Re: [patch 3/3] PTP: add kvm PTP driver

From: Marcelo Tosatti
Date: Wed Jan 18 2017 - 07:26:13 EST

On Wed, Jan 18, 2017 at 10:17:38AM -0200, Marcelo Tosatti wrote:
> On Tue, Jan 17, 2017 at 04:36:21PM +0100, Radim Krcmar wrote:
> > 2017-01-17 09:30-0200, Marcelo Tosatti:
> > > On Tue, Jan 17, 2017 at 09:03:27AM +0100, Miroslav Lichvar wrote:
> > >> On Mon, Jan 16, 2017 at 06:01:14PM -0200, Marcelo Tosatti wrote:
> > >> > On Mon, Jan 16, 2017 at 05:47:15PM -0200, Marcelo Tosatti wrote:
> > >> > > On Mon, Jan 16, 2017 at 05:36:55PM -0200, Marcelo Tosatti wrote:
> > >> > > > Sorry, unless i am misunderstanding how this works, it'll get the guest clock
> > >> > > > 2us behind, which is something not wanted.
> > >> > > >
> > >> > > > Miroslav, if ->gettime64 returns the host realtime at 2us in the past,
> > >> > > > this means Chrony will sync the guest clock to
> > >> > > >
> > >> > > > host realtime - 2us
> > >> > > >
> > >> > > > Is that correct?
> > >>
> > >> Probably. It depends on the error of both host and guest timestamps.
> > >> If the error is the same on both sides, it will cancel out. An
> > >> occasional spike in the delay shouldn't be a problem as the reading
> > >> will be filtered out, but for best accuracy it's necessary that the
> > >> host's timestamp is taken in the middle between the guest's
> > >> timestamps.
> > >
> > > The problem is that spikes can be far from occasional: it depends on activity of
> > > the host CPU and interrupts. Whose delay can be "intermittent": as long
> > > as interrupts are being sent to the host CPU, for example, the delay
> > > will be high (which can last minutes).
> > >
> > > The TSC reading in the guest KVM PTP driver corrects for that delay.
> > >
> > >> Users of the PTP_SYS_OFFSET ioctl assume that (ts[0]+ts[2])/2
> > >> corresponds to ts[1], (ts[2]+ts[4])/2 corresponds to ts[3], and so on.
> > >>
> > >> ts[1] ts[3]
> > >> Host time ---------+---------+........
> > >> | |
> > >> | |
> > >> Guest time ----+---------+---------+......
> > >> ts[0] ts[2] ts[4]
> >
> > KVM PTP delay moves host ts[i] to be close to guest ts[i+1] and makes
> > the offset very consistent, so the graph would look like:
> >
> > ts[1] ts[3]
> > Host time -------------+---------+........
> > | |
> > | |
> > Guest time ----+---------+---------+......
> > ts[0] ts[2] ts[4]
> >
> > which doesn't sound good if users assume that the host reading is in the
> > middle -- the guest time would be ahead of the host time.
> >
> > I'm wondering why is the PTP precision around 10ns, when the hypercall
> > takes around 2-3k cycles. Have you measured the guest<->host offset by
> > getting the output of the hypercall, i.e.
> > {host_sec @ tsc, host_nsec @ tsc, tsc}
> > and comparing it with guest time computed from the same tsc, i.e.
> > {guest_sec @ tsc, guest_nsec @ tsc}
