linux-kernel - Re: [PATCH v8 02/13] rcu: Make call

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Date:   Wed, 12 Oct 2022 01:10:53 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Joel Fernandes (Google)" <joel@...lfernandes.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        rushikesh.s.kadam@...el.com, urezki@...il.com,
        neeraj.iitr10@...il.com, paulmck@...nel.org, rostedt@...dmis.org,
        youssefesmat@...gle.com, surenb@...gle.com
Subject: Re: [PATCH v8 02/13] rcu: Make call_rcu() lazy to save power

On Tue, Oct 11, 2022 at 06:01:31PM +0000, Joel Fernandes (Google) wrote:
> @@ -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) {

No need for READ_ONCE() here.

> +		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 {
> @@ -611,6 +682,8 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
>  	unsigned long flags;
>  	bool gotcbs = false;
>  	unsigned long j = jiffies;
> +	bool lazy = false;
> +	long lazy_ncbs;

Looks like lazy_ncbs can be declared withing the loop, as well as bypass_cbs.

Anyway, apart from such boring cosmectics:

Acked-by: Frederic Weisbecker <frederic@...nel.org>

Thanks!