Commit c708b08c authored by Paul E. McKenney's avatar Paul E. McKenney
Browse files

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: default avatarSaravanan D <sarvanand@fb.com>
Signed-off-by: default avatarPaul E. McKenney <paulmck@kernel.org>
parent 90d2efe7

kernel/rcu/tree.c

+10 −0
Original line number Diff line number Diff line
@@ -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);

}

kernel/rcu/tree.h

+1 −0
Original line number Diff line number Diff line
@@ -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;

};