On Tue, Apr 01, 2014 at 05:46:34PM -0700, Andy Lutomirski wrote: > On Tue, Apr 1, 2014 at 5:29 PM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > > On Tue, Apr 01, 2014 at 12:17:16PM -0700, Andy Lutomirski wrote: > >> On Tue, Apr 1, 2014 at 11:01 AM, Marcelo Tosatti <mtosatti@xxxxxxxxxx> wrote: > >> > On Mon, Mar 31, 2014 at 10:33:41PM -0700, Andy Lutomirski wrote: > >> >> On Mar 31, 2014 8:45 PM, "Marcelo Tosatti" <mtosatti@xxxxxxxxxx> wrote: > >> >> > > >> >> > On Mon, Mar 31, 2014 at 10:52:25AM -0700, Andy Lutomirski wrote: > >> >> > > On 03/29/2014 01:47 AM, Zhanghailiang wrote: > >> >> > > > Hi, > >> >> > > > I found when Guest is idle, VDSO pvclock may increase host consumption. > >> >> > > > We can calcutate as follow, Correct me if I am wrong. > >> >> > > > (Host)250 * update_pvclock_gtod = 1500 * gettimeofday(Guest) > >> >> > > > In Host, VDSO pvclock introduce a notifier chain, pvclock_gtod_chain in timekeeping.c. It consume nearly 900 cycles per call. So in consideration of 250 Hz, it may consume 225,000 cycles per second, even no VM is created. > >> >> > > > In Guest, gettimeofday consumes 220 cycles per call with VDSO pvclock. If the no-kvmclock-vsyscall is configured, gettimeofday consumes 370 cycles per call. The feature decrease 150 cycles consumption per call. > >> >> > > > When call gettimeofday 1500 times,it decrease 225,000 cycles,equal to the host consumption. > >> >> > > > Both Host and Guest is linux-3.13.6. > >> >> > > > So, whether the host cpu consumption is a problem? > >> >> > > > >> >> > > Does pvclock serve any real purpose on systems with fully-functional > >> >> > > TSCs? The x86 guest implementation is awful, so it's about 2x slower > >> >> > > than TSC. It could be improved a lot, but I'm not sure I understand why > >> >> > > it exists in the first place. > >> >> > > >> >> > VM migration. > >> >> > >> >> Why does that need percpu stuff? Wouldn't it be sufficient to > >> >> interrupt all CPUs (or at least all cpus running in userspace) on > >> >> migration and update the normal timing data structures? > >> > > >> > Are you suggesting to allow interruption of the timekeeping code > >> > at any time to update frequency information ? > >> > >> I'm not sure what you mean by "interruption of the timekeeping code". > >> I'm suggesting sending an interrupt to the guest (via a virtio device, > >> presumably) to tell it that it has been paused and resumed. > >> > >> This is probably worth getting John's input if you actually want to do > >> this. I'm not about to :) > > > > Honestly, neither am i at the moment. But i'll think about it. > > > >> Is there any case in which the TSC is stable and the kvmclock data for > >> different cpus is actually different? > > > > No. However, kvmclock_data.flags field is an interface for watchdog > > unpause. > > > >> > Do you want to that as a special tsc clocksource driver ? > >> > > >> >> Even better: have the VM offer to invalidate the physical page > >> >> containing the kernel's clock data on migration and interrupt one CPU. > >> >> If another CPU races, it'll fault and wait for the guest kernel to > >> >> update its timing. > >> > > >> > Perhaps that is a good idea. > >> > > >> >> Does the current kvmclock stuff track CLOCK_MONOTONIC and > >> >> CLOCK_REALTIME separately? > >> > > >> > No. kvmclock counting is interrupted on vm pause (the "hw" clock does not > >> > count during vm pause). > >> > >> Makes sense. > >> > >> > > >> >> > Can you explain why you consider it so bad ? How you think it could be > >> >> > improved ? > >> >> > >> >> The second rdtsc_barrier looks unnecessary. Even better, if rdtscp is > >> >> available, then rdtscp can replace rdtsc_barrier, rdtsc, and the > >> >> getcpu call. > >> >> > >> >> It would also be nice to avoid having two sets of rescalings of the timing data. > >> > > >> > Yep, probably good improvements, patches are welcome :-) > >> > > >> > >> I may get to it at some point. No guarantees. I did just rewrite all > >> the mapping-related code for every other x86 vdso timesource, so maybe > >> I should try to add this to the pile. The fact that the data is a > >> variable number of pages makes it messy, though, and since I don't > >> understand why there's a separate structure for each CPU, I'm hesitant > >> to change it too much. > >> > >> --Andy > > > > kvmclock.data? Because each VCPU can have different .flags fields for > > example. > > It looks like the vdso kvmclock code only runs if > PVCLOCK_TSC_STABLE_BIT is set, which in turn is only the case if the > TSC is guaranteed to be monotonic across all CPUs. If we can rely on > the fact that that bit will only be set if tsc_to_system_mul and > tsc_shift are the same on all CPUs and that (system_time - > (tsc_timestamp * mul) >> shift) is the same on all CPUs, then there > should be no reason for the vdso to read the pvclock data for anything > but CPU 0. That will make it a lot faster and simpler. > > Can we rely on that? In theory yes, but you would have to handle PVCLOCK_TSC_STABLE_BIT set -> PVCLOCK_TSC_STABLE_BIT not set Transition (and the other way around as well). > I wonder what happens if the guest runs ntpd or otherwise uses > adjtimex. Presumably it starts drifting relative to the host. It should use ntpd and adjtimex. KVMCLOCK is the "hw" clock, the values returned by CLOCK_REALTIME and CLOCK_GETTIME are built by the Linux guest timekeeping subsystem on top of the "hw" clock. -- To unsubscribe from this list: send the line "unsubscribe kvm" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
