Re: [PATCH,RFC] RCU-based detection of stalled CPUs for ClassicRCU (LTTng support)
From: Mathieu Desnoyers
Date: Thu Oct 02 2008 - 14:51:27 EST
* Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) wrote:
> On Thu, Oct 02, 2008 at 11:10:46AM -0400, Mathieu Desnoyers wrote:
> > * Paul E. McKenney (paulmck@xxxxxxxxxxxxxxxxxx) wrote:
> > > On Thu, Oct 02, 2008 at 10:07:26AM +0200, Ingo Molnar wrote:
> > > >
> > > > * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx> wrote:
> > > >
> > > > > Hello!
> > > > >
> > > > > This patch adds stalled-CPU detection to Classic RCU. This capability
> > > > > is enabled by a new config variable CONFIG_RCU_CPU_STALL_DETECTOR,
> > > > > which defaults disabled. This is a debugging feature, not something
> > > > > that non-kernel-hackers would be expected to care about. This feature
> > > > > can detect looping CPUs in !PREEMPT builds and looping CPUs with
> > > > > preemption disabled in PREEMPT builds. This is essentially a port of
> > > > > this functionality from the treercu patch.
> > > > >
> > > > > One current shortcoming: on some systems, stalls are detected during
> > > > > early boot, when we normally would not care about them. My thought is
> > > > > to add a call from late initialization to suppress stall detection
> > > > > until the system is well along its way to being booted, but thought I
> > > > > should check to see if there might already be something for this
> > > > > purpose.
> > > >
> > > > could you be a bit more specific, why do those warnings show up and why
> > > > dont we care about them? There are things like networking that
> > > > occasionally do an rcu_sync() and a stall could mean a bootup hang.
> > >
> > > Hmmm...
> > >
> > > Good point, I was just falling back on my old "we don't care about RCU
> > > stalls in boot-time code" rule from long ago. It is entirely possible
> > > (in fact reasonably likely) that this rule no longer applies to Linux as
> > > it exists today. So please see below for the console output. Not all
> > > systems report this stall. I have recently been running only on Power
> > > systems, will fire off on some x86s. My kneejerk reaction was that the
> > > "stall" was really due to the clock-setting operation -- RCU stalls are
> > > based on get_seconds().
> > >
> > > So maybe I need to change the stall-detection code to use jiffies
> > > instead.
> > >
> > > Thoughts?
> > >
> >
> > You could possibly use get monotonic time, which should hopefully be a
> > bit better than its NTP-corrected alternatives.
>
> OK. I thought jiffies were monotonic (though not perfectly correlated
> to the passage of time). This is a warning timeout, so does not need
> better than a few percent accuracy, as long as time never jumps too far.
>
Well, jiffies should also be monotonic, but I don't know their exact
status wrt dyntick systems.
> But what API did you have in mind?
>
Something like do_posix_clock_monotonic_gettime() in linux/time.h. I
think it should be ok after early boot.
> > Also, do you have all the tools you need to poinpoint the precise source
> > of RCU stalls ? I haven't looked at RCU trace, but I should say adding a
> > new tracepoint to instrument RCU callback execution at boot time and to
> > trace it with LTTng is close to trivial. (actually, very early kernel
> > boot is not so trivial as we would have to create a small kernel module
> > to activate lttng tracing, but tracing the early phases of userspace
> > bootup is trivial given we can use the userspace API).
>
> I just dump stack, which historically worked pretty well.
>
> Could you please tell me more about what you think should be added?
>
I just created two patches which instrument RCU callbacks and call_rcu
in classic and preempt RCU to the -lttng tree to show how simple it is
once the LTTng infrastructure is in place.
