rcu: Add event tracing to ->gp_tasks update at GP start

There is currently event tracing to track when a task is preempted
within a preemptible RCU read-side critical section, and also when that
task subsequently reaches its outermost rcu_read_unlock(), but none
indicating when a new grace period starts when that grace period must
wait on pre-existing readers that have been been preempted at least once
since the beginning of their current RCU read-side critical sections.

This commit therefore adds an event trace at grace-period start in
the case where there are such readers.  Note that only the first
reader in the list is traced.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Acked-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index 14ba496..3e3f92e 100644 (file)
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -636,10 +636,17 @@ static int rcu_print_task_exp_stall(struct rcu_node *rnp)
  */
 static void rcu_preempt_check_blocked_tasks(struct rcu_node *rnp)
 {
+       struct task_struct *t;
+
        RCU_LOCKDEP_WARN(preemptible(), "rcu_preempt_check_blocked_tasks() invoked with preemption enabled!!!\n");
        WARN_ON_ONCE(rcu_preempt_blocked_readers_cgp(rnp));
-       if (rcu_preempt_has_tasks(rnp))
+       if (rcu_preempt_has_tasks(rnp)) {
                rnp->gp_tasks = rnp->blkd_tasks.next;
+               t = container_of(rnp->gp_tasks, struct task_struct,
+                                rcu_node_entry);
+               trace_rcu_unlock_preempted_task(TPS("rcu_preempt-GPS"),
+                                               rnp->gpnum, t->pid);
+       }
        WARN_ON_ONCE(rnp->qsmask);
 }