Re: [PATCH v3 1/1] rcu: Reduce synchronize_rcu() waiting time

[Uladzislau Rezki <urezki@gmail.com>]

On Thu, Oct 19, 2023 at 07:44:32PM +0800, Hillf Danton wrote:
> On Wed, 18 Oct 2023 13:20:49 +0200 Uladzislau Rezki (Sony) <urezki@gmail.com> wrote:
> > > On Mon, Oct 16, 2023 at 1:30 PM Uladzislau Rezki (Sony) <urezki@gmail.com> wrote:
> > > > +static void rcu_sr_normal_gp_init(void)
> > > > +{
> > > > +	struct llist_node *llnode, *rcu;
> > > > +	int ret;
> > > > +
> > > > +	if (llist_empty(&sr.curr))
> > > > +		return;
> > > 
> > > This empty check erases the curr_tail race below instead of
> > > atomic_inc_return(&sr.active), because llist_add() will never return true
> > > after this check.
> > >
> > I use "active" counter to guarantee that a tail was updated in the
> > rcu_sr_normal_add_req(), i.e. the list might be not empty whereas the
> > tail updating might be in progress. llist_add() success and the task gets
> > preemted as an example.
> 
> You are right - the preempt is what I missed.
> 
> Then another question rising - the adding order of sync requests is changed
> at wakeup time, as shown by the two functions below with sr.curr_tail and
> sr.active cut off.
> 
> static void rcu_sr_normal_gp_init(void)
> {
> 	struct llist_node *llnode, *rcu;
> 
> 	llnode = llist_del_all(&sr.curr);
> 	rcu = llist_reverse_order(llnode);
> 	if (!rcu)
> 		return;
> 	/*
> 	 * A waiting list of GP should be empty on this step,
> 	 * since a GP-kthread, rcu_gp_init() -> gp_cleanup(),
> 	 * rolls it over. If not, it is a BUG, warn a user.
> 	 */
> 	WARN_ON_ONCE(!llist_empty(&sr.wait));
> 
> 	WRITE_ONCE(sr.wait_tail, llnode);
> 
> 	llist_add_batch(rcu, llnode, &sr.wait);
> }
> 
> static void rcu_sr_normal_add_req(struct rcu_synchronize *rs)
> {
> 	llist_add((struct llist_node *) &rs->head, &sr.curr);
> }
>
This is what i was thinking of in the beginning but i had decided
to make it more complex and maintain the current tail. We will get
a slight penalty in performance on a synthetic test but i have compared
it and this is negligible in fact.

Another good thing with it is we process users in a reverse order,
i.e. the most waiting users to less ones, this is from a waiting time
point of view.

I like it better and it is more simple. Anyway we can improve it
further.

Thank you for the proposal!

--
Uladzislau Rezki