Re: RCU stall when using function_graph

Paul E. McKenney
2017-08-30T22:07:41Z

On Wed, Aug 16, 2017 at 10:58:05AM -0700, Paul E. McKenney wrote:
> On Wed, Aug 16, 2017 at 12:41:40PM -0400, Steven Rostedt wrote:
> > On Wed, 16 Aug 2017 09:32:28 -0700
> > "Paul E. McKenney" <paulmck@linux.vnet.ibm.com> wrote:
> >
> > > Let me see if I understand you... About halfway to the stall limit,
> > > RCU triggers an irq_work (on each CPU that has not yet passed through
> > > a quiescent state, IPIing them in turn?), and if the irq_work has
> > > not completed by the end of the stall limit, RCU adds that to its
> > > stall-warning message.
> >
> > Doesn't even have to be half way through. It could be done at the
> > limit, and then wait a little more.
>
> Agreed. However, I take other evasive action halfway through, so there
> is a nice place to put the code.
>
> > > Or am I missing something here?
> >
> > No, I think that's what I was suggesting.
>
> I don't see a way of canceling an irq_work.
>
> Ah, but there is an irq_work_queue_on() that fails if still pending.
> I should be able to use that instead of cancel. Plus, -I- don't have
> to send the IPIs, I can just let irq_work_queue_on() do my dirty work!
>
> This should provide sufficient entertainment for a bit! ;-)

Finally got to this -- how does the patch below look to you?

It adds a digit to the stall warning saying for how many full grace
periods interrupts have been disabled, "." if interrupts were enabled
recently, and "!" if RCU was in too much trouble to tell one way or
the other. (This last can happen if RCU's grace-period kthread has been
prevented from executing for most of the grace period.)

Thanx, Paul

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

commit 7b32575c82ed5c891b93a4a5961e608a97cdb192
Author: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Date: Thu Aug 17 17:05:59 2017 -0700

rcu: Make RCU CPU stall warnings check for irq-disabled CPUs

One common question upon seeing an RCU CPU stall warning is "did
the stalled CPUs have interrupts disabled?" However, the current
stall warnings are silent on this point. This commit therefore
uses irq_work to check whether stalled CPUs still respond to IPIs,
and flags this state in the RCU CPU stall warning console messages.

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

tree.c | 103 +++++++++++++++++++++++++++++++++++++++++++++++++++-------
tree.h | 5 ++
tree_plugin.h | 5 ++
3 files changed, 100 insertions(+), 13 deletions(-)

diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
index 5152120f11d3..307c641149d4 100644
--- a/kernel/rcu/tree.c
+++ b/kernel/rcu/tree.c
@@ -1197,6 +1197,22 @@ static int rcu_is_cpu_rrupt_from_idle(void)
}

/*
+ * We are reporting a quiescent state on behalf of some other CPU, so
+ * it is our responsibility to check for and handle potential overflow
+ * of the rcu_node ->gpnum counter with respect to the rcu_data counters.
+ * After all, the CPU might be in deep idle state, and thus executing no
+ * code whatsoever.
+ */
+static void rcu_gpnum_ovf(struct rcu_node *rnp, struct rcu_data *rdp)
+{
+ lockdep_assert_held(&rnp->lock);
+ if (ULONG_CMP_LT(READ_ONCE(rdp->gpnum) + ULONG_MAX / 4, rnp->gpnum))
+ WRITE_ONCE(rdp->gpwrap, true);
+ if (ULONG_CMP_LT(rdp->rcu_iw_gpnum + ULONG_MAX / 4, rnp->gpnum))
+ rdp->rcu_iw_gpnum = rnp->gpnum + ULONG_MAX / 4;
+}
+
+/*
* Snapshot the specified CPU's dynticks counter so that we can later
* credit them with an implicit quiescent state. Return 1 if this CPU
* is in dynticks idle mode, which is an extended quiescent state.
@@ -1206,15 +1222,34 @@ static int dyntick_save_progress_counter(struct rcu_data *rdp)
rdp->dynticks_snap = rcu_dynticks_snap(rdp->dynticks);
if (rcu_dynticks_in_eqs(rdp->dynticks_snap)) {
trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("dti"));
- if (ULONG_CMP_LT(READ_ONCE(rdp->gpnum) + ULONG_MAX / 4,
- rdp->mynode->gpnum))
- WRITE_ONCE(rdp->gpwrap, true);
+ rcu_gpnum_ovf(rdp->mynode, rdp);
return 1;
}
return 0;
}

