[PATCH v3 2/2] rcu: Simplify the code logic of rcu_init_nohz()

From: Zhen Lei
Date: Tue Aug 16 2022 - 08:50:11 EST


When CONFIG_RCU_NOCB_CPU_DEFAULT_ALL=y or CONFIG_NO_HZ_FULL=y, additional
CPUs need to be added to 'rcu_nocb_mask'. But 'rcu_nocb_mask' may be not
available now, due to 'rcu_nocbs' is not specified. Check and initialize
'rcu_nocb_mask' before using it. This code simplification strictly follows
this logic, compared with old implementations, unnecessary crossovers are
avoided and easy to understand.

Signed-off-by: Zhen Lei <thunder.leizhen@xxxxxxxxxx>
---
kernel/rcu/tree_nocb.h | 32 +++++++++-----------------------
1 file changed, 9 insertions(+), 23 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index ff763e7dc53551f..07b415e8117a10a 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1209,44 +1209,30 @@ EXPORT_SYMBOL_GPL(rcu_nocb_cpu_offload);
void __init rcu_init_nohz(void)
{
int cpu;
- bool need_rcu_nocb_mask = false;
- bool offload_all = false;
struct rcu_data *rdp;
+ struct cpumask *cpumask = NULL;

#if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL)
- if (!cpumask_available(rcu_nocb_mask)) {
- need_rcu_nocb_mask = true;
- offload_all = true;
- }
-#endif /* #if defined(CONFIG_RCU_NOCB_CPU_DEFAULT_ALL) */
-
-#if defined(CONFIG_NO_HZ_FULL)
- if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask)) {
- need_rcu_nocb_mask = true;
- offload_all = false; /* NO_HZ_FULL has its own mask. */
- }
-#endif /* #if defined(CONFIG_NO_HZ_FULL) */
+ cpumask = cpu_possible_mask;
+#elif defined(CONFIG_NO_HZ_FULL)
+ if (tick_nohz_full_running && !cpumask_empty(tick_nohz_full_mask))
+ cpumask = tick_nohz_full_mask;
+#endif

- if (need_rcu_nocb_mask) {
+ if (cpumask) {
if (!cpumask_available(rcu_nocb_mask)) {
if (!zalloc_cpumask_var(&rcu_nocb_mask, GFP_KERNEL)) {
pr_info("rcu_nocb_mask allocation failed, callback offloading disabled.\n");
return;
}
}
+
+ cpumask_or(rcu_nocb_mask, rcu_nocb_mask, cpumask);
}

if (!cpumask_available(rcu_nocb_mask))
return;

-#if defined(CONFIG_NO_HZ_FULL)
- if (tick_nohz_full_running)
- cpumask_or(rcu_nocb_mask, rcu_nocb_mask, tick_nohz_full_mask);
-#endif /* #if defined(CONFIG_NO_HZ_FULL) */
-
- if (offload_all)
- cpumask_setall(rcu_nocb_mask);
-
if (!cpumask_subset(rcu_nocb_mask, cpu_possible_mask)) {
pr_info("\tNote: kernel parameter 'rcu_nocbs=', 'nohz_full', or 'isolcpus=' contains nonexistent CPUs.\n");
cpumask_and(rcu_nocb_mask, cpu_possible_mask,
--
2.25.1