linux-kernel - Re: [PATCH v5 06/18] rcu: Introduce call_rcu

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Date:   Sun, 4 Sep 2022 23:01:33 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     Joel Fernandes <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,
        vineeth@...byteword.org, boqun.feng@...il.com
Subject: Re: [PATCH v5 06/18] rcu: Introduce call_rcu_lazy() API
 implementation

On Sat, Sep 03, 2022 at 10:00:29PM +0000, Joel Fernandes wrote:
> On Fri, Sep 02, 2022 at 05:21:32PM +0200, Frederic Weisbecker wrote:
> +
> +	raw_spin_lock_irqsave(&my_rdp->nocb_gp_lock, flags);

This is locking during the whole group iteration potentially contending call_rcu(),
idle loop, resume to userspace on all rdp in the group...

How about not overwriting timers instead and only set the RCU_NOCB_WAKE_LAZY
timer when it is previously in RCU_NOCB_WAKE_NOT state? After all if the timer is armed,
it's because we have regular callbacks queued and thus we don't need to wait
before processing the lazy callbacks since we are going to start a grace period
anyway.

Thanks.


>  	list_for_each_entry_rcu(rdp, &my_rdp->nocb_head_rdp, nocb_entry_rdp, 1) {
>  		bool needwake_state = false;
>  		bool flush_bypass = false;
> @@ -855,14 +870,15 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
>  		// If bypass list only has lazy CBs. Add a deferred
>  		// lazy wake up.
>  		if (lazy && !bypass) {
> -			wake_nocb_gp_defer(my_rdp, RCU_NOCB_WAKE_LAZY,
> +			wake_nocb_gp_defer_locked(my_rdp, RCU_NOCB_WAKE_LAZY,
>  					TPS("WakeLazyIsDeferred"));
>  		// Otherwise add a deferred bypass wake up.
>  		} else if (bypass) {
> -			wake_nocb_gp_defer(my_rdp, RCU_NOCB_WAKE_BYPASS,
> +			wake_nocb_gp_defer_locked(my_rdp, RCU_NOCB_WAKE_BYPASS,
>  					TPS("WakeBypassIsDeferred"));
>  		}
>  	}
> +	raw_spin_unlock_irqrestore(&my_rdp->nocb_gp_lock, flags);
>  
>  	if (rcu_nocb_poll) {
>  		/* Polling, so trace if first poll in the series. */
> -- 
> 2.37.2.789.g6183377224-goog
>