Re: rcu_preempt caused oom

From: Paul E. McKenney
Date: Mon Dec 10 2018 - 23:53:22 EST

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On Mon, Dec 10, 2018 at 04:38:38PM -0800, Paul E. McKenney wrote:
> On Mon, Dec 10, 2018 at 06:56:18AM +0000, He, Bo wrote:
> > Hi,
> > We have start the test with the CONFIG_PROVE_RCU=y, and also add one 2s to detect the preempt rcu hang, hope we can get more useful logs tomorrow.
> > I also enclosed the config and the debug patches for you review.
>
> I instead suggest the (lightly tested) debug patch shown below, which
> tracks wakeups of RCU's grace-period kthreads and dumps them out if a
> given requested grace period fails to start. Again, it is necessary to
> build with CONFIG_PROVE_RCU=y, that is, with CONFIG_PROVE_LOCKING=y.

Right. This time without commenting out the wakeup as a test of the
diagnostic. :-/

Please use the patch below instead of the one that I sent in my
previous email.

Thanx, Paul

------------------------------------------------------------------------

commit adfc7dff659495a3433d5084256be59eee0ac6df
Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
Date: Mon Dec 10 16:33:59 2018 -0800

rcu: Improve diagnostics for failed RCU grace-period start

Backported from v4.21/v5.0

If a grace period fails to start (for example, because you commented
out the last two lines of rcu_accelerate_cbs_unlocked()), rcu_core()
will invoke rcu_check_gp_start_stall(), which will notice and complain.
However, this complaint is lacking crucial debugging information such
as when the last wakeup executed and what the value of ->gp_seq was at
that time. This commit therefore removes the current pr_alert() from
rcu_check_gp_start_stall(), instead invoking show_rcu_gp_kthreads(),
which has been updated to print the needed information, which is collected
by rcu_gp_kthread_wake().

Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 0b760c1369f7..4bcd8753e293 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -626,25 +626,57 @@ void rcu_sched_force_quiescent_state(void)
}
EXPORT_SYMBOL_GPL(rcu_sched_force_quiescent_state);

+/*
+ * Convert a ->gp_state value to a character string.
+ */
+static const char *gp_state_getname(short gs)
+{
+ if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
+ return "???";
+ return gp_state_names[gs];
+}
+
+/*
+ * Return the root node of the specified rcu_state structure.
+ */
+static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
+{
+ return &rsp->node[0];
+}
+
/*
* Show the state of the grace-period kthreads.
*/
void show_rcu_gp_kthreads(void)
{
int cpu;
+ unsigned long j;
+ unsigned long ja;
+ unsigned long jr;
+ unsigned long jw;
struct rcu_data *rdp;
struct rcu_node *rnp;
struct rcu_state *rsp;

+ j = jiffies;
for_each_rcu_flavor(rsp) {
- pr_info("%s: wait state: %d ->state: %#lx\n",
- rsp->name, rsp->gp_state, rsp->gp_kthread->state);
+ ja = j - READ_ONCE(rsp->gp_activity);
+ jr = j - READ_ONCE(rsp->gp_req_activity);
+ jw = j - READ_ONCE(rsp->gp_wake_time);
+ pr_info("%s: wait state: %s(%d) ->state: %#lx delta ->gp_activity %lu ->gp_req_activity %lu ->gp_wake_time %lu ->gp_wake_seq %ld ->gp_seq %ld ->gp_seq_needed %ld ->gp_flags %#x\n",
+ rsp->name, gp_state_getname(rsp->gp_state),
+ rsp->gp_state,
+ rsp->gp_kthread ? rsp->gp_kthread->state : 0x1ffffL,
+ ja, jr, jw, (long)READ_ONCE(rsp->gp_wake_seq),
+ (long)READ_ONCE(rsp->gp_seq),
+ (long)READ_ONCE(rcu_get_root(rsp)->gp_seq_needed),
+ READ_ONCE(rsp->gp_flags));
rcu_for_each_node_breadth_first(rsp, rnp) {
if (ULONG_CMP_GE(rsp->gp_seq, rnp->gp_seq_needed))
continue;
- pr_info("\trcu_node %d:%d ->gp_seq %lu ->gp_seq_needed %lu\n",
- rnp->grplo, rnp->grphi, rnp->gp_seq,
- rnp->gp_seq_needed);
+ pr_info("\trcu_node %d:%d ->gp_seq %ld ->gp_seq_needed %ld\n",
+ rnp->grplo, rnp->grphi, (long)rnp->gp_seq,
+ (long)rnp->gp_seq_needed);
if (!rcu_is_leaf_node(rnp))
continue;
for_each_leaf_node_possible_cpu(rnp, cpu) {
@@ -653,8 +685,8 @@ void show_rcu_gp_kthreads(void)
ULONG_CMP_GE(rsp->gp_seq,
rdp->gp_seq_needed))
continue;
- pr_info("\tcpu %d ->gp_seq_needed %lu\n",
- cpu, rdp->gp_seq_needed);
+ pr_info("\tcpu %d ->gp_seq_needed %ld\n",
+ cpu, (long)rdp->gp_seq_needed);
}
}
/* sched_show_task(rsp->gp_kthread); */
@@ -690,14 +722,6 @@ void rcutorture_get_gp_data(enum rcutorture_type test_type, int *flags,
}
EXPORT_SYMBOL_GPL(rcutorture_get_gp_data);

