linux-kernel - Re: [PATCH] stacktrace: do not trace user stack for user

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Message-ID: <ddcbdaa0-479a-4821-9230-d3207be20b3c@kernel.dk>
Date: Wed, 25 Jun 2025 16:30:55 -0600
From: Jens Axboe <axboe@...nel.dk>
To: Steven Rostedt <rostedt@...dmis.org>
Cc: Jiazi Li <jqqlijiazi@...il.com>, linux-kernel@...r.kernel.org,
 "peixuan.qiu" <peixuan.qiu@...nssion.com>, io-uring@...r.kernel.org,
 Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] stacktrace: do not trace user stack for user_worker tasks

On 6/25/25 2:50 PM, Steven Rostedt wrote:
> [
>   Adding Peter Zijlstra as he has been telling me to test against
>   PF_KTHREAD instead of current->mm to tell if it is a kernel thread.
>   But that seems to not be enough!
> ]

Not sure I follow - if current->mm is NULL, then it's PF_KTHREAD too.
Unless it's used kthread_use_mm().

PF_USER_WORKER will have current->mm of the user task that it was cloned
from.

> On Wed, 25 Jun 2025 10:23:28 -0600
> Jens Axboe <axboe@...nel.dk> wrote:
> 
>> On 6/24/25 11:07 AM, Steven Rostedt wrote:
>>> On Mon, 23 Jun 2025 19:59:11 +0800
>>> Jiazi Li <jqqlijiazi@...il.com> wrote:
>>>   
>>>> Tasks with PF_USER_WORKER flag also only run in kernel space,
>>>> so do not trace user stack for these tasks.  
>>>
>>> What exactly is the difference between PF_KTHREAD and PF_USER_WORKER?  
>>
>> One is a kernel thread (eg no mm, etc), the other is basically a user
>> thread. None of them exit to userspace, that's basically the only
>> thing they have in common.
> 
> Was it ever in user space? Because exiting isn't the issue for getting
> a user space stack. If it never was in user space than sure, there's no
> reason to look at the user space stack.

It was never in userspace.

>>> Has all the locations that test for PF_KTHREAD been audited to make
>>> sure that PF_USER_WORKER isn't also needed?  
>>
>> I did when adding it, to the best of my knowledge. But there certainly
>> could still be gaps. Sometimes not easy to see why code checks for
>> PF_KTHREAD in the first place.
>>
>>> I'm working on other code that needs to differentiate between user
>>> tasks and kernel tasks, and having to have multiple flags to test is
>>> becoming quite a burden.  
>>
>> None of them are user tasks, but PF_USER_WORKER does look like a
>> user thread and acts like one, except it wasn't created by eg
>> pthread_create() and it never returns to userspace. When it's done,
>> it's simply reaped.
>>
> 
> I'm assuming that it also never was in user space, which is where we
> don't want to do any user space stack trace.

It was not.

> This looks like more rationale for having a kernel_task() user_task()
> helper functions:
> 
>   https://lore.kernel.org/linux-trace-kernel/20250425204120.639530125@goodmis.org/
> 
> Where one returns true for both PF_KERNEL and PF_USER_WORKER and the
> other returns false.

On vacation right now, but you can just CC me on the next iteration and
I'll take a look.

-- 
Jens Axboe