[PATCH 08/10] rcu: Allow to deactivate nocb on a CPU

Frederic Weisbecker
2020-05-13T16:47:39Z

Allow a CPU's rdp to quit the callback offlined mode.
The switch happens on the target with IRQs disabled and rdp->nocb_lock
held to avoid races between local callbacks handling and kthread
offloaded callbacks handling.

nocb_cb kthread is first parked to avoid any future race with
concurrent rcu_do_batch() executions. Then the cblist is set to offloaded
so that the nocb_gp kthread ignores this rdp.

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>
---
include/linux/rcupdate.h | 2 ++
kernel/rcu/rcu_segcblist.c | 4 +--
kernel/rcu/rcu_segcblist.h | 2 +-
kernel/rcu/tree_plugin.h | 50 +++++++++++++++++++++++++++++++++++++-
4 files changed, 54 insertions(+), 4 deletions(-)

diff --git a/include/linux/rcupdate.h b/include/linux/rcupdate.h
index 2678a37c3169..1d3a4c37c3c1 100644
--- a/include/linux/rcupdate.h
+++ b/include/linux/rcupdate.h
@@ -96,8 +96,10 @@ static inline void rcu_user_exit(void) { }

#ifdef CONFIG_RCU_NOCB_CPU
void rcu_init_nohz(void);
+void rcu_nocb_cpu_deoffload(int cpu);
#else /* #ifdef CONFIG_RCU_NOCB_CPU */
static inline void rcu_init_nohz(void) { }
+static inline void rcu_nocb_cpu_deoffload(int cpu) { }
#endif /* #else #ifdef CONFIG_RCU_NOCB_CPU */

/**
diff --git a/kernel/rcu/rcu_segcblist.c b/kernel/rcu/rcu_segcblist.c
index d8ea2bef5574..4bed48da7702 100644
--- a/kernel/rcu/rcu_segcblist.c
+++ b/kernel/rcu/rcu_segcblist.c
@@ -171,9 +171,9 @@ void rcu_segcblist_disable(struct rcu_segcblist *rsclp)
* Mark the specified rcu_segcblist structure as offloaded. This
* structure must be empty.
*/
-void rcu_segcblist_offload(struct rcu_segcblist *rsclp)
+void rcu_segcblist_offload(struct rcu_segcblist *rsclp, bool offload)
{
- rsclp->offloaded = 1;
+ rsclp->offloaded = offload;
}
#endif

diff --git a/kernel/rcu/rcu_segcblist.h b/kernel/rcu/rcu_segcblist.h
index 4c1503a82492..8f7c6c34cb1b 100644
--- a/kernel/rcu/rcu_segcblist.h
+++ b/kernel/rcu/rcu_segcblist.h
@@ -83,7 +83,7 @@ void rcu_segcblist_inc_len(struct rcu_segcblist *rsclp);
void rcu_segcblist_init(struct rcu_segcblist *rsclp);
void rcu_segcblist_disable(struct rcu_segcblist *rsclp);
#ifdef CONFIG_RCU_NOCB_CPU
-void rcu_segcblist_offload(struct rcu_segcblist *rsclp);
+void rcu_segcblist_offload(struct rcu_segcblist *rsclp, bool offload);
#endif
bool rcu_segcblist_ready_cbs(struct rcu_segcblist *rsclp);
bool rcu_segcblist_pend_cbs(struct rcu_segcblist *rsclp);
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index f19e81e0c691..c74a4df8d5f2 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1943,6 +1943,10 @@ static void nocb_gp_wait(struct rcu_data *my_rdp)
for (rdp = my_rdp; rdp; rdp = rdp->nocb_next_cb_rdp) {
trace_rcu_nocb_wake(rcu_state.name, rdp->cpu, TPS("Check"));
raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
+ if (!rcu_segcblist_is_offloaded(&rdp->cblist)) {
+ raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
+ continue;
+ }
bypass_ncbs = rcu_cblist_n_cbs(&rdp->nocb_bypass);
if (bypass_ncbs &&
(time_after(j, READ_ONCE(rdp->nocb_bypass_first) + 1) ||
@@ -2176,6 +2180,50 @@ static void do_nocb_deferred_wakeup(struct rcu_data *rdp)
do_nocb_deferred_wakeup_common(rdp);
}

+static void __rcu_nocb_rdp_deoffload(struct rcu_data *rdp)
+{
+ unsigned long flags;
+ struct rcu_node *rnp = rdp->mynode;
+
+ printk("De-offloading %d\n", rdp->cpu);
+ kthread_park(rdp->nocb_cb_kthread);
+
+ raw_spin_lock_irqsave(&rdp->nocb_lock, flags);
+ rcu_nocb_flush_bypass(rdp, NULL, jiffies);
+ raw_spin_lock_rcu_node(rnp);
+ rcu_segcblist_offload(&rdp->cblist, false);
+ raw_spin_unlock_rcu_node(rnp);
+ raw_spin_unlock_irqrestore(&rdp->nocb_lock, flags);
+}
+
+static long rcu_nocb_rdp_deoffload(void *arg)
+{
+ struct rcu_data *rdp = arg;
+
+ WARN_ON_ONCE(rdp->cpu != raw_smp_processor_id());
+ __rcu_nocb_rdp_deoffload(rdp);
+
+ return 0;
+}
+
+void rcu_nocb_cpu_deoffload(int cpu)
+{
+ struct rcu_data *rdp = per_cpu_ptr(&rcu_data, cpu);
+
+ mutex_lock(&rcu_state.barrier_mutex);
+ cpus_read_lock();
+ if (rcu_segcblist_is_offloaded(&rdp->cblist)) {
+ if (cpu_online(cpu)) {
+ work_on_cpu(cpu, rcu_nocb_rdp_deoffload, rdp);
+ } else {
+ __rcu_nocb_rdp_deoffload(rdp);
+ }
+ cpumask_clear_cpu(cpu, rcu_nocb_mask);
+ }
+ cpus_read_unlock();
+ mutex_unlock(&rcu_state.barrier_mutex);
+}
+
void __init rcu_init_nohz(void)
{
int cpu;
@@ -2218,7 +2266,7 @@ void __init rcu_init_nohz(void)
rdp = per_cpu_ptr(&rcu_data, cpu);
if (rcu_segcblist_empty(&rdp->cblist))
rcu_segcblist_init(&rdp->cblist);
- rcu_segcblist_offload(&rdp->cblist);
+ rcu_segcblist_offload(&rdp->cblist, true);
}
rcu_organize_nocb_kthreads();
}
--
2.25.0