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Subject[RFC PATCH 10/12] rcu: Process batch locally as long as offloading isn't complete
Date
During the offloading or de-offloading process, make sure to process
the callbacks batch locally whenever the segcblist isn't entirely
offloaded. This enforces callback service processing while we are still
in intermediate (de-)offloading state.

FIXME: Note that __call_rcu_core() isn't called during these intermediate
states. Some pieces there may still be necessary.

Inspired-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Cc: Lai Jiangshan <jiangshanlai@gmail.com>
Cc: Joel Fernandes <joel@joelfernandes.org>
---
kernel/rcu/rcu_segcblist.h | 12 ++++++++++++
kernel/rcu/tree.c | 3 ++-
2 files changed, 14 insertions(+), 1 deletion(-)

diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index 00ebeb8d39b7..f7da3d535888 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -92,6 +92,18 @@ static inline bool rcu_segcblist_is_offloaded(struct rcu_segcblist *rsclp)
return false;
}

+static inline bool rcu_segcblist_completely_offloaded(struct rcu_segcblist *rsclp)
+{
+ int flags = SEGCBLIST_KTHREAD_CB | SEGCBLIST_KTHREAD_GP | SEGCBLIST_OFFLOADED;
+
+ if (IS_ENABLED(CONFIG_RCU_NOCB_CPU)) {
+ if ((rsclp->flags & flags) == flags)
+ return true;
+ }
+
+ return false;
+}
+
/*
* Are all segments following the specified segment of the specified
* rcu_segcblist structure empty of callbacks? (The specified
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index b4292489db0c..928907e9ba94 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -2620,6 +2620,7 @@ static __latent_entropy void rcu_core(void)
struct rcu_data *rdp = raw_cpu_ptr(&rcu_data);
struct rcu_node *rnp = rdp->mynode;
const bool offloaded = rcu_segcblist_is_offloaded(&rdp->cblist);
+ const bool do_batch = !rcu_segcblist_completely_offloaded(&rdp->cblist);

if (cpu_is_offline(smp_processor_id()))
return;
@@ -2649,7 +2650,7 @@ static __latent_entropy void rcu_core(void)
rcu_check_gp_start_stall(rnp, rdp, rcu_jiffies_till_stall_check());

/* If there are callbacks ready, invoke them. */
- if (!offloaded && rcu_segcblist_ready_cbs(&rdp->cblist) &&
+ if (do_batch && rcu_segcblist_ready_cbs(&rdp->cblist) &&
likely(READ_ONCE(rcu_scheduler_fully_active)))
rcu_do_batch(rdp);

--
2.28.0
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