[PATCH][v3] hung_task: Panic after fixed number of hung tasks
Lance Yang
lance.yang at linux.dev
Tue Oct 14 16:23:58 AEDT 2025
Thanks for the patch!
I noticed the implementation panics only when N tasks are detected
within a single scan, because total_hung_task is reset for each
check_hung_uninterruptible_tasks() run.
So some suggestions to align the documentation with the code's
behavior below :)
On 2025/10/12 19:50, lirongqing wrote:
> From: Li RongQing <lirongqing at baidu.com>
>
> Currently, when 'hung_task_panic' is enabled, the kernel panics
> immediately upon detecting the first hung task. However, some hung
> tasks are transient and the system can recover, while others are
> persistent and may accumulate progressively.
>
> This patch extends the 'hung_task_panic' sysctl to allow specifying
> the number of hung tasks that must be detected before triggering
> a kernel panic. This provides finer control for environments where
> transient hangs may occur but persistent hangs should still be fatal.
>
> The sysctl can be set to:
> - 0: disabled (never panic)
> - 1: original behavior (panic on first hung task)
> - N: panic when N hung tasks are detected
>
> This maintains backward compatibility while providing more flexibility
> for handling different hang scenarios.
>
> Signed-off-by: Li RongQing <lirongqing at baidu.com>
> ---
> Diff with v2: not add new sysctl, extend hung_task_panic
>
> Documentation/admin-guide/kernel-parameters.txt | 20 +++++++++++++-------
> Documentation/admin-guide/sysctl/kernel.rst | 3 ++-
> arch/arm/configs/aspeed_g5_defconfig | 2 +-
> kernel/configs/debug.config | 2 +-
> kernel/hung_task.c | 16 +++++++++++-----
> lib/Kconfig.debug | 10 ++++++----
> tools/testing/selftests/wireguard/qemu/kernel.config | 2 +-
> 7 files changed, 35 insertions(+), 20 deletions(-)
>
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index a51ab46..7d9a8ee 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -1992,14 +1992,20 @@
> the added memory block itself do not be affected.
>
> hung_task_panic=
> - [KNL] Should the hung task detector generate panics.
> - Format: 0 | 1
> + [KNL] Number of hung tasks to trigger kernel panic.
> + Format: <int>
> +
> + Set this to the number of hung tasks that must be
> + detected before triggering a kernel panic.
> +
> + 0: don't panic
> + 1: panic immediately on first hung task
> + N: panic after N hung tasks are detect
The description should be more specific :)
N: panic after N hung tasks are detected in a single scan
Would it be better and cleaner?
>
> - A value of 1 instructs the kernel to panic when a
> - hung task is detected. The default value is controlled
> - by the CONFIG_BOOTPARAM_HUNG_TASK_PANIC build-time
> - option. The value selected by this boot parameter can
> - be changed later by the kernel.hung_task_panic sysctl.
> + The default value is controlled by the
> + CONFIG_BOOTPARAM_HUNG_TASK_PANIC build-time option. The value
> + selected by this boot parameter can be changed later by the
> + kernel.hung_task_panic sysctl.
>
> hvc_iucv= [S390] Number of z/VM IUCV hypervisor console (HVC)
> terminal devices. Valid values: 0..8
> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index f3ee807..0a8dfab 100644
> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -397,7 +397,8 @@ a hung task is detected.
> hung_task_panic
> ===============
>
> -Controls the kernel's behavior when a hung task is detected.
> +When set to a non-zero value, a kernel panic will be triggered if the
> +number of detected hung tasks reaches this value
Hmm... that is also ambiguous ...
+When set to a non-zero value, a kernel panic will be triggered if the
+number of hung tasks found during a single scan reaches this value.