-/*
- * Return the root node of the specified rcu_state structure.
- */
-static struct rcu_node *rcu_get_root(struct rcu_state *rsp)
-{
- return &rsp->node[0];
-}
-
/*
* Enter an RCU extended quiescent state, which can be either the
* idle loop or adaptive-tickless usermode execution.
@@ -1285,16 +1309,6 @@ static void record_gp_stall_check_time(struct rcu_state *rsp)
rsp->n_force_qs_gpstart = READ_ONCE(rsp->n_force_qs);
}

-/*
- * Convert a ->gp_state value to a character string.
- */
-static const char *gp_state_getname(short gs)
-{
- if (gs < 0 || gs >= ARRAY_SIZE(gp_state_names))
- return "???";
- return gp_state_names[gs];
-}
-
/*
* Complain about starvation of grace-period kthread.
*/
@@ -1693,7 +1707,8 @@ static bool rcu_future_gp_cleanup(struct rcu_state *rsp, struct rcu_node *rnp)
* Don't do a self-awaken, and don't bother awakening when there is
* nothing for the grace-period kthread to do (as in several CPUs
* raced to awaken, and we lost), and finally don't try to awaken
- * a kthread that has not yet been created.
+ * a kthread that has not yet been created. If all those checks are
+ * passed, track some debug information and awaken.
*/
static void rcu_gp_kthread_wake(struct rcu_state *rsp)
{
@@ -1701,6 +1716,8 @@ static void rcu_gp_kthread_wake(struct rcu_state *rsp)
!READ_ONCE(rsp->gp_flags) ||
!rsp->gp_kthread)
return;
+ WRITE_ONCE(rsp->gp_wake_time, jiffies);
+ WRITE_ONCE(rsp->gp_wake_seq, READ_ONCE(rsp->gp_seq));
swake_up_one(&rsp->gp_wq);
}

@@ -2802,16 +2819,11 @@ rcu_check_gp_start_stall(struct rcu_state *rsp, struct rcu_node *rnp,
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
return;
}
- pr_alert("%s: g%ld->%ld gar:%lu ga:%lu f%#x gs:%d %s->state:%#lx\n",
- __func__, (long)READ_ONCE(rsp->gp_seq),
- (long)READ_ONCE(rnp_root->gp_seq_needed),
- j - rsp->gp_req_activity, j - rsp->gp_activity,
- rsp->gp_flags, rsp->gp_state, rsp->name,
- rsp->gp_kthread ? rsp->gp_kthread->state : 0x1ffffL);
WARN_ON(1);
if (rnp_root != rnp)
raw_spin_unlock_rcu_node(rnp_root);
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+ show_rcu_gp_kthreads();
}

/*
diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 4e74df768c57..0e051d9b5f1a 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -327,6 +327,8 @@ struct rcu_state {
struct swait_queue_head gp_wq; /* Where GP task waits. */
short gp_flags; /* Commands for GP task. */
short gp_state; /* GP kthread sleep state. */
+ unsigned long gp_wake_time; /* Last GP kthread wake. */
+ unsigned long gp_wake_seq; /* ->gp_seq at ^^^. */

/* End of fields guarded by root rcu_node's lock. */

Next message: Peter Xu: "Re: [PATCH] mm: thp: fix soft dirty for migration when split"
Previous message: Tony Lindgren: "Re: [PATCH 0/2] Graph fixes for using multiple endpoints per port"
In reply to: Paul E. McKenney: "Re: rcu_preempt caused oom"
Next in thread: He, Bo: "RE: rcu_preempt caused oom"
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