Re: [PATCH v2] hung_task: Add per-round stack trace deduplication

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

On Sat, Jun 20, 2026 at 01:54:56PM -0400, Aaron Tomlin wrote:
[...]
>On Sat, Jun 20, 2026 at 11:37:15AM +0800, Lance Yang wrote:
>> Hi Aaron,
>>
>> On Fri, Jun 19, 2026 at 09:35:59PM -0400, Aaron Tomlin wrote:
>> >Currently, when multiple tasks hang in the exact same location (e.g.,
>> >such as severe contention for a mutex), khungtaskd indiscriminately
>> >reports every single instance. This wastes ring buffer space with
>> >identical stack traces up to the defined warning limit (i.e.,
>> >kernel.hung_task_warnings), obscuring the root cause without providing
>> >any additional diagnostic value.
>> >
>> >Introduce a lightweight, hash-based stack trace deduplicator for
>> >khungtaskd to ensure only unique stack traces are reported during
>> >a single detection interval.
>> >
>> >Technical details of the implementation:
>> > - Uses a 12-bit hash table (4096 slots), consuming just 16 KB of
>> >   static memory to prevent cache thrashing during massive hangs.
>> >
>> > - Operates purely serially within the single khungtaskd thread,
>> >   requiring zero atomic operations or concurrent locking overhead.
>> >
>> > - Flushes the lossy cache via memset() at the beginning of each
>> >   detection round. This ensures the immediate "thundering herd" of
>> >   duplicates is suppressed, but guarantees the system will not
>> >   permanently suppress identical hangs that occur in future rounds.
>> >
>> > - Introduces a new sysctl, kernel.hung_task_dedup, which defaults to 1
>> >   (enabled). The sysctl is locally cached at the outset of each
>> >   interval to prevent tearing caused by concurrent userspace toggling.
>> >
>>
>> Thanks for working on this, but ... guess I'll be the bad guy here, not
>> convinced this should go in ...
>>
>> When khungtaskd fires, somthing is already wrong, no? I don't see why it
>> should grow a new sysctl, a stack hash table, and extra filtering logic
>> just ot hide part of the report ...
>>
>> Emm ... do you have real cases where duplicate hung-task stacks caused
>> serious pain?
>>
>> If many tasks hang at once, usually one root cause, not a bunch of
>> different bugs. At least from what I've seen, any one of those stacks is
>> enough to start debugging ...
>>
>> We already have hung_task_detect_count and trace_sched_process_hang() for
>> basic counting/observability. Even if hung_task_warnings is finite and
>> the warning budget runs out, we still don't lose detections: counter gets
>> bumped and tracepoint fires before printk output is gated :)
>>
>> If someone wants stack grouping, I'd rather leave that to a tool than add
>> another policy knob to khungtaskd. Once it lands, maintainers have to
>> carry it forever. Not every nice-to-have feature is worth that cost, IMHO
>>
>> And if someone really wants more hung-task stacks in the log, we already
>> have hung_task_warnings for that. Raise it, or set it to -1.
>>
>> Also, looking at the v1 thread, I don't think the concerns there have
>> really settled yet ... If nobody replies, maybe give it a week before
>> sending a new version.
>
>Hi Lance,
>
>Thank you for taking the time to review the patch and for your candour.

I think your reply still misses most of my concerns ...

>You raise an entirely fair point regarding maintainability; every new
>control knob indeed carries a permanent cost for the maintainers, and I
>respect your caution.

Yeah, that matters, IMHO.

>To answer your question regarding real world pain: the primary issue is not
>merely visual clutter, but the premature exhaustion of the warning budget
>and the preservation of the kernel ring buffer during cascading failures.

Right, but that still sounds like a very specific case.

When khungtaskd fires, something is already wrong, no?

Even with one identical stack, per-round dedup only helps inside one
scan. The same stack can still come back in later rounds and burn through
hung_task_warnings anyway.

And under heavy contention, I would not expect only one stack anyway.
Different tasks can hang behind different locks or different callers, and
those stacks can still burn through the warning budget.

>In our production environments, we typically leave
>kernel.hung_task_warnings at its default value of 10. If a severe lock
>contention occurs, a single bottleneck can easily cause 10 tasks to hang
>simultaneously with the exact same stack trace. Under the current logic,

Not sure I buy this premise :)

Same bottleneck does not necessarily mean exactly the same stack.
Different callers can block on the same lock, and exact-stack dedup won't
help there.

At least from cases I've looked at, I can't really recall seeing this
exact pattern often enough to justify a new khungtaskd knob.

>those 10 identical traces will completely exhaust the warning budget.
>Consequently, the kernel is left entirely blind to any subsequent or
>completely unrelated deadlocks that might be occurring concurrently, as all
>further reports are silenced.

I don't think "entirely blind" is accurate.

hung_task_warnings *only* gates printk. We still bump
hung_task_detect_count and hit trace_sched_process_hang() before that
gate.

>Furthermore, dumping a full stack trace for every duplicate rapidly injects
>several of lines of identical noise into dmesg. We have found that this
>sudden burst frequently rolls the circular ring buffer.
>
>Userspace tooling is unfortunately unable to group or analyse logs that
>have already been evicted before the tool could read them, nor can it
>recover traces the kernel silently dropped due to an exhausted budget.
>
>The deduplicator acts as a telemetry filter, ensuring that the limited
>warning budget is spent strictly on unique traces rather than redundant
>noise, thereby preserving the history of the crash and ensuring secondary
>failures are not obscured.
>
>I wanted to clarify the exact operational context and the limitation of
>relying on userspace. Please let me know if this operational context alters
>your perspective at all.
[...]

Aaron, you've done good work in khungtaskd, and some of it is upstream
already. I do appreciate that!

But this one feels different. Useful locally, maybe, but not something
the kernel should carry forever.

Anyway, I'll stop here. Still a nack from my side.

Thanks, Lance