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rcu-tasks: Disable CPU hotplug across RCU tasks trace scans

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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>
This commit is contained in:
Paul E. McKenney 2020-03-22 10:10:07 -07:00
parent b38f57c1fe
commit 81b4a7bc3b
1 changed files with 14 additions and 4 deletions
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18
kernel/rcu/tasks.h
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@ -910,16 +910,16 @@ 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. */
@ -935,6 +935,9 @@ 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();
@ -979,6 +982,9 @@ 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 &&
@ -991,6 +997,10 @@ 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");