From: "Guilherme G. Piccoli" <gpiccoli@xxxxxxxxxxxxx>
Subject: 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.
Link: http://lkml.kernel.org/r/20200327223646.20779-1-gpiccoli@xxxxxxxxxxxxx
Signed-off-by: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxxxxx>
Reviewed-by: Kees Cook <keescook@xxxxxxxxxxxx>
Cc: Tetsuo Handa <penguin-kernel@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
Documentation/admin-guide/sysctl/kernel.rst | 14 ++++++++++++
include/linux/sched/sysctl.h | 7 ++++++
kernel/hung_task.c | 20 ++++++++++++++++--
kernel/sysctl.c | 11 +++++++++
4 files changed, 50 insertions(+), 2 deletions(-)
--- a/Documentation/admin-guide/sysctl/kernel.rst~kernel-hung_taskc-introduce-sysctl-to-print-all-traces-when-a-hung-task-is-detected
+++ a/Documentation/admin-guide/sysctl/kernel.rst
@@ -335,6 +335,20 @@ Path for the hotplug policy agent.
Default value is "``/sbin/hotplug``".
+hung_task_all_cpu_backtrace:
+================
+
+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
+CONFIG_DETECT_HUNG_TASK and CONFIG_SMP are enabled.
+
+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.
+
+
hung_task_panic
===============
--- a/include/linux/sched/sysctl.h~kernel-hung_taskc-introduce-sysctl-to-print-all-traces-when-a-hung-task-is-detected
+++ a/include/linux/sched/sysctl.h
@@ -7,6 +7,13 @@
struct ctl_table;
#ifdef CONFIG_DETECT_HUNG_TASK
+
+#ifdef CONFIG_SMP
+extern unsigned int sysctl_hung_task_all_cpu_backtrace;
+#else
+#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;
--- a/kernel/hung_task.c~kernel-hung_taskc-introduce-sysctl-to-print-all-traces-when-a-hung-task-is-detected
+++ a/kernel/hung_task.c
@@ -53,9 +53,18 @@ int __read_mostly sysctl_hung_task_warni
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:
@@ -127,6 +136,9 @@ static void check_hung_task(struct task_
" disables this message.\n");
sched_show_task(t);
hung_task_show_lock = true;
+
+ if (sysctl_hung_task_all_cpu_backtrace)
+ hung_task_show_all_bt = true;
}
touch_nmi_watchdog();
@@ -191,10 +203,14 @@ static void check_hung_uninterruptible_t
rcu_read_unlock();
if (hung_task_show_lock)
debug_show_all_locks();
- if (hung_task_call_panic) {
+
+ if (hung_task_show_all_bt) {
+ hung_task_show_all_bt = false;
trigger_all_cpu_backtrace();
- panic("hung_task: blocked tasks");
}
+
+ if (hung_task_call_panic)
+ panic("hung_task: blocked tasks");
}
static long hung_timeout_jiffies(unsigned long last_checked,
--- a/kernel/sysctl.c~kernel-hung_taskc-introduce-sysctl-to-print-all-traces-when-a-hung-task-is-detected
+++ a/kernel/sysctl.c
@@ -2437,6 +2437,17 @@ static struct ctl_table kern_table[] = {
},
#endif
#ifdef CONFIG_DETECT_HUNG_TASK
+#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,
_
