linux-kernel - Re: [PATCH] hung_task: Migrate hung_task_detect

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Message-ID: <03ea687c-2533-4aa4-bdb4-81128c907723@linux.dev>
Date: Mon, 1 Dec 2025 11:08:54 +0800
From: Lance Yang <lance.yang@...ux.dev>
To: Aaron Tomlin <atomlin@...mlin.com>
Cc: linux-kernel@...r.kernel.org, mhiramat@...nel.org,
 akpm@...ux-foundation.org, gregkh@...uxfoundation.org,
 Petr Mladek <pmladek@...e.com>
Subject: Re: [PATCH] hung_task: Migrate hung_task_detect_count to sysfs



On 2025/12/1 01:11, Aaron Tomlin wrote:
> On Sun, Nov 30, 2025 at 01:43:51PM +0800, Lance Yang wrote:
>> Hi Aaron,
> 
> Hi Lance,
> 
> 
>> Thanks for the patch.
> 
> No problem.
> 
>> Emm, I don't think we should do this :)
>>
>> Removing the sysctl file is going to break userspace tools that
>> currently read it ...
> 
> Thank you for your feedback. I appreciate the concern regarding user-space
> compatibility and the potential maintenance overhead of dual interfaces.
> 
> I agree that removing the sysctl entry entirely right now is too
> disruptive, as it would immediately break existing user-space tools relying
> on /proc/sys/kernel/hung_task_detect_count. Your point about avoiding
> unnecessary churn and maintaining two interfaces is well-taken.
> 
> However, I believe that moving the counter to sysfs is still a worthwhile
> long-term goal for the following reasons:
> 
>      1. Consistency and standardisation: Moving read-only counters to
>         /sys/kernel/ aligns with current kernel conventions and provides a
>         more consistent location for performance and diagnostic metrics.
>         This follows the pattern of existing counters like
>         /sys/kernel/softlockup_count.
> 
>      2. Graceful Migration Path: By adding the
>         /sys/kernel/hung_task_detect_count entry first (as a read-only
>         counter) and keeping the legacy
>         /proc/sys/kernel/hung_task_detect_count file in place, we provide
>         the necessary backwards compatibility and offer a grace period for
>         user-space tools to migrate. The old sysctl file can then be flagged
>         for deprecation in a future release.
> 

Deprecating established sysctl ABIs is often a very long process(sometimes
indefinite) ... and I would prefer to avoid introducing redundancy unless
strictly necessary or the current interface is functionally deficient :)

So, it's a NACK from my side regarding the interface migration.

> I am currently working on some additional updates for kernel/hung_task.c.

Please let's keep it simple and leave the interface as is. I look forward
to your other updates for hung_task.c!

[...]

Thanks,
Lance