Re: [PATCH rcu 3/9] rcu/tree: Reduce wake up for synchronize_rcu() common case

Frederic Weisbecker
2024-06-05T16:35:11Z

Le Tue, Jun 04, 2024 at 03:23:49PM -0700, Paul E. McKenney a écrit :
> From: "Joel Fernandes (Google)" <joel@joelfernandes.org>
>
> 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>
> Reviewed-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
> Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
> ---
> 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 6ba36d9c09bde..2fe08e6186b4d 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. */
> @@ -1633,8 +1634,11 @@ 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) {
> + /* See comments below. */
> + atomic_dec_return_release(&rcu_state.srs_cleanups_pending);

This condition is not supposed to happen. If the work is scheduled,
there has to be a wait_queue in rcu_state.srs_done_tail. And decrementing
may make things worse.

So this should be:

if (WARN_ON_ONCE(!done))
return;

Thanks.