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Subject[PATCH V3] kernel/hung_task.c: Introduce sysctl to print all traces when a hung task is detected
Commit 401c636a0eeb ("kernel/hung_task.c: show all hung tasks before panic")
introduced a change in that we started to show all CPUs backtraces when a
hung task is detected _and_ the sysctl/kernel parameter "hung_task_panic"
is set. The idea is good, because usually when observing deadlocks (that
may lead to hung tasks), the culprit is another task holding a lock and
not necessarily the task detected as hung.

The problem with this approach is that dumping backtraces is a slightly
expensive task, specially printing that on console (and specially in many
CPU machines, as servers commonly found nowadays). So, users that plan to
collect a kdump to investigate the hung tasks and narrow down the deadlock
definitely don't need the CPUs backtrace on dmesg/console, which will delay
the panic and pollute the log (crash tool would easily grab all CPUs traces
with 'bt -a' command).
Also, there's the reciprocal scenario: some users may be interested in
seeing the CPUs backtraces but not have the system panic when a hung task
is detected. The current approach hence is almost as embedding a policy in
the kernel, by forcing the CPUs backtraces' dump (only) on hung_task_panic.

This patch decouples the panic event on hung task from the CPUs
backtraces dump, by creating (and documenting) a new sysctl called
"hung_task_all_cpu_backtrace", analog to the approach taken on soft/hard
lockups, that have both a panic and an "all_cpu_backtrace" sysctl to allow
individual control. The new mechanism for dumping the CPUs backtraces on
hung task detection respects "hung_task_warnings" by not dumping the
traces in case there's no warnings left.

Cc: Tetsuo Handa <>
Reviewed-by: Kees Cook <>
Signed-off-by: Guilherme G. Piccoli <>

V3: Following the suggestion from Vlastimil, removed the kernel parameter
since soon we (hopefully) will have a generic approach to set sysctls via
kernel parameters[0] - thanks Vlastimil, great idea!




Documentation/admin-guide/sysctl/kernel.rst | 15 +++++++++++++++
include/linux/sched/sysctl.h | 7 +++++++
kernel/hung_task.c | 20 ++++++++++++++++++--
kernel/sysctl.c | 11 +++++++++++
4 files changed, 51 insertions(+), 2 deletions(-)

diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index def074807cee..8b4ff69d2348 100644
--- a/Documentation/admin-guide/sysctl/kernel.rst
+++ b/Documentation/admin-guide/sysctl/kernel.rst
@@ -40,6 +40,7 @@ show up in /proc/sys/kernel:
- hotplug
- hardlockup_all_cpu_backtrace
- hardlockup_panic
+- hung_task_all_cpu_backtrace
- hung_task_panic
- hung_task_check_count
- hung_task_timeout_secs
@@ -338,6 +339,20 @@ Path for the hotplug policy agent.
Default value is "/sbin/hotplug".

+If this option is set, the kernel will send an NMI to all CPUs to dump
+their backtraces when a hung task is detected. This file shows up if
+0: Won't show all CPUs backtraces when a hung task is detected.
+This is the default behavior.
+1: Will non-maskably interrupt all CPUs and dump their backtraces when
+a hung task is detected.

diff --git a/include/linux/sched/sysctl.h b/include/linux/sched/sysctl.h
index d4f6215ee03f..8cd29440ec8a 100644
--- a/include/linux/sched/sysctl.h
+++ b/include/linux/sched/sysctl.h
@@ -7,6 +7,13 @@
struct ctl_table;

+#ifdef CONFIG_SMP
+extern unsigned int sysctl_hung_task_all_cpu_backtrace;
+#define sysctl_hung_task_all_cpu_backtrace 0
+#endif /* CONFIG_SMP */
extern int sysctl_hung_task_check_count;
extern unsigned int sysctl_hung_task_panic;
extern unsigned long sysctl_hung_task_timeout_secs;
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 14a625c16cb3..9a774aee1a44 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -53,9 +53,18 @@ int __read_mostly sysctl_hung_task_warnings = 10;
static int __read_mostly did_panic;
static bool hung_task_show_lock;
static bool hung_task_call_panic;
+static bool hung_task_show_all_bt;

static struct task_struct *watchdog_task;

+#ifdef CONFIG_SMP
+ * Should we dump all CPUs backtraces in a hung task event?
+ * Defaults to 0, can be changed via sysctl.
+ */
+unsigned int __read_mostly sysctl_hung_task_all_cpu_backtrace;
+#endif /* CONFIG_SMP */
* Should we panic (and reboot, if panic_timeout= is set) when a
* hung task is detected:
@@ -137,6 +146,9 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
" disables this message.\n");
hung_task_show_lock = true;
+ if (sysctl_hung_task_all_cpu_backtrace)
+ hung_task_show_all_bt = true;

@@ -201,10 +213,14 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
if (hung_task_show_lock)
- if (hung_task_call_panic) {
+ if (hung_task_show_all_bt) {
+ hung_task_show_all_bt = false;
+ }
+ if (hung_task_call_panic)
panic("hung_task: blocked tasks");
- }

static long hung_timeout_jiffies(unsigned long last_checked,
diff --git a/kernel/sysctl.c b/kernel/sysctl.c
index ad5b88a53c5a..238f268de486 100644
--- a/kernel/sysctl.c
+++ b/kernel/sysctl.c
@@ -1098,6 +1098,17 @@ static struct ctl_table kern_table[] = {
+#ifdef CONFIG_SMP
+ {
+ .procname = "hung_task_all_cpu_backtrace",
+ .data = &sysctl_hung_task_all_cpu_backtrace,
+ .maxlen = sizeof(int),
+ .mode = 0644,
+ .proc_handler = proc_dointvec_minmax,
+ .extra1 = SYSCTL_ZERO,
+ .extra2 = SYSCTL_ONE,
+ },
+#endif /* CONFIG_SMP */
.procname = "hung_task_panic",
.data = &sysctl_hung_task_panic,
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