Re: [PATCH v1 1/3] kernel/panic: generalize panic_print's function to show sys info

[Lance Yang <lance.yang@linux.dev>]

On 2025/5/15 18:32, Petr Mladek wrote:
> On Tue 2025-05-13 21:23:25, Feng Tang wrote:
>> Hi Petr,
>>
>> Thanks for the review!
>>
>> On Tue, May 13, 2025 at 11:57:19AM +0200, Petr Mladek wrote:
>>> On Sun 2025-05-11 16:52:52, Feng Tang wrote:
>>>> panic_print was introduced to help debugging kernel panic by dumping
>>>> different kinds of system information like tasks' call stack, memory,
>>>> ftrace buffer etc. Acutually this function could help debugging cases
>>>> like task-hung, soft/hard lockup too, where user may need the snapshot
>>>> of system info at that time.
>>>>
>>> The generic approach might deserve a separate source file,
>>> for example:
>>>
>>>      include/linux/sys_info.h
>>>      lib/sys_info.c
>>
>> Thanks for the suggestion! I'm really not good at naming.
>>
>> As panic.c is always built-in, I'll put sys_info.c as obj-y.
> 
> Makes sense.
> 
>>> Also I always considered the bitmask as a horrible user interface.
>>> It might be used internally. But it would be better to allow a human
>>> readable parameter, for example:
>>>
>>> 	panic_sys_info=task,mem,timer,lock,ftrace,bt,all_bt,blocked_tasks
>>
>> Yes, it's convenient for developers, as a cmdline parameter being parsed
>> at boot time.
>>
>> But I think bitmask may be easier for runtime changing as a core parameter
>> under /proc/ or /sys/, or from sysctl interface. There are also some other
>> modules use debug bitmask controlling printking info for different
>> sub-components.
> 
> Good to know. Could you please give me a pointer to some other modules
> using the bitmask? I believe that they exist but I can't find any.
> I wonder how common it is...
> 
> Anyway, I personally find words/names easier to use. For example,
> I like the following interfaces:
> 
>    #> cat /sys/power/pm_test
>    [none] core processors platform devices freezer
> 
>    #> cat /sys/kernel/debug/tracing/available_tracers
>    blk function_graph wakeup_dl wakeup_rt wakeup function nop
> 
>    #> cat /proc/sys/kernel/seccomp/actions_avail
>    kill_process kill_thread trap errno user_notif trace log allow
>    # cat /proc/sys/kernel/seccomp/actions_logged
>    kill_process kill_thread trap errno user_notif trace log

Clean and straightforward! I prefer this too ;p

> 
>> And we have similar control knobs for hung, lockup etc.
>>
>> Or should I change the name from 'xxx_print_mask' to 'xxx_sysinfo_flag'
>> in patch 2/3 ?
>>
>>>
>>> The console reply might be handled by a separate:
>>>
>>> 	panic_console_reply=1
>>>
>>> And it would obsolete the existing "panic_print" which is an
>>> ugly name and interface from my POV.
>>
>> Agree it's ugly :). But besides a kernel parameter,  'panic_print' is
>> also a sysctl interface, I'm afraid renaming it might break user ABI.
> 
> A solution would be to keep it and create "panic_sys_info="
> with the human readable parameters in parallel. They would
> store the request in the same bitmap.

Nice! Seems like a well-balanced solution ;)

> 
> We could print a message that "panic_print" has been obsoleted
> by "panic_sys_info" when people use it.

Indeed, we should add a deprecation warning. Perhaps the message could say:

"'panic_print=' is deprecated and will be removed. Use 'panic_sys_info=' 
instead."

e.g. pr_warn_ratelimited("block device autoloading is deprecated and will be
removed.\n") in blkdev_get_no_open().

> 
> Both parameters would override the current bitmap. So the later
> used parameter or procfs/sysfs write would win.

Makes sense to me. BTW, this override behavior needs to be documented
in kernel-doc.

> 
> Note:
> 
> One question is whether to use sysctl or module parameters.
> 
> An advantage of sysctl is the "systcl" userspace tool. Some people
> might like it. But the API is very old and a bit cumbersome for
> implementing.
> 
> The sysfs, aka include/linux/moduleparam.h, API looks cleaner to me.
> But the parameters are hidden in the /sys/... jungle ;-)
> 
> I would slightly prefer "sysctl" because these parameters are easier
> to find.

+1 for sysctl.

Thanks,
Lance

> 
> Best Regards,
> Petr