/*
+ * Handler for the irq_work request posted when a grace period has
+ * gone on for too long, but not yet long enough for an RCU CPU
+ * stall warning. Set state appropriately, but just complain if
+ * there is unexpected state on entry.
+ */
+static void rcu_iw_handler(struct irq_work *iwp)
+{
+ struct rcu_data *rdp;
+ struct rcu_node *rnp;
+
+ rdp = container_of(iwp, struct rcu_data, rcu_iw);
+ rnp = rdp->mynode;
+ raw_spin_lock_rcu_node(rnp);
+ if (!WARN_ON_ONCE(!rdp->rcu_iw_pending)) {
+ rdp->rcu_iw_gpnum = rnp->gpnum;
+ rdp->rcu_iw_pending = false;
+ }
+ raw_spin_unlock_rcu_node(rnp);
+}
+
+/*
* Return true if the specified CPU has passed through a quiescent
* state by virtue of being in or having passed through an dynticks
* idle state since the last call to dyntick_save_progress_counter()
@@ -1225,7 +1260,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
unsigned long jtsq;
bool *rnhqp;
bool *ruqp;
- struct rcu_node *rnp;
+ struct rcu_node *rnp = rdp->mynode;

/*
* If the CPU passed through or entered a dynticks idle phase with
@@ -1238,6 +1273,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
if (rcu_dynticks_in_eqs_since(rdp->dynticks, rdp->dynticks_snap)) {
trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("dti"));
rdp->dynticks_fqs++;
+ rcu_gpnum_ovf(rnp, rdp);
return 1;
}

@@ -1248,12 +1284,12 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
* might not be the case for nohz_full CPUs looping in the kernel.
*/
jtsq = jiffies_till_sched_qs;
- rnp = rdp->mynode;
ruqp = per_cpu_ptr(&rcu_dynticks.rcu_urgent_qs, rdp->cpu);
if (time_after(jiffies, rdp->rsp->gp_start + jtsq) &&
READ_ONCE(rdp->rcu_qs_ctr_snap) != per_cpu(rcu_dynticks.rcu_qs_ctr, rdp->cpu) &&
READ_ONCE(rdp->gpnum) == rnp->gpnum && !rdp->gpwrap) {
trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("rqc"));
+ rcu_gpnum_ovf(rnp, rdp);
return 1;
} else if (time_after(jiffies, rdp->rsp->gp_start + jtsq)) {
/* Load rcu_qs_ctr before store to rcu_urgent_qs. */
@@ -1264,6 +1300,7 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
if (!(rdp->grpmask & rcu_rnp_online_cpus(rnp))) {
trace_rcu_fqs(rdp->rsp->name, rdp->gpnum, rdp->cpu, TPS("ofl"));
rdp->offline_fqs++;
+ rcu_gpnum_ovf(rnp, rdp);
return 1;
}

@@ -1295,11 +1332,21 @@ static int rcu_implicit_dynticks_qs(struct rcu_data *rdp)
}

