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Re: [Qemu-ppc] [Qemu-devel] [RFC PATCH v2 3/5] spapr: Set stable_cpu_id
From: |
David Gibson |
Subject: |
Re: [Qemu-ppc] [Qemu-devel] [RFC PATCH v2 3/5] spapr: Set stable_cpu_id for threads of CPU cores |
Date: |
Tue, 12 Jul 2016 14:41:53 +1000 |
User-agent: |
Mutt/1.6.1 (2016-04-27) |
On Mon, Jul 11, 2016 at 10:15:57AM +0200, Igor Mammedov wrote:
> On Mon, 11 Jul 2016 13:26:01 +1000
> David Gibson <address@hidden> wrote:
>
> > On Fri, Jul 08, 2016 at 12:59:59PM +0200, Igor Mammedov wrote:
> > > On Fri, 8 Jul 2016 17:39:52 +1000
> > > David Gibson <address@hidden> wrote:
> > >
> > > > On Fri, Jul 08, 2016 at 12:11:12PM +0530, Bharata B Rao wrote:
> > > > > On Fri, Jul 08, 2016 at 03:24:13PM +1000, David Gibson wrote:
> > > > > > On Thu, Jul 07, 2016 at 08:20:23PM +0530, Bharata B Rao wrote:
> > > > > > > Conditonally set stable_cpu_id for CPU threads that are created
> > > > > > > as part
> > > > > > > of spapr CPU cores. The use of stable_cpu_id is enabled for
> > > > > > > pseries-2.7
> > > > > > > onwards.
> > > > > > >
> > > > > > > Signed-off-by: Bharata B Rao <address@hidden>
> > > > > > > ---
> > > > > > > hw/ppc/spapr_cpu_core.c | 7 +++++++
> > > > > > > 1 file changed, 7 insertions(+)
> > > > > > >
> > > > > > > diff --git a/hw/ppc/spapr_cpu_core.c b/hw/ppc/spapr_cpu_core.c
> > > > > > > index b104778..0ec3513 100644
> > > > > > > --- a/hw/ppc/spapr_cpu_core.c
> > > > > > > +++ b/hw/ppc/spapr_cpu_core.c
> > > > > > > @@ -293,8 +293,15 @@ static void
> > > > > > > spapr_cpu_core_realize(DeviceState *dev, Error **errp)
> > > > > > > for (i = 0; i < cc->nr_threads; i++) {
> > > > > > > char id[32];
> > > > > > > obj = sc->threads + i * size;
> > > > > > > + CPUState *cs;
> > > > > > >
> > > > > > > object_initialize(obj, size, typename);
> > > > > > > + cs = CPU(obj);
> > > > > > > +
> > > > > > > + /* Use core_id (which is actually cpu_dt_id) as stable
> > > > > > > CPU id */
> > > > > > > + if (cs->has_stable_cpu_id) {
> > > > > > > + cs->stable_cpu_id = cc->core_id + i;
> > > > > > > + }
> > > > > >
> > > > > > Testing cs->has_stable_cpu_id here in machine type specific code
> > > > > > seems
> > > > > > really weird. It's the machine type that knows whether it has a
> > > > > > stable ID to give to the CPU or not, rather than the other way
> > > > > > around.
> > > > > >
> > > > > > Since we haven't yet had a release with cpu cores, I think the right
> > > > > > thing is for cpu_core to unconditionally set the stable ID (and set
> > > > > > has_stable_id to true).
> > > > >
> > > > > Right, we can set cpu_stable_id unconditionally here since this code
> > > > > path
> > > > > (core realize) will be taken only for pseries-2.7 onwards.
> > > > > has_stable_id
> > > > > will get set as part of the property we defined in SPAPR_COMPAT_2_7.
> > > > >
> > > >
> > > > Hrm, that's true. But when you describe it like that it sounds like a
> > > > really non-obvious and fragile dependency between different components.
> > > >
> > > that's how compat stuff is typically done for devices,
> > > CPUs shouldn't be an exception.
