On Wed, 2015-01-07 at 11:20 +0100, Martin Schwidefsky wrote: > On Tue, 06 Jan 2015 13:44:01 +1100 > Cyril Bur <cyril...@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 <cyril...@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. > -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/