Re: Gracefully killing kswapd, or any kernel thread

[linux-os (Dick Johnson)]

On Wed, 7 Sep 2005, Kristis Makris wrote:

> Hello,
>
> I'm trying to kill a kernel thread gracefully, in particular kswapd,
> without any success.
>
> The goal is to start another kernel thread that contains updated kswapd
> functionality, through a loadable module; no kernel recompilation.
>
> I noticed that kernel threads block SIGKILL. Hence, on module load I'm
> running:
>
> task = find_task_by_name("kswapd");
> if (task != NULL) {
>    spin_lock_irq(&task->sigmask_lock);
>    sigdelset(&task->blocked, SIGKILL);
>    recalc_sigpending(task);
>    spin_unlock_irq(&task->sigmask_lock);
>    // Also tried issuing here a: kill_proc(task->pid, SIGKILL, 1);
> }
>
> Then from userspace I issue:
>
> # ps aux |grep -i swap
> root         4  0.0  0.0     0    0 ?        SW   18:36   0:00 [kswapd]
> $ kill -9 4
>
> After the kill is issued, kswapd is taking up 99.9% of CPU time and
> remains at a runnable state:
> # ps aux |grep -i swap
> root         4  0.2  0.0     0    0 ?        RW   18:36   0:02 [kswapd]
>
>
> Can anyone explain why this is happening ? I've tried this with linux
> kernels 2.2.19 and 2.4.27 (with patch kdb-4.3). What is the proper way
> of gracefully killing a kernel thread launched from the original kernel
> image (not a module) in kernels < 2.6 (ie. without the new kernel thread
> API that contains the stop_kthread call documented in
> http://www.scs.ch/~frey/linux/kernelthreads.html)
>
> I've also tried the same with kflushd, kupdate, and keventd in 2.2.19.
> When I do issue a "kill -9" for them I see:
>
> # ps aux
> USER       PID %CPU %MEM   VSZ  RSS TTY      STAT START   TIME COMMAND
> root         2  0.0  0.0     0    0 ?        SW   12:18   0:00 [kflushd]
> root         3  1.5  0.0     0    0 ?        RW   12:18   0:16 [kupdate]
> root         5  0.0  0.0     0    0 ?        SW   12:18   0:00 [keventd]
>
> All 3 kernel threads remain in the process list. kupdate also appears to
> be in a running state consuming 99.9% of the CPU when killed. What's so
> special about kupdate and kswapd that makes them stay at a running
> state, and why do all kernel threads seem unkillable?
>
> Thanks,
> Kristis

To kill a kernel thread, you need to make __it__ call exit(). It must be
CODED to do that! You can't do it externally although you can send
it a signal, after which it will spin forever....

// Main loop of the thread

     for(;;)
     {
         set_current_state(TASK_INTERRUPTIBLE);
         if(signal_pending(current))
             complete_and_exit(&info->quit, 0);  // <--- HERE!!!
         interruptible_sleep_on(&info->twait);
         do_work();
     }

// Start a thread

     info->pid = kernel_thread(local_thread, NULL, CLONE_FS|CLONE_FILES);



// Stop a thread

     if(info->pid)
     {
         (void)kill_proc(info->pid, SIGTERM, 1);
         wait_for_completion(&info->quit);   // MUST wait!
     }

With newer kernels, we use the allow_signal(SIGNAME) macro as one
of the first things executed to allow the kernel thread to respond
to a signal.

Cheers,
Dick Johnson
Penguin : Linux version 2.6.13 on an i686 machine (5589.51 BogoMips).
Warning : 98.36% of all statistics are fiction.
.
I apologize for the following. I tried to kill it with the above dot :

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