Re: [PATCH 2/3] powerpc/spinlock: support vcpu preempted check

[xinhui]

On 2016å06æ27æ 22:58, Boqun Feng wrote:
> Hi Xinhui,
>
> On Mon, Jun 27, 2016 at 01:41:29PM -0400, Pan Xinhui wrote:
> > This is to fix some holder preemption issues. Spinning at one
> > vcpu which is preempted is meaningless.
> >
> > Kernel need such interfaces, So lets support it.
> >
> > We also should suooprt both the shared and dedicated mode.
> > So add lppaca_dedicated_proc macro in lppaca.h
> >
> > Suggested-by: Boqun Feng <boqun.feng@xxxxxxxxx>
> > Signed-off-by: Pan Xinhui <xinhui.pan@xxxxxxxxxxxxxxxxxx>
> > ---
> >   arch/powerpc/include/asm/lppaca.h   |  6 ++++++
> >   arch/powerpc/include/asm/spinlock.h | 15 +++++++++++++++
> >   2 files changed, 21 insertions(+)
> >
> > diff --git a/arch/powerpc/include/asm/lppaca.h b/arch/powerpc/include/asm/lppaca.h
> > index d0a2a2f..0a263d3 100644
> > --- a/arch/powerpc/include/asm/lppaca.h
> > +++ b/arch/powerpc/include/asm/lppaca.h
> > @@ -111,12 +111,18 @@ extern struct lppaca lppaca[];
> >    * we will have to transition to something better.
> >    */
> >   #define LPPACA_OLD_SHARED_PROC		2
> > +#define LPPACA_OLD_DEDICATED_PROC      (1 << 6)
>
> I think you should describe a little bit about the magic number here,
right.

> i.e. what document/specification says this should work, and how this
> works.
yep, I need add some comments here. for example, this bit is firmware reserved...
thanks, will do that.

> >   static inline bool lppaca_shared_proc(struct lppaca *l)
> >   {
> >   	return !!(l->__old_status & LPPACA_OLD_SHARED_PROC);
> >   }
> >
> > +static inline bool lppaca_dedicated_proc(struct lppaca *l)
> > +{
> > +	return !!(l->__old_status & LPPACA_OLD_DEDICATED_PROC);
> > +}
> > +
> >   /*
> >    * SLB shadow buffer structure as defined in the PAPR.  The save_area
> >    * contains adjacent ESID and VSID pairs for each shadowed SLB.  The
> > diff --git a/arch/powerpc/include/asm/spinlock.h b/arch/powerpc/include/asm/spinlock.h
> > index 523673d..ae938ee 100644
> > --- a/arch/powerpc/include/asm/spinlock.h
> > +++ b/arch/powerpc/include/asm/spinlock.h
> > @@ -52,6 +52,21 @@
> >   #define SYNC_IO
> >   #endif
> >
> > +/* For fixing some spinning issues in a guest.
> > + * kernel would check if vcpu is preempted during a spin loop.
> > + * we support that.
> > + */
> > +#define arch_vcpu_is_preempted arch_vcpu_is_preempted
> > +static inline bool arch_vcpu_is_preempted(int cpu)
>
> This function should be guarded by #ifdef PPC_PSERIES .. #endif, right?
> Because if the kernel is not compiled with guest support,
> vcpu_is_preempted() should always be false, right?
oh, I forgot that. thanks for pointing it out.

> > +{
> > +	struct lppaca *lp = &lppaca_of(cpu);
> > +
> > +	if (unlikely(!(lppaca_shared_proc(lp) ||
> > +			lppaca_dedicated_proc(lp))))
>
> Do you want to detect whether we are running in a guest(ie. pseries
> kernel) here? Then I wonder whether "machine_is(pseries)" works here.
I tried as you said yesterday. but .h file has dependencies.
As you said, if we add #ifdef PPC_PSERIES, this is not a big problem. only powernv will be affected as they are built into same kernel img.

> Regards,
> Boqun
>
> > +		return false;
> > +	return !!(be32_to_cpu(lp->yield_count) & 1);
> > +}
> > +
> >   static __always_inline int arch_spin_value_unlocked(arch_spinlock_t lock)
> >   {
> >   	return lock.slock == 0;
> > --
> > 2.4.11