Basically, all you need to do is to pull this tree (branch
2.6.27-rc8-lttng-0.31) :
git://git.kernel.org/pub/scm/linux/kernel/git/compudj/linux-2.6-lttng.git
And follow :
http://ltt.polymtl.ca/svn/trunk/lttv/QUICKSTART
Which will basically tell you to enable markers/tracepoints and leave
most ltt-related options to default (all in general setup menu).
wget http://ltt.polymtl.ca/lttng/ltt-control-0.53-02102008.tar.gz
extract, ./configure, make, make install
wget http://ltt.polymtl.ca/packages/lttv-0.10.0-pre15-12082008.tar.gz
extract, ./configure, make, make install
(see QUICKSTART for package dependencies. Will be automatically checked
by configure)
You may have to add a mount point for debugfs and load some kernel
modules (as explained in the QUICKSTART). Don't forget rcu-trace.ko.
To take a trace, once you are booted in the kernel :
ltt-armall
(check that the rcu_* markers are enabled by looking at the "state"
field of cat /proc/ltt | grep rcu)
lttctl -n trace -d -l /mnt/debugfs/ltt -t /tmp/trace
[Reproduce behavior to trace]
lttctl -n trace -R
# Produce a text dump
lttv -m textDump -t /tmp/trace
# Produce a text dump filtering only rcu events :
lttv -m textDump -t /tmp/trace \
-e "event.name=rcu_classic_callback|event.name=rcu_classic_call_rcu|\
event.name=rcu_classic_call_rcu_bh|event.name=rcu_preempt_callback|\
event.name=rcu_preempt_call_rcu_sched|event.name=rcu_preempt_call_rcu"
Then some scripting could be required to turn the function pointers into
symbols. Keep you kernel vmlinux around to perform symbol resolution.
A quick trick is to use objdump -S --start-address=0x.... vmlinux|less
but I am certain there are much much better ways to do it. ;)
There is also the gui you can use to dig into the trace :
lttv-gui -t /tmp/trace
That should help you pinpoint which are the functions that took so much
time and which is their associated call_rcu caller IP.
If you run a trace with preempt rcu, the textdump should give you an
output like this :
[...]
rcu_preempt_call_rcu: 130.773191675 (/tmp/trace/cpu_5), 3977, 3977, lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AE105 }
rcu_preempt_call_rcu: 130.773194003 (/tmp/trace/cpu_5), 3977, 3977, lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AE105 }
rcu_preempt_call_rcu: 130.773195929 (/tmp/trace/cpu_5), 3977, 3977, lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AE105 }
rcu_preempt_call_rcu: 130.773197399 (/tmp/trace/cpu_5), 3977, 3977, lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AE105 }
rcu_preempt_call_rcu: 130.773200429 (/tmp/trace/cpu_5), 3977, 3977, lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802c6670, ip = 0xFFFFFFFF8023A45B }
rcu_preempt_call_rcu: 130.773213907 (/tmp/trace/cpu_5), 3961, 3961, bash, , 3960, 0x0, SYSCALL { func = 0xffffffff8024a980, ip = 0xFFFFFFFF8023937D }
rcu_preempt_call_rcu: 130.773215527 (/tmp/trace/cpu_5), 3961, 3961, bash, , 3960, 0x0, SYSCALL { func = 0xffffffff80238550, ip = 0xFFFFFFFF80239985 }
rcu_preempt_callback: 130.789025735 (/tmp/trace/cpu_2), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 130.789026731 (/tmp/trace/cpu_2), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 130.789050610 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 130.789051096 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802c6670 }
rcu_preempt_callback: 130.789052083 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff8024a980 }
rcu_preempt_callback: 130.789052713 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff80238550 }
