3 files changed, 34 insertions, 5 deletions

diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index ed186a902d31..55f9707fe60a 100644
--- a/Documentation/RCU/stallwarn.txt
+++ b/Documentation/RCU/stallwarn.txt
@@ -187,6 +187,11 @@ o	For !CONFIG_PREEMPT kernels, a CPU looping anywhere in the
         behavior, you might need to replace some of the cond_resched()
         calls with calls to cond_resched_rcu_qs().
 
+o       Anything that prevents RCU's grace-period kthreads from running.
+        This can result in the "All QSes seen" console-log message.
+        This message will include information on when the kthread last
+        ran and how often it should be expected to run.
+
 o       A CPU-bound real-time task in a CONFIG_PREEMPT kernel, which might
         happen to preempt a low-priority task in the middle of an RCU
         read-side critical section.   This is especially damaging if
diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 654b15be1e36..a2ceb66bcd67 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1066,11 +1066,13 @@ static void rcu_dump_cpu_stacks(struct rcu_state *rsp)
         }
 }
 
-static void print_other_cpu_stall(struct rcu_state *rsp)
+static void print_other_cpu_stall(struct rcu_state *rsp, unsigned long gpnum)
 {
         int cpu;
         long delta;
         unsigned long flags;
+        unsigned long gpa;
+        unsigned long j;
         int ndetected = 0;
         struct rcu_node *rnp = rcu_get_root(rsp);
         long totqlen = 0;
@@ -1123,10 +1125,22 @@ static void print_other_cpu_stall(struct rcu_state *rsp)
         pr_cont("(detected by %d, t=%ld jiffies, g=%ld, c=%ld, q=%lu)\n",
                smp_processor_id(), (long)(jiffies - rsp->gp_start),
                (long)rsp->gpnum, (long)rsp->completed, totqlen);
-        if (ndetected == 0)
-                pr_err("INFO: Stall ended before state dump start\n");
-        else
+        if (ndetected) {
                 rcu_dump_cpu_stacks(rsp);
+        } else {
+                if (ACCESS_ONCE(rsp->gpnum) != gpnum ||
+                    ACCESS_ONCE(rsp->completed) == gpnum) {
+                        pr_err("INFO: Stall ended before state dump start\n");
+                } else {
+                        j = jiffies;
+                        gpa = ACCESS_ONCE(rsp->gp_activity);
+                        pr_err("All QSes seen, last %s kthread activity %ld (%ld-%ld), jiffies_till_next_fqs=%ld\n",
+                               rsp->name, j - gpa, j, gpa,
+                               jiffies_till_next_fqs);
+                        /* In this case, the current CPU might be at fault. */
+                        sched_show_task(current);
+                }
+        }
 
         /* Complain about tasks blocking the grace period. */
 
@@ -1226,7 +1240,7 @@ static void check_cpu_stall(struct rcu_state *rsp, struct rcu_data *rdp)
                    ULONG_CMP_GE(j, js + RCU_STALL_RAT_DELAY)) {
 
                 /* They had a few time units to dump stack, so complain. */
-                print_other_cpu_stall(rsp);
+                print_other_cpu_stall(rsp, gpnum);
         }
 }
 
@@ -1622,6 +1636,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
         struct rcu_data *rdp;
         struct rcu_node *rnp = rcu_get_root(rsp);
 
+        ACCESS_ONCE(rsp->gp_activity) = jiffies;
         rcu_bind_gp_kthread();
         raw_spin_lock_irq(&rnp->lock);
         smp_mb__after_unlock_lock();
@@ -1682,6 +1697,7 @@ static int rcu_gp_init(struct rcu_state *rsp)
                                             rnp->grphi, rnp->qsmask);
                 raw_spin_unlock_irq(&rnp->lock);
                 cond_resched_rcu_qs();
+                ACCESS_ONCE(rsp->gp_activity) = jiffies;
         }
 
         mutex_unlock(&rsp->onoff_mutex);
@@ -1698,6 +1714,7 @@ static int rcu_gp_fqs(struct rcu_state *rsp, int fqs_state_in)
         unsigned long maxj;
         struct rcu_node *rnp = rcu_get_root(rsp);
 
+        ACCESS_ONCE(rsp->gp_activity) = jiffies;
         rsp->n_force_qs++;
         if (fqs_state == RCU_SAVE_DYNTICK) {
                 /* Collect dyntick-idle snapshots. */
@@ -1736,6 +1753,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
         struct rcu_data *rdp;
         struct rcu_node *rnp = rcu_get_root(rsp);
 
+        ACCESS_ONCE(rsp->gp_activity) = jiffies;
         raw_spin_lock_irq(&rnp->lock);
         smp_mb__after_unlock_lock();
         gp_duration = jiffies - rsp->gp_start;
@@ -1772,6 +1790,7 @@ static void rcu_gp_cleanup(struct rcu_state *rsp)
                 nocb += rcu_future_gp_cleanup(rsp, rnp);
                 raw_spin_unlock_irq(&rnp->lock);
                 cond_resched_rcu_qs();
+                ACCESS_ONCE(rsp->gp_activity) = jiffies;
         }
         rnp = rcu_get_root(rsp);
         raw_spin_lock_irq(&rnp->lock);
@@ -1821,6 +1840,7 @@ static int __noreturn rcu_gp_kthread(void *arg)
                         if (rcu_gp_init(rsp))
                                 break;
                         cond_resched_rcu_qs();
+                        ACCESS_ONCE(rsp->gp_activity) = jiffies;
                         WARN_ON(signal_pending(current));
                         trace_rcu_grace_period(rsp->name,
                                                ACCESS_ONCE(rsp->gpnum),
@@ -1864,9 +1884,11 @@ static int __noreturn rcu_gp_kthread(void *arg)
                                                        ACCESS_ONCE(rsp->gpnum),
                                                        TPS("fqsend"));
                                 cond_resched_rcu_qs();
+                                ACCESS_ONCE(rsp->gp_activity) = jiffies;
                         } else {
                                 /* Deal with stray signal. */
                                 cond_resched_rcu_qs();
+                                ACCESS_ONCE(rsp->gp_activity) = jiffies;
                                 WARN_ON(signal_pending(current));
                                 trace_rcu_grace_period(rsp->name,
                                                        ACCESS_ONCE(rsp->gpnum),
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 */