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Message-ID: <2toaalyem4v3rmr5d5qt5lrnahr7ecbizzuue5oexrjmlo5ix6@yiw5k3cpeamf>
Date: Tue, 6 Jan 2026 12:51:14 +0100
From: Joel Granados <joel.granados@...nel.org>
To: Aaron Tomlin <atomlin@...mlin.com>
Cc: akpm@...ux-foundation.org, lance.yang@...ux.dev, mhiramat@...nel.org,
gregkh@...uxfoundation.org, pmladek@...e.com, sean@...e.io, linux-kernel@...r.kernel.org
Subject: Re: [v5 PATCH 2/2] hung_task: Enable runtime reset of
hung_task_detect_count
On Tue, Dec 30, 2025 at 07:41:25PM -0500, Aaron Tomlin wrote:
> Introduce support for writing to /proc/sys/kernel/hung_task_detect_count.
>
> Writing a value of zero to this file atomically resets the counter of
> detected hung tasks. This grants system administrators the ability to
> clear the cumulative diagnostic history after resolving an incident,
> simplifying monitoring without requiring a system restart.
>
> Signed-off-by: Aaron Tomlin <atomlin@...mlin.com>
> ---
> Documentation/admin-guide/sysctl/kernel.rst | 3 +-
> kernel/hung_task.c | 82 ++++++++++++++++++---
> 2 files changed, 73 insertions(+), 12 deletions(-)
>
> diff --git a/Documentation/admin-guide/sysctl/kernel.rst b/Documentation/admin-guide/sysctl/kernel.rst
> index 239da22c4e28..68da4235225a 100644
> --- a/Documentation/admin-guide/sysctl/kernel.rst
> +++ b/Documentation/admin-guide/sysctl/kernel.rst
> @@ -418,7 +418,8 @@ hung_task_detect_count
> ======================
>
> Indicates the total number of tasks that have been detected as hung since
> -the system boot.
> +the system boot or since the counter was reset. The counter is zeroed when
> +a value of 0 is written.
>
> This file shows up if ``CONFIG_DETECT_HUNG_TASK`` is enabled.
>
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index 00c3296fd692..3bc72a4e4032 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -17,6 +17,7 @@
> #include <linux/export.h>
> #include <linux/panic_notifier.h>
> #include <linux/sysctl.h>
> +#include <linux/atomic.h>
> #include <linux/suspend.h>
> #include <linux/utsname.h>
> #include <linux/sched/signal.h>
> @@ -36,7 +37,7 @@ static int __read_mostly sysctl_hung_task_check_count = PID_MAX_LIMIT;
> /*
> * Total number of tasks detected as hung since boot:
> */
> -static unsigned long __read_mostly sysctl_hung_task_detect_count;
> +static atomic_long_t sysctl_hung_task_detect_count = ATOMIC_LONG_INIT(0);
>
> /*
> * Limit number of tasks checked in a batch.
> @@ -246,20 +247,26 @@ static inline void hung_task_diagnostics(struct task_struct *t)
> }
>
> static void check_hung_task(struct task_struct *t, unsigned long timeout,
> - unsigned long prev_detect_count)
> + unsigned long prev_detect_count)
> {
> - unsigned long total_hung_task;
> + unsigned long total_hung_task, cur_detect_count;
>
> if (!task_is_hung(t, timeout))
> return;
>
> /*
> * This counter tracks the total number of tasks detected as hung
> - * since boot.
> + * since boot. If a reset occurred during the scan, we treat the
> + * current count as the new delta to avoid an underflow error.
> + * Ensure hang details are globally visible before the counter
> + * update.
