Re: [PATCH 1/3] rcu: Reduce synchronize_rcu() waiting time

[Uladzislau Rezki <urezki@gmail.com>]

On Wed, Oct 25, 2023 at 05:13:27PM +0200, Frederic Weisbecker wrote:
> 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.
> 
No problem i can fix it. Initially i had a check first!

> > +
> > +	llist_for_each_safe(pos, head, head) {
> 
> Two times head intended here? There should be some
> temporary storage in the middle.
> 
Yes. It is intentially done. The head is updated, i.e. shifted to a next,
because we directly process users from a GP. The number is limited to 5
all the rest is deferred.

> > +		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?
>
If too many move to done and kick the helper. The sr.wait can not
be touched until the rcu_sr_normal_gp_cleanup() is completed, i.e.:

<snip>
GP-kthread(same and one task context):
    rcu_sr_normal_gp_cleanup();
    wait for a grace period;
    rcu_sr_normal_gp_cleanup();
<snip>

Am i missing your point?

--
Uladzislau Rezki