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

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Date:   Mon, 10 May 2021 16:57:47 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     linux-kernel@...r.kernel.org
Cc:     mingo@...nel.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

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?

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()).