Re: [RFC] Restrict kernel spawning of threads to a specified set ofcpus.

[Christoph Lameter]

Let me just say that the user space approach does not work because the
kernel sets the cpumask to all and then spawns a thread f.e. for
usermodehelper.

This mean we would have to run a daemon that keeps scanning for errand
threads and then move them. But at that point the damage would already
have been done. Short term threads would never be caught.

So I think the kernel based approach is unavoidable.

Look at this in kernel/kmod.c:

static int ____call_usermodehelper(void *data)
{
        struct subprocess_info *sub_info = data;
        struct cred *new;
        int retval;

        spin_lock_irq(&current->sighand->siglock);
        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);


!!!!! No chance to catch this from user space.



	....

        retval = do_execve(sub_info->path,
                       (const char __user *const __user *)sub_info->argv,
                           (const char __user *const __user *)sub_info->envp);
        if (!retval)


	....

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