From: Frederic Weisbecker <frederic@kernel.org>
To: "Joel Fernandes (Google)" <joel@joelfernandes.org>
Cc: linux-kernel@vger.kernel.org,
"Paul E. McKenney" <paulmck@kernel.org>,
Neeraj Upadhyay <neeraj.upadhyay@kernel.org>,
Josh Triplett <josh@joshtriplett.org>,
Boqun Feng <boqun.feng@gmail.com>,
Steven Rostedt <rostedt@goodmis.org>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Lai Jiangshan <jiangshanlai@gmail.com>,
Zqiang <qiang.zhang1211@gmail.com>,
urezki@gmail.com, neeraj.iitr10@gmail.com, rcu@vger.kernel.org
Subject: Re: [PATCH v4] rcu/tree: Reduce wake up for synchronize_rcu() common case
Date: Thu, 4 Apr 2024 22:19:11 +0200 [thread overview]
Message-ID: <Zg8LP9fEHlDPeq5L@pavilion.home> (raw)
In-Reply-To: <20240319185458.3968308-1-joel@joelfernandes.org>
Le Tue, Mar 19, 2024 at 02:54:57PM -0400, Joel Fernandes (Google) a écrit :
> In the synchronize_rcu() common case, we will have less than
> SR_MAX_USERS_WAKE_FROM_GP number of users per GP. Waking up the kworker
> is pointless just to free the last injected wait head since at that point,
> all the users have already been awakened.
>
> Introduce a new counter to track this and prevent the wakeup in the
> common case.
>
> Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
> ---
> v1->v2: Rebase on paul/dev
> v2->v3: Additional optimization for wait_tail->next == NULL case.
> v3->v4: Apply clean ups from Vlad. Tested rcutorture all scenarios.
> ---
> kernel/rcu/tree.c | 35 ++++++++++++++++++++++++++++++-----
> kernel/rcu/tree.h | 1 +
> 2 files changed, 31 insertions(+), 5 deletions(-)
>
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 9fbb5ab57c84..f3193670fe42 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -96,6 +96,7 @@ static struct rcu_state rcu_state = {
> .ofl_lock = __ARCH_SPIN_LOCK_UNLOCKED,
> .srs_cleanup_work = __WORK_INITIALIZER(rcu_state.srs_cleanup_work,
> rcu_sr_normal_gp_cleanup_work),
> + .srs_cleanups_pending = ATOMIC_INIT(0),
> };
>
> /* Dump rcu_node combining tree at boot to verify correct setup. */
> @@ -1642,8 +1643,11 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
> * the done tail list manipulations are protected here.
> */
> done = smp_load_acquire(&rcu_state.srs_done_tail);
> - if (!done)
> + if (!done) {
Can this really happen? And if not should we warn?
> + /* See comments below. */
> + atomic_dec_return_release(&rcu_state.srs_cleanups_pending);
And if not should we really do that?
> return;
> + }
>
> WARN_ON_ONCE(!rcu_sr_is_wait_head(done));
> head = done->next;
> @@ -1666,6 +1670,9 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
>
> rcu_sr_put_wait_head(rcu);
> }
> +
> + /* Order list manipulations with atomic access. */
> + atomic_dec_return_release(&rcu_state.srs_cleanups_pending);
Can we have a sanity check here ensuring this never goes negative?
Thanks.
prev parent reply other threads:[~2024-04-04 20:19 UTC|newest]
Thread overview: 4+ messages / expand[flat|nested] mbox.gz Atom feed top
2024-03-19 18:54 [PATCH v4] rcu/tree: Reduce wake up for synchronize_rcu() common case Joel Fernandes (Google)
2024-03-20 14:36 ` Uladzislau Rezki
2024-03-20 23:26 ` Paul E. McKenney
2024-04-04 20:19 ` Frederic Weisbecker [this message]
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