[RFC PATCH 1/2] rcu: sysctl: Panic on RCU Stall

[Daniel Bristot de Oliveira]

It is not always easy to define the cause of an RCU stall just by
analysing the RCU stall messages, mainly when the problem is caused
by the indirect starvation of rcu threads. For example, when preempt_rcu
is not awakened due to the starvation of a timer softirq.

We have been hard coding panic() in the RCU stall functions for
some time while testing the kernel-rt. But this is not possible in
some scenarios, like when supporting customers.

This patch implements the sysctl kerner.panic_on_rcu_stall. If
set to 1, the system will panic() when an RCU stall takes place,
enabling the capture of a vmcore. The vmcore provides a way to analyze
all kernel/tasks states, helping out to point to the culprit and the
solution for the stall.

The kerner.panic_on_rcu_stall sysctl is disabled by default.

Cc: Jonathan Corbet <corbet@xxxxxxx>
Cc: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
Cc: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
Cc: Steven Rostedt <rostedt@xxxxxxxxxxx>
Cc: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>
Cc: Lai Jiangshan <jiangshanlai@xxxxxxxxx>
Reviewed-by: Arnaldo Carvalho de Melo <acme@xxxxxxxxxx>
Tested-by: "Luis Claudio R. Goncalves" <lgoncalv@xxxxxxxxxx>
Signed-off-by: Daniel Bristot de Oliveira <bristot@xxxxxxxxxx>
---
 Documentation/sysctl/kernel.txt | 12 ++++++++++++
 include/linux/kernel.h          |  1 +
 kernel/rcu/tree.c               |  8 ++++++++
 kernel/sysctl.c                 | 11 +++++++++++
 4 files changed, 32 insertions(+)

diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
index a3683ce..29ec4bb 100644
--- a/Documentation/sysctl/kernel.txt
+++ b/Documentation/sysctl/kernel.txt
@@ -58,6 +58,7 @@ show up in /proc/sys/kernel:
 - panic_on_stackoverflow
 - panic_on_unrecovered_nmi
 - panic_on_warn
+- panic_on_rcu_stall
 - perf_cpu_time_max_percent
 - perf_event_paranoid
 - perf_event_max_stack
@@ -618,6 +619,17 @@ a kernel rebuild when attempting to kdump at the location of a WARN().
 
 ==============================================================
 
+panic_on_rcu_stall:
+
+When set to 1, calls panic() after RCU stall detection messages. This
+is useful to define the root cause of RCU stalls using a vmcore.
+
+0: do not panic() when RCU stall takes place, default behavior.
+
+1: panic() after printing RCU stall messages.
+
+==============================================================
+
 perf_cpu_time_max_percent:
 
 Hints to the kernel how much CPU time it should be allowed to
diff --git a/include/linux/kernel.h b/include/linux/kernel.h
index 94aa10f..c420821 100644
--- a/include/linux/kernel.h
+++ b/include/linux/kernel.h
@@ -451,6 +451,7 @@ extern int panic_on_oops;
 extern int panic_on_unrecovered_nmi;
 extern int panic_on_io_nmi;
 extern int panic_on_warn;
+extern int sysctl_panic_on_rcu_stall;
 extern int sysctl_panic_on_stackoverflow;
 
 extern bool crash_kexec_post_notifiers;
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index c7f1bc4..296db5d 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -125,6 +125,8 @@ int rcu_num_lvls __read_mostly = RCU_NUM_LVLS;
 /* Number of rcu_nodes at specified level. */
 static int num_rcu_lvl[] = NUM_RCU_LVL_INIT;
 int rcu_num_nodes __read_mostly = NUM_RCU_NODES; /* Total # rcu_nodes in use. */
+/* panic() on RCU Stall sysctl*/
+int sysctl_panic_on_rcu_stall;
 
 /*
  * The rcu_scheduler_active variable transitions from zero to one just
@@ -1390,6 +1392,9 @@ static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 
 	rcu_check_gp_kthread_starvation(rsp);
 
+	if (sysctl_panic_on_rcu_stall)
+		panic("RCU Stall\n");
+
 	force_quiescent_state(rsp);  /* Kick them all. */
 }
 
@@ -1430,6 +1435,9 @@ static void print_cpu_stall(struct rcu_state *rsp)
 			   jiffies + 3 * rcu_jiffies_till_stall_check() + 3);
 	raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
 
+	if (sysctl_panic_on_rcu_stall)
+		panic("RCU Stall\n");
+
 	/*
 	 * Attempt to revive the RCU machinery by forcing a context switch.
 	 *
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index 87b2fc3..d3b93aa 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1205,6 +1205,17 @@ static struct ctl_table kern_table[] = {
 		.extra2		= &one,
 	},
 #endif
+#ifdef CONFIG_RCU_STALL_COMMON
+	{
+		.procname	= "panic_on_rcu_stall",
+		.data		= &sysctl_panic_on_rcu_stall,
+		.maxlen		= sizeof(sysctl_panic_on_rcu_stall),
+		.mode		= 0644,
+		.proc_handler	= proc_dointvec_minmax,
+		.extra1		= &zero,
+		.extra2		= &one,
+	},
+#endif
 	{ }
 };
 
-- 
2.5.5