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Message-Id: <20200310155650.17968-1-gpiccoli@canonical.com>
Date: Tue, 10 Mar 2020 12:56:50 -0300
From: "Guilherme G. Piccoli" <gpiccoli@...onical.com>
To: linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Cc: linux-doc@...r.kernel.org, mcgrof@...nel.org,
keescook@...omium.org, yzaikin@...gle.com, tglx@...utronix.de,
gpiccoli@...onical.com, kernel@...ccoli.net,
Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
Subject: [PATCH] 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/kernel parameter 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 <penguin-kernel@...ove.SAKURA.ne.jp>
Signed-off-by: Guilherme G. Piccoli <gpiccoli@...onical.com>
---
.../admin-guide/kernel-parameters.txt | 6 ++++
Documentation/admin-guide/sysctl/kernel.rst | 15 ++++++++++
include/linux/sched/sysctl.h | 7 +++++
kernel/hung_task.c | 30 +++++++++++++++++--
kernel/sysctl.c | 11 +++++++
5 files changed, 67 insertions(+), 2 deletions(-)
diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index adf77ead02c3..4c6595b5f6c8 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -1453,6 +1453,12 @@
x86-64 are 2M (when the CPU supports "pse") and 1G
(when the CPU supports the "pdpe1gb" cpuinfo flag).
+ hung_task_all_cpu_backtrace=
+ [KNL] Should kernel generates backtraces on all cpus
+ when a hung task is detected. Defaults to 0 and can
+ be controlled by hung_task_all_cpu_backtrace sysctl.
+ Format: <integer>
+
hung_task_panic=
[KNL] Should the hung task detector generate panics.
Format: <integer>
diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
index 95b2f3256323..218c717c1354 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
@@ -339,6 +340,20 @@ Path for the hotplug policy agent.
Default value is "/sbin/hotplug".
+hung_task_all_cpu_backtrace:
+================
+
+Determines if kernel should NMI 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 NMI all CPUs and dump their backtraces when a hung task
+is detected.
+
+
hung_task_panic:
================
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_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;
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 14a625c16cb3..54152b26117e 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -53,9 +53,28 @@ 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_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 either via cmdline or sysctl.
+ */
+unsigned int __read_mostly sysctl_hung_task_all_cpu_backtrace;
+
+static int __init hung_task_backtrace_setup(char *str)
+{
+ int rc = kstrtouint(str, 0, &sysctl_hung_task_all_cpu_backtrace);
+
+ if (rc)
+ return rc;
+ return 1;
+}
+__setup("hung_task_all_cpu_backtrace=", hung_task_backtrace_setup);
+#endif /* CONFIG_SMP */
+
/*
* Should we panic (and reboot, if panic_timeout= is set) when a
* hung task is detected:
@@ -137,6 +156,9 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
" disables this message.\n");
sched_show_task(t);
hung_task_show_lock = true;
+
+ if (sysctl_hung_task_all_cpu_backtrace)
+ hung_task_show_bt = true;
}
touch_nmi_watchdog();
@@ -201,10 +223,14 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
rcu_read_unlock();
if (hung_task_show_lock)
debug_show_all_locks();
- if (hung_task_call_panic) {
+
+ if (hung_task_show_bt) {
+ hung_task_show_bt = false;
trigger_all_cpu_backtrace();
+ }
+
+ 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[] = {
},
#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,
--
2.25.1
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