/*
- * If more than halfway to RCU CPU stall-warning time, do
- * a resched_cpu() to try to loosen things up a bit.
+ * If more than halfway to RCU CPU stall-warning time, do a
+ * resched_cpu() to try to loosen things up a bit. Also check to
+ * see if the CPU is getting hammered with interrupts, but only
+ * once per grace period, just to keep the IPIs down to a dull roar.
*/
- if (jiffies - rdp->rsp->gp_start > rcu_jiffies_till_stall_check() / 2)
+ if (jiffies - rdp->rsp->gp_start > rcu_jiffies_till_stall_check() / 2) {
resched_cpu(rdp->cpu);
+ if (!rdp->rcu_iw_pending && rdp->rcu_iw_gpnum != rnp->gpnum &&
+ (rnp->ffmask & rdp->grpmask)) {
+ init_irq_work(&rdp->rcu_iw, rcu_iw_handler);
+ rdp->rcu_iw_pending = true;
+ rdp->rcu_iw_gpnum = rnp->gpnum;
+ irq_work_queue_on(&rdp->rcu_iw, rdp->cpu);
+ }
+ }

return 0;
}
@@ -1488,6 +1535,7 @@ static void print_cpu_stall(struct rcu_state *rsp)
{
int cpu;
unsigned long flags;
+ struct rcu_data *rdp = this_cpu_ptr(rsp->rda);
struct rcu_node *rnp = rcu_get_root(rsp);
long totqlen = 0;

@@ -1503,7 +1551,9 @@ static void print_cpu_stall(struct rcu_state *rsp)
*/
pr_err("INFO: %s self-detected stall on CPU", rsp->name);
print_cpu_stall_info_begin();
+ raw_spin_lock_irqsave_rcu_node(rdp->mynode, flags);
print_cpu_stall_info(rsp, smp_processor_id());
+ raw_spin_unlock_irqrestore_rcu_node(rdp->mynode, flags);
print_cpu_stall_info_end();
for_each_possible_cpu(cpu)
totqlen += rcu_segcblist_n_cbs(&per_cpu_ptr(rsp->rda,
@@ -1897,6 +1947,7 @@ static bool __note_gp_changes(struct rcu_state *rsp, struct rcu_node *rnp,
rdp->core_needs_qs = need_gp;
zero_cpu_stall_ticks(rdp);
WRITE_ONCE(rdp->gpwrap, false);
+ rcu_gpnum_ovf(rnp, rdp);
}
return ret;
}
@@ -3675,6 +3726,8 @@ rcu_init_percpu_data(int cpu, struct rcu_state *rsp)
rdp->cpu_no_qs.b.norm = true;
rdp->rcu_qs_ctr_snap = per_cpu(rcu_dynticks.rcu_qs_ctr, cpu);
rdp->core_needs_qs = false;
+ rdp->rcu_iw_pending = false;
+ rdp->rcu_iw_gpnum = rnp->gpnum - 1;
trace_rcu_grace_period(rsp->name, rdp->gpnum, TPS("cpuonl"));
raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
}
@@ -3712,10 +3765,24 @@ static void rcutree_affinity_setting(unsigned int cpu, int outgoing)
*/
int rcutree_online_cpu(unsigned int cpu)
{
- sync_sched_exp_online_cleanup(cpu);
- rcutree_affinity_setting(cpu, -1);
+ unsigned long flags;
+ struct rcu_data *rdp;
+ struct rcu_node *rnp;
+ struct rcu_state *rsp;
+
+ for_each_rcu_flavor(rsp) {
+ rdp = per_cpu_ptr(rsp->rda, cpu);
+ rnp = rdp->mynode;
+ raw_spin_lock_irqsave_rcu_node(rnp, flags);
+ rnp->ffmask |= rdp->grpmask;
+ raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+ }
if (IS_ENABLED(CONFIG_TREE_SRCU))
srcu_online_cpu(cpu);
+ if (rcu_scheduler_active == RCU_SCHEDULER_INACTIVE)
+ return 0; /* Too early in boot for scheduler work. */
+ sync_sched_exp_online_cleanup(cpu);
+ rcutree_affinity_setting(cpu, -1);
return 0;
}

