rcu: RCU-based detection of stalled CPUs for Classic RCU

This patch adds stalled-CPU detection to Classic RCU.  This capability
is enabled by a new config variable CONFIG_RCU_CPU_STALL_DETECTOR, which
defaults disabled.

This is a debugging feature to detect infinite loops in kernel code, not
something that non-kernel-hackers would be expected to care about.

This feature can detect looping CPUs in !PREEMPT builds and looping CPUs
with preemption disabled in PREEMPT builds.  This is essentially a port of
this functionality from the treercu patch, replacing the stall debug patch
that is already in tip/core/rcu (commit 67182ae1c4).

The changes from the patch in tip/core/rcu include making the config
variable name match that in treercu, changing from seconds to jiffies to
avoid spurious warnings, and printing a boot message when this feature
is enabled.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Signed-off-by: Ingo Molnar <mingo@elte.hu>

1 files changed, 9 insertions, 3 deletions

diff --git a/include/linux/rcuclassic.h b/include/linux/rcuclassic.h
index 29bf528c7dcc..5f89b62e6983 100644
--- a/include/linux/rcuclassic.h
+++ b/include/linux/rcuclassic.h
@@ -40,15 +40,21 @@
 #include <linux/cpumask.h>
 #include <linux/seqlock.h>
 
+#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
+#define RCU_SECONDS_TILL_STALL_CHECK    ( 3 * HZ) /* for rcp->jiffies_stall */
+#define RCU_SECONDS_TILL_STALL_RECHECK  (30 * HZ) /* for rcp->jiffies_stall */
+#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
 
 /* Global control variables for rcupdate callback mechanism. */
 struct rcu_ctrlblk {
         long    cur;            /* Current batch number.                      */
         long    completed;      /* Number of the last completed batch         */
         long    pending;        /* Number of the last pending batch           */
-#ifdef CONFIG_DEBUG_RCU_STALL
-        unsigned long gp_check; /* Time grace period should end, in seconds.  */
-#endif /* #ifdef CONFIG_DEBUG_RCU_STALL */
+#ifdef CONFIG_RCU_CPU_STALL_DETECTOR
+        unsigned long gp_start; /* Time at which GP started in jiffies. */
+        unsigned long jiffies_stall;
+                                /* Time at which to check for CPU stalls. */
+#endif /* #ifdef CONFIG_RCU_CPU_STALL_DETECTOR */
 
         int     signaled;