Re: VDSO pvclock may increase host cpu consumption, is this a problem?

[Marcelo Tosatti <mtosatti@redhat.com>]

On Mon, Mar 31, 2014 at 10:33:41PM -0700, Andy Lutomirski wrote:
> On Mar 31, 2014 8:45 PM, "Marcelo Tosatti" <mtosatti@redhat.com> 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 ?

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).

> > 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 :-)