linux-kernel - Re: [PATCH 1/1] usermodehelper: kill ____call_usermodehelper()->set_cpus_allowed

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Date:	Thu, 5 Dec 2013 15:21:53 +0100
From:	Frederic Weisbecker <fweisbec@...il.com>
To:	Oleg Nesterov <oleg@...hat.com>, Christoph Lameter <cl@...ux.com>
Cc:	Peter Zijlstra <peterz@...radead.org>,
	Tejun Heo <htejun@...il.com>, zhang.yi20@....com.cn,
	lkml <linux-kernel@...r.kernel.org>,
	Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
	Ingo Molnar <mingo@...hat.com>
Subject: Re: [PATCH 1/1] usermodehelper: kill ____call_usermodehelper()->set_cpus_allowed_ptr()

2013/11/28 Oleg Nesterov <oleg@...hat.com>:
> ____call_usermodehelper() does set_cpus_allowed_ptr(cpu_all_mask),
> this (and the comment) is misleading. We no longer have keventd_wq,
> and kmod.c switched to khelper_wq a long ago.
>
> And more importantly, "unlike our parent" is no longer true too,
> this thread was created by WQ_UNBOUND worker thread which has the
> full ->cpus_allowed mask, so this set_cpus_allowed_ptr() is simply
> unnecessary.
>
> Perhaps we will change this later, so that userspace can control
> the affinity of the usermode helper tasks, but this is yet another
> reason to remove this set_cpus_allowed_ptr().
>
> To some degree this also applies to set_user_nice(), but this
> patch only updates the comment.
>
> Signed-off-by: Oleg Nesterov <oleg@...hat.com>

I'm adding Christophe in Cc because he is interested in tweaking the
affinity of call_usermodehelper for cpu isolation. This welcome
cleanup confirms that we want to take the direction of being able to
change the affinity of workqueue themselves instead of just
call_usermodehelper() alone.

> ---
>  kernel/kmod.c |    8 ++------
>  1 files changed, 2 insertions(+), 6 deletions(-)
>
> diff --git a/kernel/kmod.c b/kernel/kmod.c
> index b086006..2fe4544 100644
> --- a/kernel/kmod.c
> +++ b/kernel/kmod.c
> @@ -208,13 +208,9 @@ static int ____call_usermodehelper(void *data)
>         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);
> -
>         /*
> -        * Our parent is keventd, which runs with elevated scheduling priority.
> -        * Avoid propagating that into the userspace child.
> +        * Our parent is a workqueue thread, which can run with elevated
> +        * scheduling priority. Avoid propagating that into the userspace.
>          */
>         set_user_nice(current, 0);
>
> --
> 1.5.5.1
>
>
> --
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