[PATCH v4 tip/core/rcu 29/38] rcu-tasks: Disable CPU hotplug across RCU tasks trace scans

paulmck
2020-04-15T18:22:11Z

This commit disables CPU hotplug across RCU tasks trace scans, which
is a first step towards correctly recognizing idle tasks "running" on
offline CPUs.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
---
kernel/rcu/tasks.h | 18 ++++++++++++++----
1 file changed, 14 insertions(+), 4 deletions(-)

diff --git a/kernel/rcu/tasks.h b/kernel/rcu/tasks.h
index 0cdeeb3..e8c03bd 100644
--- a/kernel/rcu/tasks.h
+++ b/kernel/rcu/tasks.h
@@ -906,16 +906,16 @@ static void rcu_tasks_trace_pregp_step(void)
{
int cpu;

- // Wait for CPU-hotplug paths to complete.
- cpus_read_lock();
- cpus_read_unlock();
-
// Allow for fast-acting IPIs.
atomic_set(&trc_n_readers_need_end, 1);

// There shouldn't be any old IPIs, but...
for_each_possible_cpu(cpu)
WARN_ON_ONCE(per_cpu(trc_ipi_to_cpu, cpu));
+
+ // Disable CPU hotplug across the tasklist scan.
+ // This also waits for all readers in CPU-hotplug code paths.
+ cpus_read_lock();
}

/* Do first-round processing for the specified task. */
@@ -931,6 +931,9 @@ static void rcu_tasks_trace_pertask(struct task_struct *t,
/* Do intermediate processing between task and holdout scans. */
static void rcu_tasks_trace_postscan(void)
{
+ // Re-enable CPU hotplug now that the tasklist scan has completed.
+ cpus_read_unlock();
+
// Wait for late-stage exiting tasks to finish exiting.
// These might have passed the call to exit_tasks_rcu_finish().
synchronize_rcu();
@@ -975,6 +978,9 @@ static void check_all_holdout_tasks_trace(struct list_head *hop,
{
struct task_struct *g, *t;

+ // Disable CPU hotplug across the holdout list scan.
+ cpus_read_lock();
+
list_for_each_entry_safe(t, g, hop, trc_holdout_list) {
// If safe and needed, try to check the current task.
if (READ_ONCE(t->trc_ipi_to_cpu) == -1 &&
@@ -987,6 +993,10 @@ static void check_all_holdout_tasks_trace(struct list_head *hop,
else if (needreport)
show_stalled_task_trace(t, firstreport);
}
+
+ // Re-enable CPU hotplug now that the holdout list scan has completed.
+ cpus_read_unlock();
+
if (needreport) {
if (firstreport)
pr_err("INFO: rcu_tasks_trace detected stalls? (Late IPI?)\n");
--
2.9.5