linux-kernel - Re: [RFC] cpu/hotplug: allow the cpu in UP

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Date:   Fri, 19 Nov 2021 18:36:33 +0100
From:   Sebastian Andrzej Siewior <bigeasy@...utronix.de>
To:     "Longpeng (Mike, Cloud Infrastructure Service Product Dept.)" 
        <longpeng2@...wei.com>
Cc:     "peterz@...radead.org" <peterz@...radead.org>,
        "valentin.schneider@....com" <valentin.schneider@....com>,
        "mingo@...nel.org" <mingo@...nel.org>,
        "tglx@...utronix.de" <tglx@...utronix.de>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "Gonglei (Arei)" <arei.gonglei@...wei.com>
Subject: Re: [RFC] cpu/hotplug: allow the cpu in UP_PREPARE state to bringup
 again

Sorry for forgetting…

On 2021-10-08 03:10:34 [+0000], Longpeng (Mike, Cloud Infrastructure Service Product Dept.) wrote:
> > -----Original Message-----
> > From: Sebastian Andrzej Siewior [mailto:bigeasy@...utronix.de]
> > Sent: Thursday, September 30, 2021 10:01 PM
> > To: Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
> > <longpeng2@...wei.com>
> > Cc: peterz@...radead.org; valentin.schneider@....com; mingo@...nel.org;
> > tglx@...utronix.de; linux-kernel@...r.kernel.org; Gonglei (Arei)
> > <arei.gonglei@...wei.com>
> > Subject: Re: [RFC] cpu/hotplug: allow the cpu in UP_PREPARE state to bringup
> > again
> > 
> > On 2021-09-01 13:11:43 [+0800], Longpeng(Mike) wrote:
> > > The cpu's cpu_hotplug_state will be set to CPU_UP_PREPARE before
> > > the cpu is waken up, but it won't be reset when the failure occurs.
> > > Then the user cannot to make the cpu online anymore, because the
> > > CPU_UP_PREPARE state makes cpu_check_up_prepare() unhappy.
> > >
> > > We should allow the user to try again in this case.
> > 
> > Can you please describe where it failed / what did you reach that state?
> > 
> 
> native_cpu_up
>   cpu_check_up_prepare
>   do_boot_cpu
>     /* Wait 10s total for first sign of life from AP */
> 
> It will fail if the AP doesn't response in 10s and then cpu_hotplug_state
> will stay in CPU_UP_PREPARE state.
> 
> This could happen on a virtualized system, especially in some special usages,
> e.g. Software Enclaves [1][2]

So wakeup_cpu_via_init_nmi() / wakeup_secondary_cpu() succeeds but the
CPU does not show up with 10 seconds.
Does the CPU come in later and spins in wait_for_master_cpu() or is the
CPU completely missing?

> [1] https://docs.aws.amazon.com/enclaves/latest/user/nitro-enclave.html
> [2] https://www.alibabacloud.com/help/doc-detail/203433.htm?spm=a3c0i.23986742.6981761520.1.7e30715eZCRXmk
> 
> 
> > > Signed-off-by: Longpeng(Mike) <longpeng2@...wei.com>
> > > ---
> > >  kernel/smpboot.c | 7 +++++++
> > >  1 file changed, 7 insertions(+)
> > >
> > > diff --git a/kernel/smpboot.c b/kernel/smpboot.c
> > > index f6bc0bc..d18f8ff 100644
> > > --- a/kernel/smpboot.c
> > > +++ b/kernel/smpboot.c
> > > @@ -392,6 +392,13 @@ int cpu_check_up_prepare(int cpu)
> > >  		 */
> > >  		return -EAGAIN;
> > >
> > > +	case CPU_UP_PREPARE:
> > > +		/*
> > > +		 * The CPU failed to bringup last time, allow the user
> > > +		 * continue to try to start it up.
> > > +		 */
> > > +		return 0;
> > > +
> > >  	default:
> > >
> > >  		/* Should not happen.  Famous last words. */

Sebastian