rcu_preempt_callback: 130.805051097 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 130.805052318 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 130.805052822 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 130.805053434 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 130.805054226 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802c6670 }
rcu_preempt_callback: 130.805055339 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff8024a980 }
rcu_preempt_callback: 130.805056014 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff80238550 }
rcu_preempt_call_rcu: 131.442990122 (/tmp/trace/cpu_7), 3685, 3685, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_call_rcu: 131.443615323 (/tmp/trace/cpu_6), 3687, 3687, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_call_rcu: 131.446861604 (/tmp/trace/cpu_7), 3683, 3683, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_call_rcu: 131.447487425 (/tmp/trace/cpu_5), 3681, 3681, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_callback: 131.473052317 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 131.473078626 (/tmp/trace/cpu_6), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 131.477001607 (/tmp/trace/cpu_7), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 131.477002435 (/tmp/trace/cpu_7), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_call_rcu: 133.443017217 (/tmp/trace/cpu_5), 3681, 3681, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_call_rcu: 133.443636900 (/tmp/trace/cpu_6), 3687, 3687, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_call_rcu: 133.446887423 (/tmp/trace/cpu_7), 3683, 3683, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_call_rcu: 133.447509515 (/tmp/trace/cpu_7), 3685, 3685, hald-addon-stor, , 3634, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AA8D9 }
rcu_preempt_callback: 133.473044696 (/tmp/trace/cpu_5), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 133.473070649 (/tmp/trace/cpu_6), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 133.476994292 (/tmp/trace/cpu_7), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_callback: 133.476994961 (/tmp/trace/cpu_7), 0, 0, swapper, , 0, 0x0, SOFTIRQ { func = 0xffffffff802addc0 }
rcu_preempt_call_rcu: 134.085511140 (/tmp/trace/cpu_5), 3961, 3961, bash, , 3960, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AE105 }
rcu_preempt_call_rcu: 134.085545237 (/tmp/trace/cpu_5), 3981, 3981, bash, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AE105 }
rcu_preempt_call_rcu: 134.086124869 (/tmp/trace/cpu_2), 3981, 3981, ./lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802B8E2D }
rcu_preempt_call_rcu: 134.086135078 (/tmp/trace/cpu_2), 3981, 3981, ./lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802B8E2D }
rcu_preempt_call_rcu: 134.086143519 (/tmp/trace/cpu_2), 3981, 3981, ./lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802B8E2D }
rcu_preempt_call_rcu: 134.086206323 (/tmp/trace/cpu_2), 3981, 3981, ./lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802B8E2D }
rcu_preempt_call_rcu: 134.086526166 (/tmp/trace/cpu_2), 3981, 3981, ./lttctl, , 3961, 0x0, SYSCALL { func = 0xffffffff802addc0, ip = 0xFFFFFFFF802AE105 }
[...]
Basically, once you have the LTTng infrastructure in place, all that
needs to be added to RCU to do that is this (which is already in the
-lttng tree), plus a small LTTng "probe" module. If you want, you could
add a tracepoint in your stalled CPU detector and send the data through
LTTng so you have a starting point to dig from in large traces. From
that point, finding the root cause of long RCU-based CPU stalls becomes
trivial.
RCU : tracepoint instrumentation
Instrument RCU classic and RCU preempt callback execution and call_rcu.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
