[PATCH 3/6] rcu/nocb: Optimize kthreads and rdp initialization

Frederic Weisbecker
2021-11-17T15:56:57Z

Currently cpumask_available() is used to prevent from unwanted
NOCB initialization. However if "rcu_nocbs=" isn't passed but
CONFIG_CPUMASK_OFFSTACK=n, the initialization path is still taken,
running through all sorts of needless operations and iterations on an
empty cpumask.

Fix this with relying on a real initialization state instead. This
also optimize kthreads creation, sparing iteration over all online CPUs
when nocb isn't initialized.

Signed-off-by: Frederic Weisbecker <frederic@kernel.org>
Cc: Neeraj Upadhyay <quic_neeraju@quicinc.com>
Cc: Boqun Feng <boqun.feng@gmail.com>
Cc: Uladzislau Rezki <urezki@gmail.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Joel Fernandes <joel@joelfernandes.org>
---
kernel/rcu/tree_nocb.h | 24 +++++++++++++++++-------
1 file changed, 17 insertions(+), 7 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 0543d5e913bb..9fe4be10fde7 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -60,6 +60,9 @@ static inline bool rcu_current_is_nocb_kthread(struct rcu_data *rdp)
* Parse the boot-time rcu_nocb_mask CPU list from the kernel parameters.
* If the list is invalid, a warning is emitted and all CPUs are offloaded.
*/
+
+static bool rcu_nocb_is_setup;
+
static int __init rcu_nocb_setup(char *str)
{
alloc_bootmem_cpumask_var(&rcu_nocb_mask);
@@ -67,6 +70,7 @@ static int __init rcu_nocb_setup(char *str)
pr_warn("rcu_nocbs= bad CPU range, all CPUs set\n");
cpumask_setall(rcu_nocb_mask);
}
+ rcu_nocb_is_setup = true;
return 1;
}
__setup("rcu_nocbs=", rcu_nocb_setup);
@@ -1151,13 +1155,17 @@ void __init rcu_init_nohz(void)
need_rcu_nocb_mask = true;
#endif /* #if defined(CONFIG_NO_HZ_FULL) */

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

#if defined(CONFIG_NO_HZ_FULL)
@@ -1259,8 +1267,10 @@ static void __init rcu_spawn_nocb_kthreads(void)
{
int cpu;

- for_each_online_cpu(cpu)
- rcu_spawn_cpu_nocb_kthread(cpu);
+ if (rcu_nocb_is_setup) {
+ for_each_online_cpu(cpu)
+ rcu_spawn_cpu_nocb_kthread(cpu);
+ }
}

/* How many CB CPU IDs per GP kthread? Default of -1 for sqrt(nr_cpu_ids). */
--
2.25.1