rcu: Check for jiffies going backwards

A report of a 12-jiffy normal RCU CPU stall warning raises interesting
questions about the nature of time on the offending system.  This commit
instruments rcu_sched_clock_irq(), which is RCU's hook into the
scheduling-clock interrupt, checking for the jiffies counter going
backwards.

Reported-by: Saravanan D <sarvanand@fb.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index a4b8189455d5eca9694101d779346a269597994b..a5ea674546404d2702e053bb9efc9ebd4dd1d4dc 100644 (file)
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1679,6 +1679,8 @@ static bool __note_gp_changes(struct rcu_node *rnp, struct rcu_data *rdp)
        rdp->gp_seq = rnp->gp_seq;  /* Remember new grace-period state. */
        if (ULONG_CMP_LT(rdp->gp_seq_needed, rnp->gp_seq_needed) || rdp->gpwrap)
                WRITE_ONCE(rdp->gp_seq_needed, rnp->gp_seq_needed);
+       if (IS_ENABLED(CONFIG_PROVE_RCU) && READ_ONCE(rdp->gpwrap))
+               WRITE_ONCE(rdp->last_sched_clock, jiffies);
        WRITE_ONCE(rdp->gpwrap, false);
        rcu_gpnum_ovf(rnp, rdp);
        return ret;
@@ -2609,6 +2611,13 @@ static void rcu_do_batch(struct rcu_data *rdp)
  */
 void rcu_sched_clock_irq(int user)
 {
+       unsigned long j;
+
+       if (IS_ENABLED(CONFIG_PROVE_RCU)) {
+               j = jiffies;
+               WARN_ON_ONCE(time_before(j, __this_cpu_read(rcu_data.last_sched_clock)));
+               __this_cpu_write(rcu_data.last_sched_clock, j);
+       }
        trace_rcu_utilization(TPS("Start scheduler-tick"));
        lockdep_assert_irqs_disabled();
        raw_cpu_inc(rcu_data.ticks_this_gp);
@@ -4179,6 +4188,7 @@ rcu_boot_init_percpu_data(int cpu)
        rdp->rcu_ofl_gp_flags = RCU_GP_CLEANED;
        rdp->rcu_onl_gp_seq = rcu_state.gp_seq;
        rdp->rcu_onl_gp_flags = RCU_GP_CLEANED;
+       rdp->last_sched_clock = jiffies;
        rdp->cpu = cpu;
        rcu_boot_init_nocb_percpu_data(rdp);
 }

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 926673ebe355f123c1b0a76d6aaed4c211f9dcd6..94b55f669915355d5fe067120774849391dac9bb 100644 (file)
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -254,6 +254,7 @@ struct rcu_data {
        unsigned long rcu_onl_gp_seq;   /* ->gp_seq at last online. */
        short rcu_onl_gp_flags;         /* ->gp_flags at last online. */
        unsigned long last_fqs_resched; /* Time of last rcu_resched(). */
+       unsigned long last_sched_clock; /* Jiffies of last rcu_sched_clock_irq(). */
 
        int cpu;
 };