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Message-ID: <d98c6c9f-b50d-4818-848f-326f6ab01439@linux.dev>
Date: Fri, 9 May 2025 12:44:14 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Feng Tang <feng.tang@...ux.alibaba.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>, Petr Mladek
<pmladek@...e.com>, Steven Rostedt <rostedt@...dmis.org>,
linux-kernel@...r.kernel.org, llong@...hat.com, mhiramat@...nel.org,
amaindex@...look.com
Subject: Re: [PATCH RFC 2/3] kernel/hung_task: add option to dump system info
when hung task detected
On 2025/5/8 13:45, Feng Tang wrote:
> Hi Lance,
>
> Many thanks for the review!
>
> On Thu, May 08, 2025 at 11:02:22AM +0800, Lance Yang wrote:
>> Hi Feng,
>>
>> Thanks for the patch series!
>>
>> On 2025/5/7 18:43, Feng Tang wrote:
>>> Kernel panic code utilizes sys_show_info() to dump needed system
>>> information to help debugging. Similarly, add this debug option for
>>> task hung case, and 'hungtask_print' is the knob to control what
>>> information should be printed out.
>>>
>>> Also clean up the code about dumping locks and triggering backtrace
>>> for all CPUs. One todo may be to merge this 'hungtask_print' with
>>> some sysctl knobs in hung_task.c.
>>>
>>> Signed-off-by: Feng Tang <feng.tang@...ux.alibaba.com>
>>> ---
>>> kernel/hung_task.c | 29 ++++++++++++++++-------------
>>> 1 file changed, 16 insertions(+), 13 deletions(-)
>>>
>>> diff --git a/kernel/hung_task.c b/kernel/hung_task.c
>>> index dc898ec93463..8229637be2c7 100644
>>> --- a/kernel/hung_task.c
>>> +++ b/kernel/hung_task.c
>>> @@ -58,12 +58,20 @@ static unsigned long __read_mostly sysctl_hung_task_check_interval_secs;
>>> static 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_all_bt;
>>> static struct task_struct *watchdog_task;
>>> +/*
>>> + * A bitmask to control what kinds of system info to be printed when a
>>> + * hung task is detected, it could be task, memory, lock etc. Refer panic.h
>>> + * for details of bit definition.
>>> + */
>>> +unsigned long hungtask_print;
>>> +core_param(hungtask_print, hungtask_print, ulong, 0644);
>>
>> how about lockup_debug_print_mask?
Oops, typo: hungtask_* (not lockup_*)
>
> The 3/3 patch has a 'lockup_print' as it is for soft/hard lockup :).
> The name follows the existing 'panic_print', and indeed it's actually
> a bitmask, how about 'hung_print_mask'?
Yep, we should be following ’panic_print‘ pattern like 'hungtask_print',
but I‘d rather go with 'hungtask_print_mask' ;)
>
>>
>> It could be useful for debugging, but there are a few concerns:
>>
>> 1) SYS_PRINT_* vs. hung_task_* priority conflict
>> - If SYS_PRINT_ALL_CPU_BT is set on the command line but
>> hung_task_all_cpu_backtrace is disabled, which one wins?
>> - Or should SYS_PRINT_ALL_CPU_BT force-enable hung_task_all_cpu_backtrace?
>
> With this patch, the 'hungtask_print' and hung_task_all_cpu_backtrace
> will be ORed, so yes, if user sets SYS_PRINT_ALL_CPU_BT explicitly, the
> all-cpu-backtrace will be printed.
>
> While the default value for hungtask_print is 0, and no system info will
> be dumped by default.
>
> Long term wise, I'm not sure if sysctl_hung_task_all_cpu_backtracemay
> could be removed as its function can be covered by this print_mask knob.
Afraid we cannot remove that knob — it would break user-space. Note that
hungtask_print_mask should act as an extension (to provide more details
when investigating hangs), and it must still follow hung-task detector's
rules, IIUC.
Hmm... SYS_PRINT_ALL_CPU_BT is a bit tricky here. Maybe we can directly
enable hung_task_all_cpu_backtrace when SYS_PRINT_ALL_CPU_BT is set in
hungtask_print_mask, while still allowing manual disabling to dump
backtraces for all CPUs via hung_task_all_cpu_backtrace. This way, we
keep its original semantics ;)
>
>> 2) Duplicate prints
>> With SYS_PRINT_BLOCKED_TASKS enabled, processes in D state will be printed
>> twice, right?
>
> Good point. As sys_show_info() is a general API helper, the user may chose
> not to set SYS_PRINT_BLOCKED_TASKS when debugging task hung.
>
> In one recent bug we debugged with this patch, when the first "task hung" was
> shown, there were already dozens of tasks were in D state, which just hadn't
> hit the 120 seconds limit yet, and dumping them all helped in that case.
Makes sense to me. Right, SYS_PRINT_BLOCKED_TASKS doesn’t duplicate
prints, and
catches all D-state tasks - even the ones not yet timed out.
>
>> Also, we really should document how those command-line parameters work ;)
>
> Exactly! It currently just said 'refer panic.h' in code comment, maybe I
> should copy those definitions here as comments. How do you think?
Yes. Both comments and kernel-doc are needed ;)
Thanks,
Lance
>
> Thanks,
> Feng
>
>> Thansk,
>> Lance
>>
>>> +
>>> +static unsigned long cur_hungtask_print;
>>> +
>>> #ifdef CONFIG_SMP
>>> /*
>>> * Should we dump all CPUs backtraces in a hung task event?
>>> @@ -163,11 +171,12 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
>>> */
>>> sysctl_hung_task_detect_count++;
>>> + cur_hungtask_print = hungtask_print;
>>> trace_sched_process_hang(t);
>>> if (sysctl_hung_task_panic) {
>>> console_verbose();
>>> - hung_task_show_lock = true;
>>> + cur_hungtask_print |= SYS_PRINT_LOCK_INFO;
>>> hung_task_call_panic = true;
>>> }
>>> @@ -190,10 +199,10 @@ static void check_hung_task(struct task_struct *t, unsigned long timeout)
>>> " disables this message.\n");
>>> sched_show_task(t);
>>> debug_show_blocker(t);
>>> - hung_task_show_lock = true;
>>> + cur_hungtask_print |= SYS_PRINT_LOCK_INFO;
>>> if (sysctl_hung_task_all_cpu_backtrace)
>>> - hung_task_show_all_bt = true;
>>> + cur_hungtask_print |= SYS_PRINT_ALL_CPU_BT;
>>> if (!sysctl_hung_task_warnings)
>>> pr_info("Future hung task reports are suppressed, see sysctl kernel.hung_task_warnings\n");
>>> }
>>> @@ -242,7 +251,7 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
>>> if (test_taint(TAINT_DIE) || did_panic)
>>> return;
>>> - hung_task_show_lock = false;
>>> + cur_hungtask_print = 0;
>>> rcu_read_lock();
>>> for_each_process_thread(g, t) {
>>> unsigned int state;
>>> @@ -266,14 +275,8 @@ static void check_hung_uninterruptible_tasks(unsigned long timeout)
>>> }
>>> unlock:
>>> rcu_read_unlock();
>>> - if (hung_task_show_lock)
>>> - debug_show_all_locks();
>>> -
>>> - if (hung_task_show_all_bt) {
>>> - hung_task_show_all_bt = false;
>>> - trigger_all_cpu_backtrace();
>>> - }
>>> + sys_show_info(cur_hungtask_print);
>>> if (hung_task_call_panic)
>>> panic("hung_task: blocked tasks");
>>> }
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