Re: [PATCH v2 2/2] powerpc: add running_clock for powerpc to prevent spurious softlockup warnings

[Cyril Bur <cyrilbur@gmail.com>]

On Wed, 2015-02-04 at 11:42 +0100, Paul Bolle wrote:
> On Fri, 2015-01-09 at 14:34 +1100, Cyril Bur wrote:
> > On POWER8 virtualised kernels the VTB register can be read to have a view of
> > time that only increases while the guest is running. This will prevent guests
> > from seeing time jump if a guest is paused for significant amounts of time.
> > 
> > On POWER7 and below virtualised kernels stolen time is subtracted from
> > local_clock as a best effort approximation. This will not eliminate spurious
> > warnings in the case of a suspended guest but may reduce the occurance in the
> > case of softlockups due to host over commit.
> > 
> > Bare metal kernels should avoid reading the VTB as KVM does not restore sane
> > values when not executing, the approxmation is fine as host kernels won't
> > observe any stolen time.
> > 
> > Signed-off-by: Cyril Bur <cyrilbur@gmail.com>
> > ---
> > V2:
> >    Replaced the use of sched_clock_with local_clock it was used originally in
> > the softlockup detector.
> >    Added #ifdef CONFIG_PPC_PSERIES and optimised the non lpar + vtb cases.
> 
> This became commit 3e5aba51e929 ("powerpc: add running_clock for powerpc
> to prevent spurious softlockup warnings") in today's linux-next (ie,
> next-20150204). I noticed because a script I use to check linux-next
> spotted a trivial issues with it.
> 
> > ---
> >  arch/powerpc/kernel/time.c | 32 ++++++++++++++++++++++++++++++++
> >  1 file changed, 32 insertions(+)
> > 
> > diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
> > index fa7c4f1..fd35e5b 100644
> > --- a/arch/powerpc/kernel/time.c
> > +++ b/arch/powerpc/kernel/time.c
> > @@ -621,6 +621,38 @@ unsigned long long sched_clock(void)
> >  	return mulhdu(get_tb() - boot_tb, tb_to_ns_scale) << tb_to_ns_shift;
> >  }
> >  
> > +
> > +#ifdef CONFIG_PPC_PSERIES
> > +
> > +/*
> > + * Running clock - attempts to give a view of time passing for a virtualised
> > + * kernels.
> > + * Uses the VTB register if available otherwise a next best guess.
> > + */
> > +unsigned long long running_clock(void)
> > +{
> > +	/*
> > +	 * Don't read the VTB as a host since KVM does not switch in host timebase
> > +	 * into the VTB when it takes a guest off the CPU, reading the VTB would
> > +	 * result in reading 'last switched out' guest VTB.
> > +	 *
> > +	 * Host kernels are often compiled with CONFIG_PSERIES checked, it would be
> 
> You obviously wanted to use CONFIG_PPC_PSERIES here.
> 
Yep, sorry.

> Should I submit a trivial patch to fix that typo or do you prefer to do
> that yourself?
> 
Indeed a trivial fixup, I've pasted the hunk below. Andrew are you ok to
fold it in?

Thanks,

Cyril


diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index fd35e5b..770dc16 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -636,8 +636,8 @@ unsigned long long running_clock(void)
         * into the VTB when it takes a guest off the CPU, reading the VTB would
         * result in reading 'last switched out' guest VTB.
         *
-        * Host kernels are often compiled with CONFIG_PSERIES checked, it would be
-        * unsafe to rely only on the #ifdef above.
+        * Host kernels are often compiled with CONFIG_PPC_PSERIES checked, it
+        * would be unsafe to rely only on the #ifdef above.
         */
        if (firmware_has_feature(FW_FEATURE_LPAR) &&
            cpu_has_feature(CPU_FTR_ARCH_207S))
> Thanks,
> 
> 
> Paul Bolle
>