linux-kernel - Re: [PATCH] hung_task: add warning counter to blocked task report

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Message-ID: <83ac6ac0-a7c5-4475-8800-0beefa117164@linux.dev>
Date: Mon, 21 Jul 2025 14:19:10 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Ye Liu <ye.liu@...ux.dev>
Cc: Ye Liu <liuye@...inos.cn>, linux-kernel@...r.kernel.org,
 Andrew Morton <akpm@...ux-foundation.org>, Zi Li <zi.li@...ux.dev>
Subject: Re: [PATCH] hung_task: add warning counter to blocked task report



On 2025/7/21 13:45, Ye Liu wrote:
> 
> 
> On 2025/7/21 12:56, Lance Yang wrote:
>> Hi Ye,
>>
>> Thanks for your patch!
>>
>> On 2025/7/21 11:17, Ye Liu wrote:
>>> From: Ye Liu <liuye@...inos.cn>
>>>
>>> Add a warning counter to each hung task message to make it easier
>>> to analyze and locate issues in the logs.
>>>
>>> Signed-off-by: Ye Liu <liuye@...inos.cn>
>>> ---
>>>    kernel/hung_task.c | 6 ++++--
>>>    1 file changed, 4 insertions(+), 2 deletions(-)
>>>
>>> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
>>> index 8708a1205f82..9e5f86148d47 100644
>>> --- a/kernel/hung_task.c
>>> +++ b/kernel/hung_task.c
>>> @@ -58,6 +58,7 @@ EXPORT_SYMBOL_GPL(sysctl_hung_task_timeout_secs);
>>>    static unsigned long __read_mostly sysctl_hung_task_check_interval_secs;
>>>      static int __read_mostly sysctl_hung_task_warnings = 10;
>>> +static int hung_task_warning_count;
>>>      static int __read_mostly did_panic;
>>>    static bool hung_task_show_lock;
>>> @@ -232,8 +233,9 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
>>>        if (sysctl_hung_task_warnings || hung_task_call_panic) {
>>>            if (sysctl_hung_task_warnings > 0)
>>>                sysctl_hung_task_warnings--;
>>> -        pr_err("INFO: task %s:%d blocked for more than %ld seconds.\n",
>>> -               t->comm, t->pid, (jiffies - t->last_switch_time) / HZ);
>>> +        pr_err("INFO: task %s:%d blocked for more than %ld seconds. [Warning #%d]\n",
>>> +               t->comm, t->pid, (jiffies - t->last_switch_time) / HZ,
>>> +               ++hung_task_warning_count);
>>>            pr_err("      %s %s %.*s\n",
>>>                print_tainted(), init_utsname()->release,
>>>                (int)strcspn(init_utsname()->version, " "),
>>
>> A quick thought on this: we already have the hung_task_detect_count
>> counter, which tracks the total number of hung tasks detected since
>> boot ;)
>>
>> While this patch adds a counter inline with the warning message, the
>> existing counter already provides a way to know how many hung task
>> events have occurred.
>>
>> Could you elaborate on the specific benefit of printing this count
>> directly in the log, compared to checking the global hung_task_detect_count?
>>
>> Also, if the goal is to give each warning a unique sequence number,
>> I think the dmesg timestamp prefix serves the same purpose ;)
>>
>> Thanks,
>> Lance
> 
> Sorry for not noticing sysctl_hung_task_detect_count.
> I just thought adding it directly to the warning message would make the
> log easier to read and more intuitive than relying on timestamps.
> 
> If accepted, I will send V2, like this:

Let's step back and considet the practical use case. when we are
troubleshooting hung task issues in a production log, what information
do we actually use?

Typically, we look for:
1) The timestamp, to correlate with other system events
2) The task name and PID (%s:%d)
3) The kernel stack trace that follows, to see where it's stuck

So, my question is: in what specific troubleshooting scenario would
knowing the sequence number, like [#N], provide actionable information
that the above data points do not?

Unless there's a compelling use case I'm missing, I'd prefer to keep
the code as it is ;)
Thanks,
Lance

> 
> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
> index 8708a1205f82..231afdb68bb2 100644
> --- a/kernel/hung_task.c
> +++ b/kernel/hung_task.c
> @@ -232,8 +232,9 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
>          if (sysctl_hung_task_warnings || hung_task_call_panic) {
>                  if (sysctl_hung_task_warnings > 0)
>                          sysctl_hung_task_warnings--;
> -               pr_err("INFO: task %s:%d blocked for more than %ld seconds.\n",
> -                      t->comm, t->pid, (jiffies - t->last_switch_time) / HZ);
> +               pr_err("INFO: task %s:%d blocked for more than %ld seconds. [#%ld]\n",
> +                      t->comm, t->pid, (jiffies - t->last_switch_time) / HZ,
> +                      sysctl_hung_task_detect_count);
>                  pr_err("      %s %s %.*s\n",
>                          print_tainted(), init_utsname()->release,
>                          (int)strcspn(init_utsname()->version, " "),
> 
> 
> 
>