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>

diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index ed186a902d312a23913bb3e4ba0f272306fd1cb3..55f9707fe60aab3d44d5164d31dd89960944038b 100644 (file)
--- 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 654b15be1e365f75b31b7f8dc544e6d0a2a1c480..a2ceb66bcd6729ba2ecb9ed0ef976d5c91937189 100644 (file)
--- 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 e300848cc0cfab411f6b4d87d243d985df0b1cd5..5ec81cf938fdd33a914e5b1b2451ae915cbabc05 100644 (file)
--- 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 */