[PATCH 37/48] rcu: Support direct wake-up of synchronize_rcu() users

Uladzislau Rezki (Sony)
2024-05-07T09:36:11Z

This patch introduces a small enhancement which allows to do a
direct wake-up of synchronize_rcu() callers. It occurs after a
completion of grace period, thus by the gp-kthread.

Number of clients is limited by the hard-coded maximum allowed
threshold. The remaining part, if still exists is deferred to
a main worker.

Link: https://lore.kernel.org/lkml/Zd0ZtNu+Rt0qXkfS@lothringen/

Reviewed-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>
---
kernel/rcu/tree.c | 24 +++++++++++++++++++++++-
kernel/rcu/tree.h | 6 ++++++
2 files changed, 29 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 2e1c5be6d64b..2a270abade4d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1645,7 +1645,8 @@ static void rcu_sr_normal_gp_cleanup_work(struct work_struct *work)
*/
static void rcu_sr_normal_gp_cleanup(void)
{
- struct llist_node *wait_tail;
+ struct llist_node *wait_tail, *next, *rcu;
+ int done = 0;

wait_tail = rcu_state.srs_wait_tail;
if (wait_tail == NULL)
@@ -1653,11 +1654,32 @@ static void rcu_sr_normal_gp_cleanup(void)

rcu_state.srs_wait_tail = NULL;
ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_wait_tail);
+ WARN_ON_ONCE(!rcu_sr_is_wait_head(wait_tail));
+
+ /*
+ * Process (a) and (d) cases. See an illustration.
+ */
+ llist_for_each_safe(rcu, next, wait_tail->next) {
+ if (rcu_sr_is_wait_head(rcu))
+ break;
+
+ rcu_sr_normal_complete(rcu);
+ // It can be last, update a next on this step.
+ wait_tail->next = next;
+
+ if (++done == SR_MAX_USERS_WAKE_FROM_GP)
+ break;
+ }

// concurrent sr_normal_gp_cleanup work might observe this update.
smp_store_release(&rcu_state.srs_done_tail, wait_tail);
ASSERT_EXCLUSIVE_WRITER(rcu_state.srs_done_tail);

+ /*
+ * We schedule a work in order to perform a final processing
+ * of outstanding users(if still left) and releasing wait-heads
+ * added by rcu_sr_normal_gp_init() call.
+ */
schedule_work(&rcu_state.srs_cleanup_work);
}

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index b942b9437438..2832787cee1d 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -315,6 +315,12 @@ do { \
__set_current_state(TASK_RUNNING); \
} while (0)

+/*
+ * A max threshold for synchronize_rcu() users which are
+ * awaken directly by the rcu_gp_kthread(). Left part is
+ * deferred to the main worker.
+ */
+#define SR_MAX_USERS_WAKE_FROM_GP 5
#define SR_NORMAL_GP_WAIT_HEAD_MAX 5

struct sr_wait_node {
--
2.39.2