CC: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
---
include/trace/rcu.h | 31 +++++++++++++++++++++++++++++++
kernel/rcuclassic.c | 4 ++++
kernel/rcupreempt.c | 4 ++++
3 files changed, 39 insertions(+)
Index: linux-2.6-lttng/include/trace/rcu.h
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6-lttng/include/trace/rcu.h 2008-10-02 14:18:05.000000000 -0400
@@ -0,0 +1,31 @@
+#ifndef _TRACE_RCU_H
+#define _TRACE_RCU_H
+
+#include <linux/tracepoint.h>
+#include <linux/rcupdate.h>
+
+DEFINE_TRACE(rcu_classic_callback,
+ TPPROTO(struct rcu_head *head),
+ TPARGS(head));
+
+DEFINE_TRACE(rcu_classic_call_rcu,
+ TPPROTO(struct rcu_head *head, unsigned long ip),
+ TPARGS(head, ip));
+
+DEFINE_TRACE(rcu_classic_call_rcu_bh,
+ TPPROTO(struct rcu_head *head, unsigned long ip),
+ TPARGS(head, ip));
+
+DEFINE_TRACE(rcu_preempt_callback,
+ TPPROTO(struct rcu_head *head),
+ TPARGS(head));
+
+DEFINE_TRACE(rcu_preempt_call_rcu,
+ TPPROTO(struct rcu_head *head, unsigned long ip),
+ TPARGS(head, ip));
+
+DEFINE_TRACE(rcu_preempt_call_rcu_sched,
+ TPPROTO(struct rcu_head *head, unsigned long ip),
+ TPARGS(head, ip));
+
+#endif
Index: linux-2.6-lttng/kernel/rcuclassic.c
===================================================================
--- linux-2.6-lttng.orig/kernel/rcuclassic.c 2008-10-02 13:44:35.000000000 -0400
+++ linux-2.6-lttng/kernel/rcuclassic.c 2008-10-02 14:16:45.000000000 -0400
@@ -47,6 +47,7 @@
#include <linux/notifier.h>
#include <linux/cpu.h>
#include <linux/mutex.h>
+#include <trace/rcu.h>
#ifdef CONFIG_DEBUG_LOCK_ALLOC
static struct lock_class_key rcu_lock_key;
@@ -138,6 +139,7 @@ void call_rcu(struct rcu_head *head,
head->func = func;
head->next = NULL;
local_irq_save(flags);
+ trace_rcu_classic_call_rcu(head, _RET_IP_);
rdp = &__get_cpu_var(rcu_data);
*rdp->nxttail = head;
rdp->nxttail = &head->next;
@@ -174,6 +176,7 @@ void call_rcu_bh(struct rcu_head *head,
head->func = func;
head->next = NULL;
local_irq_save(flags);
+ trace_rcu_classic_call_rcu_bh(head, _RET_IP_);
rdp = &__get_cpu_var(rcu_bh_data);
*rdp->nxttail = head;
rdp->nxttail = &head->next;
@@ -232,6 +235,7 @@ static void rcu_do_batch(struct rcu_data
while (list) {
next = list->next;
prefetch(next);
+ trace_rcu_classic_callback(list);
list->func(list);
list = next;
if (++count >= rdp->blimit)
Index: linux-2.6-lttng/kernel/rcupreempt.c
===================================================================
--- linux-2.6-lttng.orig/kernel/rcupreempt.c 2008-10-02 13:53:41.000000000 -0400
+++ linux-2.6-lttng/kernel/rcupreempt.c 2008-10-02 14:15:47.000000000 -0400
@@ -57,6 +57,7 @@
#include <linux/byteorder/swabb.h>
#include <linux/cpumask.h>
#include <linux/rcupreempt_trace.h>
+#include <trace/rcu.h>
/*
* Macro that prevents the compiler from reordering accesses, but does
@@ -1108,6 +1109,7 @@ static void rcu_process_callbacks(struct
spin_unlock_irqrestore(&rdp->lock, flags);
while (list) {
next = list->next;
+ trace_rcu_preempt_callback(list);
list->func(list);
list = next;
RCU_TRACE_ME(rcupreempt_trace_invoke);
@@ -1122,6 +1124,7 @@ void call_rcu(struct rcu_head *head, voi
head->func = func;
head->next = NULL;
local_irq_save(flags);
+ trace_rcu_preempt_call_rcu(head, _RET_IP_);
rdp = RCU_DATA_ME();
spin_lock(&rdp->lock);
__rcu_advance_callbacks(rdp);
@@ -1141,6 +1144,7 @@ void call_rcu_sched(struct rcu_head *hea
head->func = func;
head->next = NULL;
local_irq_save(flags);
+ trace_rcu_preempt_call_rcu_sched(head, _RET_IP_);
rdp = RCU_DATA_ME();
spin_lock(&rdp->lock);
*rdp->nextschedtail = head;
The LTTng probe module looks like this :
LTTng trace RCU probe
RCU callback/call_rcu LTTng probe.
