From: Frederic Weisbecker <frederic@kernel.org>
To: Uladzislau Rezki <urezki@gmail.com>
Cc: "Paul E . McKenney" <paulmck@kernel.org>,
RCU <rcu@vger.kernel.org>,
Neeraj upadhyay <Neeraj.Upadhyay@amd.com>,
Boqun Feng <boqun.feng@gmail.com>,
Hillf Danton <hdanton@sina.com>,
Joel Fernandes <joel@joelfernandes.org>,
LKML <linux-kernel@vger.kernel.org>,
Oleksiy Avramchenko <oleksiy.avramchenko@sony.com>
Subject: Re: [PATCH v5 2/4] rcu: Reduce synchronize_rcu() latency
Date: Tue, 5 Mar 2024 12:36:29 +0100 [thread overview]
Message-ID: <ZecDvTGKErRckb2G@lothringen> (raw)
In-Reply-To: <Zebn-MvZq7NkFjq-@pc636>
On Tue, Mar 05, 2024 at 10:38:00AM +0100, Uladzislau Rezki wrote:
> On Mon, Mar 04, 2024 at 11:56:19PM +0100, Frederic Weisbecker wrote:
> > Le Mon, Mar 04, 2024 at 05:23:13PM +0100, Uladzislau Rezki a écrit :
> > > On Mon, Mar 04, 2024 at 12:55:47PM +0100, Frederic Weisbecker wrote:
> > > The easiest way is to drop the patch. To address it we can go with:
> > >
> > > <snip>
> > > diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> > > index 31f3a61f9c38..9aa2cd46583e 100644
> > > --- a/kernel/rcu/tree.c
> > > +++ b/kernel/rcu/tree.c
> > > @@ -1661,16 +1661,8 @@ static void rcu_sr_normal_gp_cleanup(void)
> > > * wait-head is released if last. The worker is not kicked.
> > > */
> > > llist_for_each_safe(rcu, next, wait_tail->next) {
> > > - if (rcu_sr_is_wait_head(rcu)) {
> > > - if (!rcu->next) {
> > > - rcu_sr_put_wait_head(rcu);
> > > - wait_tail->next = NULL;
> > > - } else {
> > > - wait_tail->next = rcu;
> > > - }
> > > -
> > > + if (rcu_sr_is_wait_head(rcu))
> > > break;
> > > - }
> > >
> > > rcu_sr_normal_complete(rcu);
> > > // It can be last, update a next on this step.
> > > <snip>
> > >
> > > i.e. the process of users from GP is still there. The work is triggered
> > > to perform a final complete(if there are users) + releasing wait-heads
> > > so we do not race anymore.
> >
> > It's worth mentioning that this doesn't avoid scheduling the workqueue.
> > Except perhaps for the very first time rcu_sr_normal_gp_cleanup() is called,
> > the workqueue will always have to be scheduled at least in order to release the
> > wait_tail of the previous rcu_sr_normal_gp_cleanup() call.
> >
> No, it does not avoid for sure :) I will add more explanation.
>
> > But indeed you keep the optimization that performs the completions themselves
> > synchronously from the GP kthread if there aren't too many of them (which
> > probably is the case most of the time).
> >
> > > I am OK with both cases. Dropping the patch will make it more simple
> > > for sure.
> >
> > I am ok with both cases as well :-)
> >
> > You choose. But note that the time spent doing the completions from the GP
> > kthread may come at the expense of delaying the start of the next grace period,
> > on which further synchronous RCU calls may in turn depend on...
> >
> That is a true point. Therefore we do it with a fixed number which should not
> influence on a GP.
Sounds good!
next prev parent reply other threads:[~2024-03-05 11:36 UTC|newest]
Thread overview: 27+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-02-20 18:31 [PATCH v5 0/4] Reduce synchronize_rcu() latency(v5) Uladzislau Rezki (Sony)
2024-02-20 18:31 ` [PATCH v5 1/4] rcu: Add data structures for synchronize_rcu() Uladzislau Rezki (Sony)
2024-02-20 18:31 ` [PATCH v5 2/4] rcu: Reduce synchronize_rcu() latency Uladzislau Rezki (Sony)
2024-02-26 23:07 ` Frederic Weisbecker
2024-02-27 6:39 ` Z qiang
2024-02-27 14:37 ` Frederic Weisbecker
2024-02-27 16:16 ` Frederic Weisbecker
2024-02-27 19:35 ` Uladzislau Rezki
2024-02-28 18:04 ` Uladzislau Rezki
2024-03-04 11:55 ` Frederic Weisbecker
2024-03-04 16:23 ` Uladzislau Rezki
2024-03-04 20:07 ` Paul E. McKenney
2024-03-05 9:35 ` Uladzislau Rezki
2024-03-04 22:56 ` Frederic Weisbecker
2024-03-05 9:38 ` Uladzislau Rezki
2024-03-05 11:36 ` Frederic Weisbecker [this message]
2024-02-27 16:15 ` Frederic Weisbecker
2024-02-27 17:03 ` Frederic Weisbecker
2024-02-27 20:51 ` Joel Fernandes
2024-02-28 9:28 ` Uladzislau Rezki
[not found] ` <4b932245-2825-4e53-87a4-44d2892e7c13@joelfernandes.org>
2024-02-27 22:50 ` Joel Fernandes
2024-02-27 22:53 ` Joel Fernandes
2024-02-28 14:32 ` Joel Fernandes
2024-02-28 16:44 ` Joel Fernandes
2024-02-20 18:31 ` [PATCH v5 3/4] rcu: Add a trace event for synchronize_rcu_normal() Uladzislau Rezki (Sony)
2024-02-20 18:31 ` [PATCH v5 4/4] rcu: Support direct wake-up of synchronize_rcu() users Uladzislau Rezki (Sony)
2024-02-21 1:53 ` [PATCH v5 0/4] Reduce synchronize_rcu() latency(v5) Paul E. McKenney
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