linux-kernel - Re: [PATCH 3/3] kmod: Remove unecessary explicit wide CPU affinity setting

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Date:	Tue, 7 Jul 2015 18:30:30 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Frederic Weisbecker <fweisbec@...il.com>
Cc:	LKML <linux-kernel@...r.kernel.org>,
	Christoph Lameter <cl@...ux.com>,
	Rik van Riel <riel@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 3/3] kmod: Remove unecessary explicit wide CPU affinity
	setting

On 07/06, Frederic Weisbecker wrote:
>
> The call_usermodehelper_exec_[a]sync() kernel threads are created by
> khelper precisely because

I think khelper should simply die. It doesn't make any sense today,
kmod.c should use some system wq instead. But see below.

> Not only useless it even breaks nohz full. The housekeeping work
> (general kernel internal code that user doesn't care much about) is
> handled by a reduced set of CPUs in nohz full, precisely those that are
> not included by nohz_full= kernel parameters. For example unbound
> workqueues are handled by housekeeping CPUs.

Confused... I do not see how workqueue_attrs->cpumask can depend on
tick_nohz_full_mask or housekeeping_mask. Could you explain?

> @@ -223,9 +223,6 @@ static int call_usermodehelper_exec_async(void *data)
>  	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);
> -

I think this is fine, ->no_numa is true for khelper.

But this means that after this change kmod.c can't use a system wq,
->no_numa is false by default. And khelper is no_numa only because
it is __WQ_ORDERED, but kmod.c doesn't need really need__WQ_ORDERED,
except, again, this implies ->no_numa == T.

So perhaps init_workqueues() should create another global
WQ_UNBOUND/no_numa workqueue_struct so that we could kill khelper_wq?
Or kmod.c can use system_unbound_wq, but then we need to keep this
set_cpus_allowed_ptr().

To me khelper_wq looks just annoying...

Oleg.

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