> */
> - sysctl_hung_task_detect_count++;
> + cur_detect_count = atomic_long_inc_return_release(&sysctl_hung_task_detect_count);
> + if (cur_detect_count >= prev_detect_count)
> + total_hung_task = cur_detect_count - prev_detect_count;
> + else
> + total_hung_task = cur_detect_count;
>
> - total_hung_task = sysctl_hung_task_detect_count - prev_detect_count;
> trace_sched_process_hang(t);
>
> if (sysctl_hung_task_panic && total_hung_task >= sysctl_hung_task_panic) {
> @@ -318,10 +325,12 @@ 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;
> + unsigned long cur_detect_count, prev_detect_count, delta;
> int need_warning = sysctl_hung_task_warnings;
> unsigned long si_mask = hung_task_si_mask;
>
> + /* Acquire prevents reordering task checks before this point. */
> + prev_detect_count = atomic_long_read_acquire(&sysctl_hung_task_detect_count);
> /*
> * If the system crashed already then all bets are off,
> * do not report extra hung tasks:
> @@ -346,7 +355,14 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
> unlock:
> rcu_read_unlock();
>
> - if (!(sysctl_hung_task_detect_count - prev_detect_count))
> + /* Ensures we see all hang details recorded during the scan. */
> + cur_detect_count = atomic_long_read_acquire(&sysctl_hung_task_detect_count);
> + if (cur_detect_count < prev_detect_count)
> + delta = cur_detect_count;
> + else
> + delta = cur_detect_count - prev_detect_count;
> +
> + if (!delta)
> return;
>
> if (need_warning || hung_task_call_panic) {
> @@ -371,6 +387,51 @@ static long hung_timeout_jiffies(unsigned long last_checked,
> }
>
> #ifdef CONFIG_SYSCTL
> +
> +/**
> + * proc_dohung_task_detect_count - proc handler for hung_task_detect_count
> + * @table: Pointer to the struct ctl_table definition for this proc entry
> + * @dir: Flag indicating the operation
> + * @buffer: User space buffer for data transfer
> + * @lenp: Pointer to the length of the data being transferred
> + * @ppos: Pointer to the current file offset
> + *
> + * This handler is used for reading the current hung task detection count
> + * and for resetting it to zero when a write operation is performed using a
> + * zero value only. Returns 0 on success or a negative error code on
> + * failure.
> + */
> +static int proc_dohung_task_detect_count(const struct ctl_table *table, int dir,
> + void *buffer, size_t *lenp, loff_t *ppos)
> +{
> + unsigned long detect_count;
> + struct ctl_table proxy_table;
> + int err;
> +
> + proxy_table = *table;
> + proxy_table.data = &detect_count;
> +
> + if (SYSCTL_KERN_TO_USER(dir)) {
> + detect_count = atomic_long_read(&sysctl_hung_task_detect_count);
> +
> + return proc_doulongvec_minmax(&proxy_table, dir, buffer, lenp, ppos);
> + }
Could you do something like this (untested):
if (SYSCTL_KERN_TO_USER(dir))
detect_count = atomic_long_read(&sysctl_hung_task_detect_count);
err = proc_doulongvec_minmax(&proxy_table, dir, buffer, lenp, ppos);
if (err < 0)
return err;
if (SYSCTL_USER_TO_KERN(dir)) {
if (detect_count)
return -EINVAL;
atomic_long_set(&sysctl_hung_task_detect_count, 0);
}
return 0;
> +
> + err = proc_doulongvec_minmax(&proxy_table, dir, buffer, lenp, ppos);
> + if (err < 0)
> + return err;
> +
> + if (SYSCTL_USER_TO_KERN(dir)) {
> + /* The only valid value for clearing is zero. */
> + if (detect_count)
> + return -EINVAL;
> + atomic_long_set(&sysctl_hung_task_detect_count, 0);
> + }
> +
> + *ppos += *lenp;
why do you advance ppos here? It is already advanced when you call
proc_doulongvec_minmax.
> + return err;
> +}
> +
> /*
> * Process updating of timeout sysctl
> */
> @@ -451,10 +512,9 @@ static const struct ctl_table hung_task_sysctls[] = {
> },
> {
> .procname = "hung_task_detect_count",
> - .data = &sysctl_hung_task_detect_count,
> .maxlen = sizeof(unsigned long),
> - .mode = 0444,
> - .proc_handler = proc_doulongvec_minmax,
> + .mode = 0644,
> + .proc_handler = proc_dohung_task_detect_count,
> },
> {
> .procname = "hung_task_sys_info",
> --
> 2.51.0
>
--
Joel Granados
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