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxx>
CC: "Paul E. McKenney" <paulmck@xxxxxxxxxxxxxxxxxx>
---
ltt/probes/Makefile | 2 -
ltt/probes/rcu-trace.c | 54 +++++++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 55 insertions(+), 1 deletion(-)
Index: linux-2.6-lttng/ltt/probes/rcu-trace.c
===================================================================
--- /dev/null 1970-01-01 00:00:00.000000000 +0000
+++ linux-2.6-lttng/ltt/probes/rcu-trace.c 2008-10-02 14:23:17.000000000 -0400
@@ -0,0 +1,54 @@
+/*
+ * ltt/probes/rcu-trace.c
+ *
+ * RCU tracepoint probes.
+ */
+
+#include <linux/module.h>
+#include <trace/rcu.h>
+
+#ifdef CONFIG_CLASSIC_RCU
+void probe_rcu_classic_callback(struct rcu_head *head)
+{
+ trace_mark_tp(rcu_classic_callback, rcu_classic_callback,
+ probe_rcu_classic_callback, "func %p", head->func);
+}
+
+void probe_rcu_classic_call_rcu(struct rcu_head *head, unsigned long ip)
+{
+ trace_mark_tp(rcu_classic_call_rcu, rcu_classic_call_rcu,
+ probe_rcu_classic_call_rcu, "func %p ip 0x%lX", head->func, ip);
+}
+
+void probe_rcu_classic_call_rcu_bh(struct rcu_head *head, unsigned long ip)
+{
+ trace_mark_tp(rcu_classic_call_rcu_bh, rcu_classic_call_rcu_bh,
+ probe_rcu_classic_call_rcu_bh, "func %p ip 0x%lX",
+ head->func, ip);
+}
+#endif
+
+#ifdef CONFIG_PREEMPT_RCU
+void probe_rcu_preempt_callback(struct rcu_head *head)
+{
+ trace_mark_tp(rcu_preempt_callback, rcu_preempt_callback,
+ probe_rcu_preempt_callback, "func %p", head->func);
+}
+
+void probe_rcu_preempt_call_rcu(struct rcu_head *head, unsigned long ip)
+{
+ trace_mark_tp(rcu_preempt_call_rcu, rcu_preempt_call_rcu,
+ probe_rcu_preempt_call_rcu, "func %p ip 0x%lX", head->func, ip);
+}
+
+void probe_rcu_preempt_call_rcu_sched(struct rcu_head *head, unsigned long ip)
+{
+ trace_mark_tp(rcu_preempt_call_rcu_sched, rcu_preempt_call_rcu_sched,
+ probe_rcu_preempt_call_rcu_sched, "func %p ip 0x%lX",
+ head->func, ip);
+}
+#endif
+
+MODULE_LICENSE("GPL");
+MODULE_AUTHOR("Mathieu Desnoyers");
+MODULE_DESCRIPTION("RCU Tracepoint Probes");
Index: linux-2.6-lttng/ltt/probes/Makefile
===================================================================
--- linux-2.6-lttng.orig/ltt/probes/Makefile 2008-10-02 14:11:20.000000000 -0400
+++ linux-2.6-lttng/ltt/probes/Makefile 2008-10-02 14:18:16.000000000 -0400
@@ -9,7 +9,7 @@ CFLAGS_REMOVE_lockdep-trace.o = -pg
endif
obj-$(CONFIG_LTT_TRACEPROBES) += kernel-trace.o mm-trace.o fs-trace.o \
- ipc-trace.o lockdep-trace.o
+ ipc-trace.o lockdep-trace.o rcu-trace.o
ifeq ($(CONFIG_NET),y)
CFLAGS_REMOVE_net-trace.o = -pg
obj-$(CONFIG_TRACEPROBES) += net-trace.o
Mathieu
--
Mathieu Desnoyers
OpenPGP key fingerprint: 8CD5 52C3 8E3C 4140 715F BA06 3F25 A8FE 3BAE 9A68
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