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Message-ID: <aYG5-GP50UtDCk7s@pathway.suse.cz>
Date: Tue, 3 Feb 2026 10:03:52 +0100
From: Petr Mladek <pmladek@...e.com>
To: Lance Yang <lance.yang@...ux.dev>
Cc: Aaron Tomlin <atomlin@...mlin.com>, neelx@...e.com, sean@...e.io,
akpm@...ux-foundation.org, mproche@...il.com, chjohnst@...il.com,
nick.lange@...il.com, linux-kernel@...r.kernel.org,
mhiramat@...nel.org, joel.granados@...nel.org,
gregkh@...uxfoundation.org
Subject: Re: [v7 PATCH 1/2] hung_task: Refactor detection logic and atomicise
detection count
On Tue 2026-02-03 11:08:33, Lance Yang wrote:
> On 2026/2/3 11:05, Lance Yang wrote:
> > On 2026/1/25 21:58, Aaron Tomlin wrote:
> > > The check_hung_task() function currently conflates two distinct
> > > responsibilities: validating whether a task is hung and handling the
> > > subsequent reporting (printing warnings, triggering panics, or
> > > tracepoints).
> > >
> > > This patch refactors the logic by introducing hung_task_info(), a
> > > function dedicated solely to reporting. The actual detection check,
> > > task_is_hung(), is hoisted into the primary loop within
> > > check_hung_uninterruptible_tasks(). This separation clearly decouples
> > > the mechanism of detection from the policy of reporting.
> > >
> > > Furthermore, to facilitate future support for concurrent hung task
> > > detection, the global sysctl_hung_task_detect_count variable is
> > > converted from unsigned long to atomic_long_t. Consequently, the
> > > counting logic is updated to accumulate the number of hung tasks locally
> > > (this_round_count) during the iteration. The global counter is then
> > > updated atomically via atomic_long_cmpxchg_relaxed() once the loop
> > > concludes, rather than incrementally during the scan.
> > >
> > > These changes are strictly preparatory and introduce no functional
> > > change to the system's runtime behaviour.
> > >
> > > Signed-off-by: Aaron Tomlin <atomlin@...mlin.com>
> > > ---
> > > kernel/hung_task.c | 58 ++++++++++++++++++++++++++--------------------
> > > 1 file changed, 33 insertions(+), 25 deletions(-)
> > >
> > > diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> > > index d2254c91450b..df10830ed9ef 100644
> > > --- a/kernel/hung_task.c
> > > +++ b/kernel/hung_task.c
> > > @@ -36,7 +36,7 @@ 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.
> > > @@ -223,31 +223,29 @@ 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,
> > > - unsigned long prev_detect_count)
> > > +/**
> > > + * hung_task_info - Print diagnostic details for a hung task
> > > + * @t: Pointer to the detected hung task.
> > > + * @timeout: Timeout threshold for detecting hung tasks
> > > + * @this_round_count: Count of hung tasks detected in the current
> > > iteration
> > > + *
> > > + * Print structured information about the specified hung task, if
> > > warnings
> > > + * are enabled or if the panic batch threshold is exceeded.
> > > + */
> > > +static void hung_task_info(struct task_struct *t, unsigned long timeout,
> > > + unsigned long this_round_count)
> > > {
> > > - unsigned long total_hung_task;
> > > -
> > > - if (!task_is_hung(t, timeout))
> > > - return;
> > > -
> > > - /*
> > > - * This counter tracks the total number of tasks detected as hung
> > > - * since boot.
> > > - */
> > > - sysctl_hung_task_detect_count++;
> >
> > Previously, the global detect count updated immediately when a hung task
> > was found. BUT now, it only updates after the full scan finishes ...
> >
> > Ideally, the count should update as soon as possible, so that userspace
> > can react in time :)
> >
> > For example, by migrating critical containers away from the node before
> > the situation gets worse - something we already do.
>
> Sorry, I should have said that earlier - just realized it ...
Better late then sorry ;-)
That said, is the delay really critical? I guess that the userspace
checks the counter in regular intervals (seconds or tens of seconds).
Or is there any way to get a notification immediately?
Anyway, I thought how the counting and barriers might work when
we update the global counter immediately. And I came up with
the following:
diff --git a/kernel/hung_task.c b/kernel/hung_task.c
index 350093de0535..8bc043fbe89c 100644
--- a/kernel/hung_task.c
+++ b/kernel/hung_task.c
@@ -302,15 +302,10 @@ 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 total_count, this_round_count;
+ unsigned long this_round_count;
int need_warning = sysctl_hung_task_warnings;
unsigned long si_mask = hung_task_si_mask;
- /*
- * The counter might get reset. Remember the initial value.
- * Acquire prevents reordering task checks before this point.
- */
- total_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:
@@ -330,6 +325,13 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
}
if (task_is_hung(t, timeout)) {
+ /*
+ * Increment the global counter so that userspace could
+ * start migrating tasks ASAP. But count the current
+ * round separately because userspace could reset
+ * the global counter at any time.
+ */
+ atomic_long_inc(&sysctl_hung_task_detect_count);
this_round_count++;
hung_task_info(t, timeout, this_round_count);
}
@@ -340,15 +342,6 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
if (!this_round_count)
return;
- /*
- * Do not count this round when the global counter has been reset
- * during this check. Release ensures we see all hang details
- * recorded during the scan.
- */
- atomic_long_cmpxchg_release(&sysctl_hung_task_detect_count,
- total_count, total_count +
- this_round_count);
-
if (need_warning || hung_task_call_panic) {
si_mask |= SYS_INFO_LOCKS;
I am not sure of the comment above the increment is needed.
Well, it might help anyone to understand the motivation without
digging in the git log history.
Best Regards,
Petr
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