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

[Cyril Bur <cyrilbur@gmail.com>]

On Wed, 2015-01-07 at 11:20 +0100, Martin Schwidefsky wrote:
> On Tue, 06 Jan 2015 13:44:01 +1100
> Cyril Bur <cyrilbur@gmail.com> wrote:
> 
> > On Mon, 2015-01-05 at 14:10 -0800, Andrew Morton wrote:
> > > On Mon, 22 Dec 2014 16:06:04 +1100 Cyril Bur <cyrilbur@gmail.com> 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
> > > > sched_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. sched_clock is returned in this case.
> > > > 
> > > > --- a/arch/powerpc/kernel/time.c
> > > > +++ b/arch/powerpc/kernel/time.c
> > > > @@ -621,6 +621,30 @@ unsigned long long sched_clock(void)
> > > >  	return mulhdu(get_tb() - boot_tb, tb_to_ns_scale) << tb_to_ns_shift;
> > > >  }
> > > >  
> > > > +unsigned long long running_clock(void)
> > > 
> > > Non-kvm kernels don't need this code.  Is there some appropriate
> > > "#ifdef CONFIG_foo" we can wrap this in?
> > CONFIG_PSERIES would work, having said that typical compilation for a
> > powernv kernel almost always includes CONFIG_PSERIES (although it
> > doesn't need to)... still, your point is valid, will add in v2.
> > > 
> > > 
> > > > +{
> > > > +	/*
> > > > +	 * 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.
> > > > +	 */
> > > > +
> > > > +	if (firmware_has_feature(FW_FEATURE_LPAR)) {
> > > > +		if (cpu_has_feature(CPU_FTR_ARCH_207S))
> > > > +			return mulhdu(get_vtb() - boot_tb, tb_to_ns_scale) << tb_to_ns_shift;
> > > > +
> > > > +		/* This is a next best approximation without a VTB. */
> > > > +		return sched_clock() - cputime_to_nsecs(kcpustat_this_cpu->cpustat[CPUTIME_STEAL]);
> > > 
> > > Why is this result dependent on FW_FEATURE_LPAR?  It's all generic code.
> > Good point, the reason it ended up there is because I wanted to avoid
> > behaviour changes.
> > > 
> > > In fact the kernel/sched/clock.c default implementation of
> > > running_clock() could use this expression.  Would that be good or bad? :)
> > For power I'm almost certain it would be fine, on platforms which don't
> > do stolen time cpustat[CPUTIME_STEAL] should always be zero and if not
> > then the value should always be sane (although as mentioned in the
> > comment, not as accurate as using the VTB).
> > 
> > Putting it in the default implementation could cause behavioural changes
> > for x86 and s390, I would want their views on doing that.
> 
> I would prefer to make sched_clock do all the work. We have been thinking
> about steal time vs sched_clock as well, our solution would be to exchange
> the time source. Right now sched_clock is based on the TOD clock, the code
> that takes steal time into account would use the CPU timer instead.
> With the subtraction of kcpustat_this_cpu->cpustat[CPUTIME_STEAL] in
> common code we would have to add the same value in the sched_clock
> implementation as the steal time is already included in the CPU timer
> deltas.

Thanks for the quick reply Martin,
Sound like you've got ideas and while I didn't really grasp all of that,
I gather we best leave the common code as is.
>