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Re: [PATCH 3/3] PowerPC/mpc85xx: Add hotplug support on E6500 cores

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  Re: [PATCH 3/3] PowerPC/mpc85xx: Add hotplug support on E6500 cores Scott Wood <scottwood@freescale.com> - 2015-08-06 05:20 +0200
    Re: [PATCH 3/3] PowerPC/mpc85xx: Add hotplug support on E6500 cores Scott Wood <scottwood@freescale.com> - 2015-08-06 07:50 +0200

#1201407 — Re: [PATCH 3/3] PowerPC/mpc85xx: Add hotplug support on E6500 cores

On Wed, 2015-08-05 at 19:08 +0800, Chenhui Zhao wrote:
> On Sat, Aug 1, 2015 at 8:22 AM, Scott Wood <scottwood@freescale.com> 
> wrote:
> > On Fri, 2015-07-31 at 17:20 +0800,  b29983@freescale.comwrote:
> > >  +             /*
> > >  +              * If both threads are offline, reset core to start.
> > >  +              * When core is up, Thread 0 always gets up first,
> > >  +              * so bind the current logical cpu with Thread 0.
> > >  +              */
> > >  +             if (hw_cpu != cpu_first_thread_sibling(hw_cpu)) {
> > >  +                     int hw_cpu1, hw_cpu2;
> > >  +
> > >  +                     hw_cpu1 = get_hard_smp_processor_id(primary);
> > >  +                     hw_cpu2 = get_hard_smp_processor_id(primary + 
> > > 1);
> > >  +                     set_hard_smp_processor_id(primary, hw_cpu2);
> > >  +                     set_hard_smp_processor_id(primary + 1, 
> > > hw_cpu1);
> > >  +                     /* get new physical cpu id */
> > >  +                     hw_cpu = get_hard_smp_processor_id(nr);
> > 
> > NACK as discussed in http://patchwork.ozlabs.org/patch/454944/
> > 
> > -Scott
> 
> You said,
> 
>     There's no need for this. I have booting from a thread1, and having 
> it
>     kick its thread0, working locally without messing with the hwid/cpu
>     mapping.
> 
> I still have questions here. After a core reset, how can you boot 
> Thread1
> of the core first. As I know, Thread0 boots up first by default.

So the issue isn't that thread1 comes up first, but that you *want* thread1 
to come up first and it won't.  I don't think this remapping is an acceptable 
answer, though.  Instead, if you need only thread1 to come up, start the 
core, have thread0 start thread1, and then send thread0 into whatever waiting 
state it would be in if thread1 had never been offlined.

-Scott

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#1201450

On Thu, 2015-08-06 at 12:32 +0800, Chenhui Zhao wrote:
> On Thu, Aug 6, 2015 at 11:16 AM, Scott Wood <scottwood@freescale.com> 
> wrote:
> > On Wed, 2015-08-05 at 19:08 +0800, Chenhui Zhao wrote:
> > >  On Sat, Aug 1, 2015 at 8:22 AM, Scott Wood <scottwood@freescale.com>
> > >  wrote:
> > >  > On Fri, 2015-07-31 at 17:20 +0800,  b29983@freescale.comwrote:
> > >  > >  +             /*
> > >  > >  +              * If both threads are offline, reset core to 
> > > start.
> > >  > >  +              * When core is up, Thread 0 always gets up 
> > > first,
> > >  > >  +              * so bind the current logical cpu with Thread 0.
> > >  > >  +              */
> > >  > >  +             if (hw_cpu != cpu_first_thread_sibling(hw_cpu)) {
> > >  > >  +                     int hw_cpu1, hw_cpu2;
> > >  > >  +
> > >  > >  +                     hw_cpu1 = 
> > > get_hard_smp_processor_id(primary);
> > >  > >  +                     hw_cpu2 = 
> > > get_hard_smp_processor_id(primary +
> > >  > > 1);
> > >  > >  +                     set_hard_smp_processor_id(primary, 
> > > hw_cpu2);
> > >  > >  +                     set_hard_smp_processor_id(primary + 1,
> > >  > > hw_cpu1);
> > >  > >  +                     /* get new physical cpu id */
> > >  > >  +                     hw_cpu = get_hard_smp_processor_id(nr);
> > >  >
> > >  > NACK as discussed in http://patchwork.ozlabs.org/patch/454944/
> > >  >
> > >  > -Scott
> > > 
> > >  You said,
> > > 
> > >      There's no need for this. I have booting from a thread1, and 
> > > having
> > >  it
> > >      kick its thread0, working locally without messing with the 
> > > hwid/cpu
> > >      mapping.
> > > 
> > >  I still have questions here. After a core reset, how can you boot
> > >  Thread1
> > >  of the core first. As I know, Thread0 boots up first by default.
> > 
> > So the issue isn't that thread1 comes up first, but that you *want* 
> > thread1
> > to come up first and it won't.  I don't think this remapping is an 
> > acceptable
> > answer, though.  Instead, if you need only thread1 to come up, start 
> > the
> > core, have thread0 start thread1, and then send thread0 into whatever 
> > waiting
> > state it would be in if thread1 had never been offlined.
> > 
> > -Scott
> 
> Remapping is a concise solution. what's the harm of it?
> Keeping things simple is good in my opinion.

Remapping is not simple.  Remapping will make debugging more complicated (I 
see an oops on CPU <n>, which CPU's registers do I dump in the debugger?), 
could expose bugs where smp_processor_id() is used where 
hard_smp_processor_id() is needed, etc.

Having thread0 start thread1 and then go wherever it would have gone if 
thread1 were up the whole time is much more straightforward.

-Scott


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