@@ -3725,6 +3792,19 @@ int rcutree_online_cpu(unsigned int cpu)
*/
int rcutree_offline_cpu(unsigned int cpu)
{
+ unsigned long flags;
+ struct rcu_data *rdp;
+ struct rcu_node *rnp;
+ struct rcu_state *rsp;
+
+ for_each_rcu_flavor(rsp) {
+ rdp = per_cpu_ptr(rsp->rda, cpu);
+ rnp = rdp->mynode;
+ raw_spin_lock_irqsave_rcu_node(rnp, flags);
+ rnp->ffmask &= ~rdp->grpmask;
+ raw_spin_unlock_irqrestore_rcu_node(rnp, flags);
+ }
+
rcutree_affinity_setting(cpu, cpu);
if (IS_ENABLED(CONFIG_TREE_SRCU))
srcu_offline_cpu(cpu);
@@ -4173,8 +4253,7 @@ void __init rcu_init(void)
for_each_online_cpu(cpu) {
rcutree_prepare_cpu(cpu);
rcu_cpu_starting(cpu);
- if (IS_ENABLED(CONFIG_TREE_SRCU))
- srcu_online_cpu(cpu);
+ rcutree_online_cpu(cpu);
}
}

diff --git a/kernel/rcu/tree.h b/kernel/rcu/tree.h
index 8e1f285f0a70..46a5d1991450 100644
--- a/kernel/rcu/tree.h
+++ b/kernel/rcu/tree.h
@@ -103,6 +103,7 @@ struct rcu_node {
/* Online CPUs for next expedited GP. */
/* Any CPU that has ever been online will */
/* have its bit set. */
+ unsigned long ffmask; /* Fully functional CPUs. */
unsigned long grpmask; /* Mask to apply to parent qsmask. */
/* Only one bit will be set in this mask. */
int grplo; /* lowest-numbered CPU or group here. */
@@ -285,6 +286,10 @@ struct rcu_data {

/* 8) RCU CPU stall data. */
unsigned int softirq_snap; /* Snapshot of softirq activity. */
+ /* ->rcu_iw* fields protected by leaf rcu_node ->lock. */
+ struct irq_work rcu_iw; /* Check for non-irq activity. */
+ bool rcu_iw_pending; /* Is ->rcu_iw pending? */
+ unsigned long rcu_iw_gpnum; /* ->gpnum associated with ->rcu_iw. */

int cpu;
struct rcu_state *rsp;
diff --git a/kernel/rcu/tree_plugin.h b/kernel/rcu/tree_plugin.h
index cfb95d3e7fbc..bca03bf2b3c9 100644
--- a/kernel/rcu/tree_plugin.h
+++ b/kernel/rcu/tree_plugin.h
@@ -1671,6 +1671,7 @@ static void print_cpu_stall_info_begin(void)
*/
static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
{
+ unsigned long delta;
char fast_no_hz[72];
struct rcu_data *rdp = per_cpu_ptr(rsp->rda, cpu);
struct rcu_dynticks *rdtp = rdp->dynticks;
@@ -1685,11 +1686,13 @@ static void print_cpu_stall_info(struct rcu_state *rsp, int cpu)
ticks_value = rsp->gpnum - rdp->gpnum;
}
print_cpu_stall_fast_no_hz(fast_no_hz, cpu);
- pr_err("\t%d-%c%c%c: (%lu %s) idle=%03x/%llx/%d softirq=%u/%u fqs=%ld %s\n",
+ delta = rdp->mynode->gpnum - rdp->rcu_iw_gpnum;
+ pr_err("\t%d-%c%c%c%c: (%lu %s) idle=%03x/%llx/%d softirq=%u/%u fqs=%ld %s\n",
cpu,
"O."[!!cpu_online(cpu)],
"o."[!!(rdp->grpmask & rdp->mynode->qsmaskinit)],
"N."[!!(rdp->grpmask & rdp->mynode->qsmaskinitnext)],
+ rdp->rcu_iw_pending ? (int)min(delta, 9UL) + '0' : "!."[!delta],
ticks_value, ticks_title,
rcu_dynticks_snap(rdtp) & 0xfff,
rdtp->dynticks_nesting, rdtp->dynticks_nmi_nesting,