> > ?
> >
> > Thanks.
>
> Testcase: run a guest and a loop sending SIGUSR1 to vcpu0 (emulating
> intense interrupts). Follows results:
>
> Without TSC delta calculation:
> =============================
>
> #* PHC0 0 3 377 2 -99ns[ +206ns] +/- 116ns
> #* PHC0 0 3 377 8 +202ns[ +249ns] +/- 111ns
> #* PHC0 0 3 377 8 -213ns[ +683ns] +/- 88ns
> #* PHC0 0 3 377 6 +77ns[ +319ns] +/- 56ns
> #* PHC0 0 3 377 4 -771ns[-1029ns] +/- 93ns
> #* PHC0 0 3 377 10 -49ns[ -58ns] +/- 121ns
> #* PHC0 0 3 377 9 +562ns[ +703ns] +/- 107ns
> #* PHC0 0 3 377 6 -2ns[ -3ns] +/- 94ns
> #* PHC0 0 3 377 4 +451ns[ +494ns] +/- 138ns
> #* PHC0 0 3 377 11 -67ns[ -74ns] +/- 113ns
> #* PHC0 0 3 377 8 +244ns[ +264ns] +/- 119ns
> #* PHC0 0 3 377 7 -696ns[ -890ns] +/- 89ns
> #* PHC0 0 3 377 4 +468ns[ +560ns] +/- 110ns
> #* PHC0 0 3 377 11 -310ns[ -430ns] +/- 72ns
> #* PHC0 0 3 377 9 +189ns[ +298ns] +/- 54ns
> #* PHC0 0 3 377 7 +594ns[ +473ns] +/- 96ns
> #* PHC0 0 3 377 5 +151ns[ +280ns] +/- 71ns
> #* PHC0 0 3 377 10 -590ns[ -696ns] +/- 94ns
> #* PHC0 0 3 377 8 +415ns[ +526ns] +/- 74ns
> #* PHC0 0 3 377 6 +1381ns[+1469ns] +/- 101ns
> #* PHC0 0 3 377 4 +571ns[+1304ns] +/- 54ns
> #* PHC0 0 3 377 8 -5ns[ +71ns] +/- 139ns
> #* PHC0 0 3 377 7 -247ns[ -502ns] +/- 69ns
> #* PHC0 0 3 377 5 -283ns[ +879ns] +/- 73ns
> #* PHC0 0 3 377 3 +148ns[ -109ns] +/- 61ns
>
> With TSC delta calculation:
> ============================
>
> #* PHC0 0 3 377 7 +379ns[ +432ns] +/- 53ns
> #* PHC0 0 3 377 9 +106ns[ +420ns] +/- 42ns
> #* PHC0 0 3 377 7 -58ns[ -136ns] +/- 62ns
> #* PHC0 0 3 377 12 +93ns[ -38ns] +/- 64ns
> #* PHC0 0 3 377 8 +84ns[ +107ns] +/- 69ns
> #* PHC0 0 3 377 3 -76ns[ -103ns] +/- 52ns
> #* PHC0 0 3 377 7 +52ns[ +63ns] +/- 50ns
> #* PHC0 0 3 377 11 +29ns[ +31ns] +/- 70ns
> #* PHC0 0 3 377 7 -47ns[ -56ns] +/- 42ns
> #* PHC0 0 3 377 10 -35ns[ -42ns] +/- 33ns
> #* PHC0 0 3 377 7 -32ns[ -34ns] +/- 42ns
> #* PHC0 0 3 377 11 -172ns[ -173ns] +/- 118ns
> #* PHC0 0 3 377 6 +65ns[ +76ns] +/- 23ns
> #* PHC0 0 3 377 9 +18ns[ +23ns] +/- 37ns
> #* PHC0 0 3 377 6 +41ns[ -60ns] +/- 30ns
> #* PHC0 0 3 377 10 +39ns[ +183ns] +/- 42ns
> #* PHC0 0 3 377 6 +50ns[ +102ns] +/- 86ns
> #* PHC0 0 3 377 11 +50ns[ +75ns] +/- 52ns
> #* PHC0 0 3 377 6 +50ns[ +116ns] +/- 100ns
> #* PHC0 0 3 377 10 +46ns[ +65ns] +/- 79ns
> #* PHC0 0 3 377 7 -38ns[ -51ns] +/- 29ns
> #* PHC0 0 3 377 10 -11ns[ -12ns] +/- 32ns
> #* PHC0 0 3 377 7 -31ns[ -32ns] +/- 99ns
> #* PHC0 0 3 377 10 +222ns[ +238ns] +/- 58ns
> #* PHC0 0 3 377 6 +185ns[ +207ns] +/- 39ns
> #* PHC0 0 3 377 10 -392ns[ -394ns] +/- 118ns
> #* PHC0 0 3 377 6 -9ns[ -50ns] +/- 35ns
> #* PHC0 0 3 377 10 -346ns[ -355ns] +/- 111ns
>
>
> Do you still want to drop it in favour of simplicity?
>

This is the output of "chronyc sources". See section "Time sources"
of https://chrony.tuxfamily.org/doc/2.4/chronyc.html.