Re: [PATCH] kmod: Run usermodehelpers only on cpus allowed for kthreadd

[Andrew Morton <akpm@linux-foundation.org>]

On Wed, 16 Oct 2013 14:44:28 +0000 Christoph Lameter <cl@linux.com> wrote:

> This is a follow on patch related to the earlier
> discussion about restricting the
> spawning of kernel threads. See https://lkml.org/lkml/2013/9/5/426
> 
> 
> 
> 
> usermodehelper() threads can currently run on all processors.
> This is an issue for low latency cores.

How much of an issue?  The severity of the problem is utterly unclear
from this description.

> Restrict usermodehelper() threads to the cores that kthreadd is
> restricted to.
> 
> The default for kthreadd is to be allowed to run on an processors.
> If the user restricts kthreadd then threads spawned by
> usermodhelper() will similarly restricted.
> 
> Before this patch there is no way to limit the cpus that usermodehelper
> can run on since the affinity is set when the thread is spawned to
> all processors.
> 
> ...
>
> --- linux.orig/include/linux/kthread.h	2013-10-10 11:00:34.167771996 -0500
> +++ linux/include/linux/kthread.h	2013-10-15 13:57:52.859056676 -0500
> @@ -51,6 +51,7 @@ void kthread_parkme(void);
>  int kthreadd(void *unused);
>  extern struct task_struct *kthreadd_task;
>  extern int tsk_fork_get_node(struct task_struct *tsk);
> +void set_kthreadd_affinity(void);
> 
>  /*
>   * Simple work processor based on kthread.
> Index: linux/kernel/kmod.c
> ===================================================================
> --- linux.orig/kernel/kmod.c	2013-10-10 11:00:39.091771917 -0500
> +++ linux/kernel/kmod.c	2013-10-15 14:02:01.904261324 -0500
> @@ -40,6 +40,7 @@
>  #include <linux/ptrace.h>
>  #include <linux/async.h>
>  #include <asm/uaccess.h>
> +#include <linux/kthread.h>
> 
>  #include <trace/events/module.h>
> 
> @@ -209,8 +210,8 @@ static int ____call_usermodehelper(void
>  	flush_signal_handlers(current, 1);
>  	spin_unlock_irq(&current->sighand->siglock);
> 
> -	/* We can run anywhere, unlike our parent keventd(). */
> -	set_cpus_allowed_ptr(current, cpu_all_mask);
> +	/* We can run anywhere kthreadd can run */

This is a poor comment - it explains "what" (which was utterly obvious)
but doesn't explain "why".  The reader will want to know *why*
call_usermodehelper() only runs on kthreadd CPUs, but we didn't tell
him.

> +	set_kthreadd_affinity();
>