distinguish kernel thread / user task

distinguish kernel thread / user task

[Sylvain]

Hi all,

I have little question while doing some kernel implementation.
How can I distinguish whether a task_struct is actually kernel thread
or mere user task?

My idea was to look at task_struct "mm" field to discriminate them,
but that was wrong...

Thanks,

Sylvain

Re: distinguish kernel thread / user task

[Brian Gerst]

Sylvain wrote:
> Hi all,
> 
> I have little question while doing some kernel implementation.
> How can I distinguish whether a task_struct is actually kernel thread
> or mere user task?
> 
> My idea was to look at task_struct "mm" field to discriminate them,
> but that was wrong...
> 
> Thanks,
> 
> Sylvain

To the scheduler, a thread is a thread.  It doesn't care if it's a 
kernel thread or not.  The difference is execution context, which is 
cpu-dependant.  For example, on x86 the difference is in the code 
segment the task runs in.  Kernel threads run in KERNEL_CS (ring 0), and 
user threads run USER_CS (or any other ring 3 code segment, or vm86 mode 
set in eflags).  Other cpus might have a flag in the status register.

What are you trying to do that you need to know whether a thread is 
kernel or user?  I suppose if there were a compelling enough reason, a 
kernel/user flag could be added to the task struct, set in do_fork() for 
kernel threads, and cleared by execve().

--
				Brian Gerst

Re: distinguish kernel thread / user task

[Sylvain]

I am trying to do a tool to record task switching...separating also
kernel/user tasks, but I got some trouble with that last case.

I confused since "ps" is actually able to distinguish kernel thread
from user task.
I wouldn't had a flag if It 's not necessary

Sylvain


On Fri, 03 Dec 2004 14:24:27 -0500, Brian Gerst <bgerst@didntduck.org> wrote:
> Sylvain wrote:
> 
> 
> > Hi all,
> >
> > I have little question while doing some kernel implementation.
> > How can I distinguish whether a task_struct is actually kernel thread
> > or mere user task?
> >
> > My idea was to look at task_struct "mm" field to discriminate them,
> > but that was wrong...
> >
> > Thanks,
> >
> > Sylvain
> 
> To the scheduler, a thread is a thread.  It doesn't care if it's a
> kernel thread or not.  The difference is execution context, which is
> cpu-dependant.  For example, on x86 the difference is in the code
> segment the task runs in.  Kernel threads run in KERNEL_CS (ring 0), and
> user threads run USER_CS (or any other ring 3 code segment, or vm86 mode
> set in eflags).  Other cpus might have a flag in the status register.
> 
> What are you trying to do that you need to know whether a thread is
> kernel or user?  I suppose if there were a compelling enough reason, a
> kernel/user flag could be added to the task struct, set in do_fork() for
> kernel threads, and cleared by execve().
> 
> --
>                                Brian Gerst
>

Re: distinguish kernel thread / user task

[Brian Gerst]

Sylvain wrote:
> I am trying to do a tool to record task switching...separating also
> kernel/user tasks, but I got some trouble with that last case.
> 
> I confused since "ps" is actually able to distinguish kernel thread
> from user task.
> I wouldn't had a flag if It 's not necessary
> 
> Sylvain
> 

Pstools doesn't really know the difference between user and kernel 
threads.  It only shows kernel threads as swapped out (in brackets) 
because they have an RSS of zero (since kernel threads have no mm struct).

--
				Brian Gerst

Re: distinguish kernel thread / user task

[Sylvain]

kernel threads have no mm struct, right.
but it appears some programs (user tasks) haven't either ?! actually,
that what I notice..

Sylvain



On Fri, 03 Dec 2004 15:50:19 -0500, Brian Gerst <bgerst@didntduck.org> wrote:
> Sylvain wrote:
> > I am trying to do a tool to record task switching...separating also
> > kernel/user tasks, but I got some trouble with that last case.
> >
> > I confused since "ps" is actually able to distinguish kernel thread
> > from user task.
> > I wouldn't had a flag if It 's not necessary
> >
> > Sylvain
> >
> 
> Pstools doesn't really know the difference between user and kernel
> threads.  It only shows kernel threads as swapped out (in brackets)
> because they have an RSS of zero (since kernel threads have no mm struct).
> 
> --
>                                Brian Gerst
>