Re: rcu_sched stall detected, but no state dump
From: Paul E. McKenney
Date: Thu Dec 11 2014 - 14:10:10 EST
On Thu, Dec 11, 2014 at 08:50:10AM -0800, Paul E. McKenney wrote:
> On Thu, Dec 11, 2014 at 10:35:15AM +0100, Miroslav Benes wrote:
> > On Wed, 10 Dec 2014, Paul E. McKenney wrote:
> >
> > > On Wed, Dec 10, 2014 at 01:52:02PM +0100, Miroslav Benes wrote:
> > > >
> > > > Hi,
> > > >
> > > > today I came across RCU stall which was correctly detected, but there is
> > > > no state dump. This is a bit suspicious, I think.
> > > >
> > > > This is the output in serial console:
> > > >
> > > > [ 105.727003] INFO: rcu_sched detected stalls on CPUs/tasks:
> > > > [ 105.727003] (detected by 0, t=21002 jiffies, g=3269, c=3268, q=138)
> > > > [ 105.727003] INFO: Stall ended before state dump start
> > > > [ 168.732006] INFO: rcu_sched detected stalls on CPUs/tasks:
> > > > [ 168.732006] (detected by 0, t=84007 jiffies, g=3269, c=3268, q=270)
> > > > [ 168.732006] INFO: Stall ended before state dump start
> > > > [ 231.737003] INFO: rcu_sched detected stalls on CPUs/tasks:
> > > > [ 231.737003] (detected by 0, t=147012 jiffies, g=3269, c=3268, q=388)
> > > > [ 231.737003] INFO: Stall ended before state dump start
> > > > [ 294.742003] INFO: rcu_sched detected stalls on CPUs/tasks:
> > > > [ 294.742003] (detected by 0, t=210017 jiffies, g=3269, c=3268, q=539)
> > > > [ 294.742003] INFO: Stall ended before state dump start
> > > > [ 357.747003] INFO: rcu_sched detected stalls on CPUs/tasks:
> > > > [ 357.747003] (detected by 0, t=273022 jiffies, g=3269, c=3268, q=693)
> > > > [ 357.747003] INFO: Stall ended before state dump start
> > > > [ 420.752003] INFO: rcu_sched detected stalls on CPUs/tasks:
> > > > [ 420.752003] (detected by 0, t=336027 jiffies, g=3269, c=3268, q=806)
> > > > [ 420.752003] INFO: Stall ended before state dump start
> > > > ...
> > > >
> > > > It can be reproduced by trivial code attached to this mail (infinite
> > > > loop in kernel thread created in kernel module). I have CONFIG_PREEMPT=n.
> > > > The kernel thread is scheduled on the same CPU which causes soft lockup
> > > > (reliably detected when lockup detector is on). There is certainly RCU
> > > > stall, but I would expect a state dump. Is this an expected behaviour?
> > > > Maybe I overlooked some config option, don't know.
> > >
> > > Definitely not expected behavior! Unless you have only one CPU, but in
> > > that case you should be running tiny RCU, not tree RCU.
> >
> > So indeed I messed up my configs somehow and run the code on uniprocessor
> > with SMP=y and tree RCU. With more processors RCU stall is detected and
> > correct state is dumped. On uniprocessor with SMP=n and tiny RCU
> > softlockup is detected, but no RCU stall in the log (is this correct?). So
> > I'm really sorry for the noise.
> >
> > Anyway I still think that running SMP kernel with tree RCU on
> > uniprocessor is possible option (albeit suboptimal and maybe improbable).
> > Should I proceed with your patch below and bisection or am I mistaken
> > completely and we can leave it because there is no problem?
>
> Not a problem, there have been some interesting RCU CPU stall warnings
> recently, and your data did add some insight.
>
> So the combination SMP=n PREEMPT=y can happen straightforwardly via
> kbuild. The combination SMP=n PREEMPT=n can happen (somewhat less)
> straightforwardly by running an SMP=y PREEMPT=n kernel on a single-CPU
> system. In both cases, what can happen is that RCU's grace-period
> kthreads are starved, which can result in those reports.
>
> And these reports are confusing. I am considering attempting to improve
> the diagnostics. If I do, would you be willing to test the resulting
> patches?
Like this one, for example. ;-)
Thanx, Paul
------------------------------------------------------------------------
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@xxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
diff --git a/Documentation/RCU/stallwarn.txt b/Documentation/RCU/stallwarn.txt
index 4f8e33952b88..8085bc133791 100644
--- 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 7497dfe6ff3f..6a43836b61b0 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1096,7 +1096,7 @@ 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;
@@ -1144,10 +1144,18 @@ 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
+ pr_err("All QSes seen, last %s kthread activity %ld/%ld, jiffies_till_next_fqs=%ld\n",
+ rsp->name,
+ ACCESS_ONCE(rsp->gp_activity), jiffies,
+ jiffies_till_next_fqs);
+ }
/* Complain about tasks blocking the grace period. */
@@ -1247,7 +1255,7 @@ 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);
}
}
@@ -1643,6 +1651,7 @@ 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();
@@ -1703,6 +1712,7 @@ 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);
@@ -1719,6 +1729,7 @@ 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. */
@@ -1757,6 +1768,7 @@ 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;
@@ -1793,6 +1805,7 @@ 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);
@@ -1842,6 +1855,7 @@ 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),
@@ -1885,9 +1899,11 @@ 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 44e9ccfe6570..856518c8dcfa 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -483,6 +483,8 @@ 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 */
--
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