> > > (consistency with other devices helps here in long run)
> > >
> > > > > > The backup path that does thread-based cpu
> > > > > > init, can set has_stable_id to false (if that's not the default).
> > > > > >
> > > > >
> > > > > Default is off, but turning it on for 2.7 will be inherited by 2.6
> > > > > and others below. Hence I have code to explicitly disable this prop
> > > > > for 2.6 and below via SPAPR_COMPAT_2_6.
> > > >
> > > > This is all seeming terribly awkward.
> > > Typically default is set the way so new machine type doesn't have
> > > to enable it explicitly.
> > >
> > > However the way it's done here helps not to touch/check every user
> > > that uses cpu_index, limiting series impact only to code that
> > > asks for it, it look a lot safer to got this rout for now.
> > >
> > >
> > > > Can we try investigating a
> > > > different approach:
> > > >
> > > > 1. Rename cpu_index to cpu_id, but it's still used in the same
> > > > places it's used.
> > > >
> > > > 2. Remove assumptions that cpu_id values are contiguous or
> > > > dense
> > > >
> > > > 3. Machine type decides whether it wants to populate the cpu_id
> > > > values explicitly, or leave it to generic code to calculate
> > > > them as cpu_index is calculated now.
> > > >
> > > > 4. Ideally, generic code enforces that the machine type populates
> > > > either all or none of the cpu_id values.
> > > >
> > > > Does that seem workable?
> > > Ideally we will get there some day (and may be get rid of cpu_index
> > > altogether),
> > > but for now it seems too invasive with a lot of chances to introduce non
> > > obvious
> > > regression.
> >
> > Yes, that's a risk. But I'm basically no longer convinced that it's
> > any higher than the risk of the same thing with the current approach.
> to me it looks much less risky as it affects only migration path and
> could be done in time for 2.7
>
> if we go to cpu_index re-factoring path then I'm afraid it quite a bit
> more complex and it's not a 2.7 material.
Yeah, ok. Having looked in a bit more depth, I agree with you.
>
>
> >
> > > So I'd keep approach used in this series.
> >
>
--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
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- Re: [Qemu-ppc] [RFC PATCH v2 3/5] spapr: Set stable_cpu_id for threads of CPU cores, (continued)
Re: [Qemu-ppc] [RFC PATCH v2 3/5] spapr: Set stable_cpu_id for threads of CPU cores, David Gibson, 2016/07/08
- Re: [Qemu-ppc] [RFC PATCH v2 3/5] spapr: Set stable_cpu_id for threads of CPU cores, Bharata B Rao, 2016/07/08
- Re: [Qemu-ppc] [RFC PATCH v2 3/5] spapr: Set stable_cpu_id for threads of CPU cores, David Gibson, 2016/07/08
- Re: [Qemu-ppc] [RFC PATCH v2 3/5] spapr: Set stable_cpu_id for threads of CPU cores, Igor Mammedov, 2016/07/08
- Re: [Qemu-ppc] [RFC PATCH v2 3/5] spapr: Set stable_cpu_id for threads of CPU cores, Bharata B Rao, 2016/07/10
- Re: [Qemu-ppc] [RFC PATCH v2 3/5] spapr: Set stable_cpu_id for threads of CPU cores, David Gibson, 2016/07/10
- Re: [Qemu-ppc] [Qemu-devel] [RFC PATCH v2 3/5] spapr: Set stable_cpu_id for threads of CPU cores, Igor Mammedov, 2016/07/11
- Re: [Qemu-ppc] [Qemu-devel] [RFC PATCH v2 3/5] spapr: Set stable_cpu_id for threads of CPU cores,
David Gibson <=
[Qemu-ppc] [RFC PATCH v2 5/5] spapr: Enable the use of stable_cpu_id from pseries-2.7 onwards, Bharata B Rao, 2016/07/07
[Qemu-ppc] [RFC PATCH v2 4/5] xics: Use stable_cpu_id instead of cpu_index in XICS code, Bharata B Rao, 2016/07/07
Re: [Qemu-ppc] [RFC PATCH v2 0/5] sPAPR: Fix migration when CPUs are removed in random order, Greg Kurz, 2016/07/07