linux-kernel - Re: [patch 17/18] sched: Enable might

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Date:   Mon, 15 May 2017 21:12:03 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Steven Rostedt <rostedt@...dmis.org>
cc:     LKML <linux-kernel@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [patch 17/18] sched: Enable might_sleep() checks early

On Mon, 15 May 2017, Steven Rostedt wrote:

> On Sun, 14 May 2017 20:27:33 +0200
> Thomas Gleixner <tglx@...utronix.de> wrote:
> 
> > might_sleep() checks are enabled after the boot process is done. That hides
> > bugs in the smp bringup and driver initialization code.
> > 
> > Enable it right when the scheduler starts working, i.e. when init task and
> > kthreadd have been created and right before the idle task enables
> > preemption.
> 
> Looking at commit b433c3d4549ae749, it appears that on very slow
> machines, there is a possibility that the init task can start running.
> Should system_state be updated before that complete() is called?

That commit is magic voodoo with exactly no effect at all.

rest_init() is called with preemption disabled and nothing can schedule
there _before_ schedule_preempt_disabled().

Both threads - init task and kthreadd - are only created and woken up. They
cannot get on the CPU simply because preemption is disabled. And this was
the case back then in 2.6.35 as well.

It does not matter at all whether the machine is slow or not. That
completion is pointless.

Peter, can you explain what the heck this patch is actually doing?

Thanks,

	tglx