linux-kernel - Re: [PATCH 1/2] sched: Make the idle task quack like a per-CPU kthread

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Message-ID: <YJozF+wmiMYuSa6/@gmail.com>
Date:   Tue, 11 May 2021 09:32:39 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, peterz@...radead.org,
        tglx@...utronix.de, bristot@...hat.com, yejune.deng@...il.com
Subject: Re: [PATCH 1/2] sched: Make the idle task quack like a per-CPU
 kthread


* Valentin Schneider <valentin.schneider@....com> wrote:

> On 10/05/21 16:10, Valentin Schneider wrote:
> > This requires some extra iffery as init_idle()
> > call be called more than once on the same idle task.
> >
> 
> While I'm at it, do we actually still need to suffer through this?

No.

> AFAICT the extra calls are due to idle_thread_get() (used in cpuhp) 
> calling init_idle(). However it looks to me that since
> 
>   3bb5d2ee396a ("smp, idle: Allocate idle thread for each possible cpu during boot")
> 
> we don't need to do that: we already have a
> 
>   for_each_possible_cpu(cpu)
>     init_idle(cpu)
> 
> issued at init. So can't we "simply" rely on that init-time creation, 
> given it's done against the possible mask? I think the only thing that 
> might need doing at later hotplug is making sure the preempt count is 
> right (secondary startups seem to all prepare the idle task by issuing a 
> preempt_disable()).

Best-case it works, worst-case we discover an unclean assumption in the 
init sequence and it works after we fix that.

Win-win. :-)

Thanks,

	Ingo