[PATCH 4/6] rcu/nocb: Create nocb kthreads on all CPUs as long as the "rcu_nocb=" is passed

Frederic Weisbecker
2021-11-17T15:57:00Z

In order to be able to (de-)offload any CPU using cpuset in the future,
create a NOCB kthread for all possible CPUs. For now this is done only
as long as the "rcu_nocb=" kernel parameter is passed to avoid
the unnecessary overhead for most users.

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 | 16 +++++++---------
1 file changed, 7 insertions(+), 9 deletions(-)

diff --git a/kernel/rcu/tree_nocb.h b/kernel/rcu/tree_nocb.h
index 9fe4be10fde7..1871f15b8472 100644
--- a/kernel/rcu/tree_nocb.h
+++ b/kernel/rcu/tree_nocb.h
@@ -1221,11 +1221,8 @@ static void rcu_spawn_one_nocb_kthread(int cpu)
struct rcu_data *rdp_gp;
struct task_struct *t;

- /*
- * If this isn't a no-CBs CPU or if it already has an rcuo kthread,
- * then nothing to do.
- */
- if (!rcu_is_nocb_cpu(cpu) || rdp->nocb_cb_kthread)
+ /* If it already has an rcuo kthread, then nothing to do. */
+ if (rdp->nocb_cb_kthread)
return;

/* If we didn't spawn the GP kthread first, reorganize! */
@@ -1253,7 +1250,7 @@ static void rcu_spawn_one_nocb_kthread(int cpu)
*/
static void rcu_spawn_cpu_nocb_kthread(int cpu)
{
- if (rcu_scheduler_fully_active)
+ if (rcu_scheduler_fully_active && rcu_nocb_is_setup)
rcu_spawn_one_nocb_kthread(cpu);
}

@@ -1268,7 +1265,7 @@ static void __init rcu_spawn_nocb_kthreads(void)
int cpu;

if (rcu_nocb_is_setup) {
- for_each_online_cpu(cpu)
+ for_each_possible_cpu(cpu)
rcu_spawn_cpu_nocb_kthread(cpu);
}
}
@@ -1303,7 +1300,7 @@ static void __init rcu_organize_nocb_kthreads(void)
* Should the corresponding CPU come online in the future, then
* we will spawn the needed set of rcu_nocb_kthread() kthreads.
*/
- for_each_cpu(cpu, rcu_nocb_mask) {
+ for_each_possible_cpu(cpu) {
rdp = per_cpu_ptr(&rcu_data, cpu);
if (rdp->cpu >= nl) {
/* New GP kthread, set up for CBs & next GP. */
@@ -1327,7 +1324,8 @@ static void __init rcu_organize_nocb_kthreads(void)
pr_cont(" %d", cpu);
}
rdp->nocb_gp_rdp = rdp_gp;
- list_add_tail(&rdp->nocb_entry_rdp, &rdp_gp->nocb_head_rdp);
+ if (cpumask_test_cpu(cpu, rcu_nocb_mask))
+ list_add_tail(&rdp->nocb_entry_rdp, &rdp_gp->nocb_head_rdp);
}
if (gotnocbs && dump_tree)
pr_cont("%s\n", gotnocbscbs ? "" : " (self only)");
--
2.25.1