linux-kernel - Re: [PATCH 1/3] rcu: Reduce synchronize

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Date:   Wed, 25 Oct 2023 17:13:27 +0200
From:   Frederic Weisbecker <frederic@...nel.org>
To:     "Uladzislau Rezki (Sony)" <urezki@...il.com>
Cc:     "Paul E . McKenney" <paulmck@...nel.org>,
        RCU <rcu@...r.kernel.org>,
        Neeraj upadhyay <Neeraj.Upadhyay@....com>,
        Boqun Feng <boqun.feng@...il.com>,
        Hillf Danton <hdanton@...a.com>,
        Joel Fernandes <joel@...lfernandes.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Oleksiy Avramchenko <oleksiy.avramchenko@...y.com>
Subject: Re: [PATCH 1/3] rcu: Reduce synchronize_rcu() waiting time

Le Wed, Oct 25, 2023 at 04:09:13PM +0200, Uladzislau Rezki (Sony) a écrit :
> +/*
> + * Helper function for rcu_gp_cleanup().
> + */
> +static void rcu_sr_normal_gp_cleanup(void)
> +{
> +	struct llist_node *head, *tail, *pos;
> +	int i = 0;
> +
> +	tail = READ_ONCE(sr.wait_tail);
> +	head = llist_del_all(&sr.wait);

This could be llist_empty() first to do a quick
cheap check. And then __llist_del_all() here because
it appears nothing else than gp kthread can touch sr.wait.

> +
> +	llist_for_each_safe(pos, head, head) {

Two times head intended here? There should be some
temporary storage in the middle.

> +		rcu_sr_normal_complete(pos);
> +
> +		if (++i == MAX_SR_WAKE_FROM_GP) {
> +			/* If last, process it also. */
> +			if (head && !head->next)
> +				continue;
> +			break;
> +		}
> +	}
> +
> +	if (head) {
> +		/* Can be not empty. */
> +		llist_add_batch(head, tail, &sr.done);
> +		queue_work(system_highpri_wq, &sr_normal_gp_cleanup);

So you can have:

* Queue to sr.curr is atomic fully ordered
* Check and move from sr.curr to sr.wait is atomic fully ordered
* Check from sr.wait can have a quick unatomic unordered
  llist_empty() check. Then extract unatomic unordered as well.
* If too many, move atomic/ordered to sr.done.

Am I missing something?