RE: [RFC] cpu/hotplug: allow the cpu in UP_PREPARE state to bringup again

[Longpeng (Mike, Cloud Infrastructure Service Product Dept.)]

> -----Original Message-----
> From: Sebastian Andrzej Siewior [mailto:bigeasy@xxxxxxxxxxxxx]
> Sent: Thursday, September 30, 2021 10:01 PM
> To: Longpeng (Mike, Cloud Infrastructure Service Product Dept.)
> <longpeng2@xxxxxxxxxx>
> Cc: peterz@xxxxxxxxxxxxx; valentin.schneider@xxxxxxx; mingo@xxxxxxxxxx;
> tglx@xxxxxxxxxxxxx; linux-kernel@xxxxxxxxxxxxxxx; Gonglei (Arei)
> <arei.gonglei@xxxxxxxxxx>
> 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]

[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@xxxxxxxxxx>
> > ---
> >  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. */
> > --
> > 1.8.3.1
> 
> Sebastian