Re: rcu self-detected stall messages on OMAP3, 4 boards

[Paul Walmsley]

Hi Paul

On Sat, 22 Sep 2012, Paul Walmsley wrote:

> On Sat, 22 Sep 2012, Paul E. McKenney wrote:
> 
> > And here is a patch.  I am still having trouble reproducing the problem,
> > but figured that I should avoid serializing things.
> 
> Thanks, testing this now on v3.6-rc6.

Looks like you solved it!

Tested v3.6-rc6 + your stall diagnostic patch:

    http://marc.info/?l=linux-arm-kernel&m=134827237215882&w=2

on OMAP4430ES2 Pandaboard using omap2plus_defconfig and 
CONFIG_RCU_CPU_STALL_INFO=y; got the stall warnings.

Then added "rcu: Fix day-one dyntick-idle stall-warning bug" from:

    http://marc.info/?l=linux-arm-kernel&m=134835120600590&w=2

Booted that, and the stall warnings did not appear within 30 minutes.

To confirm that the problem being solved matched your hypothesis, the
debugging patch below[1] was added to the RCU idle entry/exit code.

Without the bugfix patch, a boot log transcript was obtained
indicating that the idle loop was entered with tick_nohz_enabled=1
during a grace period with no callbacks present:

    http://www.pwsan.com/omap/transcripts/20120922-rcu-stall-debug-pre-fix.txt

The debugging events started to appear at 1.867370 seconds into the
boot.  ENTER was pressed about 464 seconds in; this triggered the
rcu_sched stall traceback.

With the bugfix patch, a boot log transcript was obtained that
indicated that the condition under test never occurred after waiting
about 20 minutes:

    http://www.pwsan.com/omap/transcripts/20120922-rcu-stall-debug-post-fix.txt

Thanks for being so willing to root-cause the issue, Paul; it's 
appreciated, and it's been quite instructive as well.  Will address some 
remaining loose ends in follow-up E-mails.


- Paul


[1] Debugging patch to printk() if the previous idle loop entry occurred 
with tick_nohz_enabled=1 during a grace period with no RCU callbacks 
present:


---
 kernel/rcutree.c |   17 +++++++++++++++++
 1 file changed, 17 insertions(+)

diff --git a/kernel/rcutree.c b/kernel/rcutree.c
index f1eb7ad..f42941b 100644
--- a/kernel/rcutree.c
+++ b/kernel/rcutree.c
@@ -60,6 +60,9 @@
 
 /* Data structures. */
 
+extern int tick_nohz_enabled;
+static int no_cbs_idle_entry_count;
+
 static struct lock_class_key rcu_node_class[RCU_NUM_LVLS];
 
 #define RCU_STATE_INITIALIZER(sname, cr) { \
@@ -400,8 +403,12 @@ void rcu_idle_enter(void)
 	unsigned long flags;
 	long long oldval;
 	struct rcu_dynticks *rdtp;
+	int cpu;
+	long totqlen = 0;
+	struct rcu_data *rdp;
 
 	local_irq_save(flags);
+	rdp = &__get_cpu_var(rcu_sched_data);
 	rdtp = &__get_cpu_var(rcu_dynticks);
 	oldval = rdtp->dynticks_nesting;
 	WARN_ON_ONCE((oldval & DYNTICK_TASK_NEST_MASK) == 0);
@@ -410,6 +417,12 @@ void rcu_idle_enter(void)
 	else
 		rdtp->dynticks_nesting -= DYNTICK_TASK_NEST_VALUE;
 	rcu_idle_enter_common(rdtp, oldval);
+	if (tick_nohz_enabled && rcu_gp_in_progress(rdp->rsp)) {
+		for_each_possible_cpu(cpu)
+			totqlen += per_cpu_ptr(rdp->rsp->rda, cpu)->qlen;
+		if (totqlen == 0)
+			no_cbs_idle_entry_count = 1;
+	}
 	local_irq_restore(flags);
 }
 EXPORT_SYMBOL_GPL(rcu_idle_enter);
@@ -503,6 +516,10 @@ void rcu_idle_exit(void)
 		rdtp->dynticks_nesting = DYNTICK_TASK_EXIT_IDLE;
 	rcu_idle_exit_common(rdtp, oldval);
 	local_irq_restore(flags);
+	if (no_cbs_idle_entry_count) {
+		no_cbs_idle_entry_count = 0;
+		pr_err("* Tickless idle was entered with zero RCU callbacks\n");
+	}
 }
 EXPORT_SYMBOL_GPL(rcu_idle_exit);
 
-- 
1.7.10.4

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