rcu: Improve diagnostics for spurious RCU CPU stall warnings

The current RCU CPU stall warning code will print "Stall ended before
state dump start" any time that the stall-warning code is triggered on
a CPU that has already reported a quiescent state for the current grace
period and if all quiescent states have been reported for the current
grace period.  However, a true stall can result in these symptoms, for
example, by preventing RCU's grace-period kthreads from ever running

This commit therefore checks for this condition, reporting the end of
the stall only if one of the grace-period counters has actually advanced.
Otherwise, it reports the last time that the grace-period kthread made
meaningful progress.  (In normal situations, the grace-period kthread
should make meaningful progress at least every jiffies_till_next_fqs
jiffies.)

Reported-by: Miroslav Benes <mbenes@suse.cz>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Miroslav Benes <mbenes@suse.cz>

1 files changed, 2 insertions, 0 deletions

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index e300848cc0cf..5ec81cf938fd 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -488,6 +488,8 @@ struct rcu_state {
                                                 /*  due to no GP active. */
         unsigned long gp_start;                 /* Time at which GP started, */
                                                 /*  but in jiffies. */
+        unsigned long gp_activity;              /* Time of last GP kthread */
+                                                /*  activity in jiffies. */
         unsigned long jiffies_stall;            /* Time at which to check */
                                                 /*  for CPU stalls. */
         unsigned long jiffies_resched;          /* Time at which to resched */