {
>  		atomic_inc(&rcu_state.barrier_cpu_count);
>  	} else {
> @@ -269,10 +294,14 @@ static void wake_nocb_gp_defer(struct rcu_data *rdp, int waketype,
>  	raw_spin_lock_irqsave(&rdp_gp->nocb_gp_lock, flags);
>  
>  	/*
> -	 * Bypass wakeup overrides previous deferments. In case
> -	 * of callback storm, no need to wake up too early.
> +	 * 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
> +		&& READ_ONCE(rdp->nocb_defer_wakeup) == RCU_NOCB_WAKE_NOT) {

This can be a plain READ since ->nocb_defer_wakeup is only written under ->nocb_gp_lock.

> +		mod_timer(&rdp_gp->nocb_timer, jiffies + jiffies_till_flush);
> +		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 {
> @@ -512,9 +598,16 @@ static void __call_rcu_nocb_wake(struct rcu_data *rdp, bool was_alldone,
>  	}
>  	// Need to actually to a wakeup.
>  	len = rcu_segcblist_n_cbs(&rdp->cblist);
> +	bypass_len = rcu_cblist_n_cbs(&rdp->nocb_bypass);
> +	lazy_len = READ_ONCE(rdp->lazy_len);
>  	if (was_alldone) {
>  		rdp->qlen_last_fqs_check = len;
> -		if (!irqs_disabled_flags(flags)) {
> +		// Only lazy CBs in bypass list
> +		if (lazy_len && bypass_len == lazy_len) {
> +			rcu_nocb_unlock_irqrestore(rdp, flags);
> +			wake_nocb_gp_defer(rdp, RCU_NOCB_WAKE_LAZY,
> +					   TPS("WakeLazy"));

I'm trying to think of a case where rcu_nocb_try_bypass() returns false
(queue to regular list) but then call_rcu() -> __call_rcu_nocb_wake() ends up
seeing a lazy bypass queue even though we are queueing a non-lazy callback
(should have flushed in this case).

Looks like it shouldn't happen, even with concurrent (de-offloading) but just
in case, can we add:

      WARN_ON_ONCE(lazy_len != len)

> +		} else if (!irqs_disabled_flags(flags)) {
>  			/* ... if queue was empty ... */
>  			rcu_nocb_unlock_irqrestore(rdp, flags);
>  			wake_nocb_gp(rdp, false);

Thanks.