> This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
>
> = =================================================
> diff --git a/arch/arm/configs/aspeed_g5_defconfig b/arch/arm/configs/aspeed_g5_defconfig
> index 61cee1e..c3b0d5f 100644
> --- a/arch/arm/configs/aspeed_g5_defconfig
> +++ b/arch/arm/configs/aspeed_g5_defconfig
> @@ -308,7 +308,7 @@ CONFIG_PANIC_ON_OOPS=y
> CONFIG_PANIC_TIMEOUT=-1
> CONFIG_SOFTLOCKUP_DETECTOR=y
> CONFIG_BOOTPARAM_SOFTLOCKUP_PANIC=y
> -CONFIG_BOOTPARAM_HUNG_TASK_PANIC=y
> +CONFIG_BOOTPARAM_HUNG_TASK_PANIC=1
> CONFIG_WQ_WATCHDOG=y
> # CONFIG_SCHED_DEBUG is not set
> CONFIG_FUNCTION_TRACER=y
> diff --git a/kernel/configs/debug.config b/kernel/configs/debug.config
> index e81327d..9f6ab7d 100644
> --- a/kernel/configs/debug.config
> +++ b/kernel/configs/debug.config
> @@ -83,7 +83,7 @@ CONFIG_SLUB_DEBUG_ON=y
> #
> # Debug Oops, Lockups and Hangs
> #
> -# CONFIG_BOOTPARAM_HUNG_TASK_PANIC is not set
> +CONFIG_BOOTPARAM_HUNG_TASK_PANIC=0
> # CONFIG_BOOTPARAM_SOFTLOCKUP_PANIC is not set
> CONFIG_DEBUG_ATOMIC_SLEEP=y
> CONFIG_DETECT_HUNG_TASK=y
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index b2c1f14..3929ed9 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -81,7 +81,7 @@ static unsigned int __read_mostly sysctl_hung_task_all_cpu_backtrace;
> * hung task is detected:
> */
> static unsigned int __read_mostly sysctl_hung_task_panic =
> - IS_ENABLED(CONFIG_BOOTPARAM_HUNG_TASK_PANIC);
> + CONFIG_BOOTPARAM_HUNG_TASK_PANIC;
>
> static int
> hung_task_panic(struct notifier_block *this, unsigned long event, void *ptr)
> @@ -218,8 +218,11 @@ static inline void debug_show_blocker(struct task_struct *task, unsigned long ti
> }
> #endif
>
> -static void check_hung_task(struct task_struct *t, unsigned long timeout)
> +static void check_hung_task(struct task_struct *t, unsigned long timeout,
> + unsigned long prev_detect_count)
> {
> + unsigned long total_hung_task;
> +
> if (!task_is_hung(t, timeout))
> return;
>
> @@ -229,9 +232,11 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
> */
> sysctl_hung_task_detect_count++;
>
> + total_hung_task = sysctl_hung_task_detect_count - prev_detect_count;
> trace_sched_process_hang(t);
>
> - if (sysctl_hung_task_panic) {
> + if (sysctl_hung_task_panic &&
> + (total_hung_task >= sysctl_hung_task_panic)) {
> console_verbose();
> hung_task_show_lock = true;
> hung_task_call_panic = true;
> @@ -300,6 +305,7 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
> int max_count = sysctl_hung_task_check_count;
> unsigned long last_break = jiffies;
> struct task_struct *g, *t;
> + unsigned long prev_detect_count = sysctl_hung_task_detect_count;
>
> /*
> * If the system crashed already then all bets are off,
> @@ -320,7 +326,7 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
> last_break = jiffies;
> }
>
> - check_hung_task(t, timeout);
> + check_hung_task(t, timeout, prev_detect_count);
> }
> unlock:
> rcu_read_unlock();
> @@ -389,7 +395,7 @@ static const struct ctl_table hung_task_sysctls[] = {
> .mode = 0644,
> .proc_handler = proc_dointvec_minmax,
> .extra1 = SYSCTL_ZERO,
> - .extra2 = SYSCTL_ONE,
> + .extra2 = SYSCTL_INT_MAX,
> },
> {
> .procname = "hung_task_check_count",
> diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug
> index 3034e294..077b9e4 100644
> --- a/lib/Kconfig.debug
> +++ b/lib/Kconfig.debug
> @@ -1258,12 +1258,14 @@ config DEFAULT_HUNG_TASK_TIMEOUT
> Keeping the default should be fine in most cases.
>
> config BOOTPARAM_HUNG_TASK_PANIC
> - bool "Panic (Reboot) On Hung Tasks"
> + int "Number of hung tasks to trigger kernel panic"
> depends on DETECT_HUNG_TASK
> + default 0
> help
> - Say Y here to enable the kernel to panic on "hung tasks",
> - which are bugs that cause the kernel to leave a task stuck
> - in uninterruptible "D" state.
> + The number of hung tasks must be detected to trigger kernel panic.
> +
> + - 0: Don't trigger panic
> + - N: Panic when N hung tasks are detected
+ - N: Panic when N hung tasks are detected in a single scan
With these documentation changes, this patch would accurately describe
its behavior, IMHO.
>
> The panic can be used in combination with panic_timeout,
> to cause the system to reboot automatically after a
> diff --git a/tools/testing/selftests/wireguard/qemu/kernel.config b/tools/testing/selftests/wireguard/qemu/kernel.config
> index 936b18b..0504c11 100644
> --- a/tools/testing/selftests/wireguard/qemu/kernel.config
> +++ b/tools/testing/selftests/wireguard/qemu/kernel.config
> @@ -81,7 +81,7 @@ CONFIG_WQ_WATCHDOG=y
> CONFIG_DETECT_HUNG_TASK=y
> CONFIG_BOOTPARAM_HARDLOCKUP_PANIC=y
> CONFIG_BOOTPARAM_SOFTLOCKUP_PANIC=y
> -CONFIG_BOOTPARAM_HUNG_TASK_PANIC=y
> +CONFIG_BOOTPARAM_HUNG_TASK_PANIC=1
> CONFIG_PANIC_TIMEOUT=-1
> CONFIG_STACKTRACE=y
> CONFIG_EARLY_PRINTK=y
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