linux-kernel - Re: [PATCH tip/core/rcu 22/30] rcu: Don't flag non-starting GPs before GP kthread is running

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Message-ID: <20200217152517.26cc11ea@gandalf.local.home>
Date:   Mon, 17 Feb 2020 15:25:17 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        dipankar@...ibm.com, akpm@...ux-foundation.org,
        mathieu.desnoyers@...icios.com, josh@...htriplett.org,
        tglx@...utronix.de, peterz@...radead.org, dhowells@...hat.com,
        edumazet@...gle.com, fweisbec@...il.com, oleg@...hat.com,
        joel@...lfernandes.org,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH tip/core/rcu 22/30] rcu: Don't flag non-starting GPs
 before GP kthread is running

On Sat, 15 Feb 2020 05:42:08 -0800
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> 
> And does the following V2 look better?
> 

For the issue I brought up, yes. But now I have to ask...

> @@ -1252,10 +1253,10 @@ static bool rcu_future_gp_cleanup(struct rcu_node *rnp)
>   */
>  static void rcu_gp_kthread_wake(void)
>  {
> -	if ((current == rcu_state.gp_kthread &&
> -	     !in_irq() && !in_serving_softirq()) ||
> -	    !READ_ONCE(rcu_state.gp_flags) ||
> -	    !rcu_state.gp_kthread)
> +	struct task_struct *t = READ_ONCE(rcu_state.gp_kthread);
> +
> +	if ((current == t && !in_irq() && !in_serving_softirq()) ||
> +	    !READ_ONCE(rcu_state.gp_flags) || !t)

Why not test !t first? As that is the fastest operation in the if
statement, and will shortcut all the other operations if it is true.

As I like to micro-optimize ;-), for or (||) statements, I like to add
the fastest operations first. To me, that would be:

	if (!t || READ_ONCE(rcu_state.gp_flags) ||
	    (current == t && !in_irq() && !in_serving_softirq()))
		return;

Note, in_irq() reads preempt_count which is not always a fast operation.

-- Steve


>  		return;
>  	WRITE_ONCE(rcu_state.gp_wake_time, jiffies);
>  	WRITE_ONCE(rcu_state.gp_wake_seq, READ_ONCE(rcu_state.gp_seq));
> @@ -3554,7 +3555,10 @@ static int __init rcu_spawn_gp_kthread(void)
>  	}
>  	rnp = rcu_get_root();
>  	raw_spin_lock_irqsave_rcu_node(rnp, flags);
> -	rcu_state.gp_kthread = t;
> +	WRITE_ONCE(rcu_state.gp_activity, jiffies);
> +	WRITE_ONCE(rcu_state.gp_req_activity, jiffies);
> +	// Reset .gp_activity and .gp_req_activity before setting .gp_kthread.
> +	smp_store_release(&rcu_state.gp_kthread, t);  /* ^^^ */
>  	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
>  	wake_up_process(t);
>  	rcu_spawn_nocb_kthreads();
> diff --git a/kernel/rcu/tree_stall.h b/kernel/rcu/tree_stall.h
> index 488b71d..16ad7ad 100644
\