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

From: Paolo Bonzini
Date: Wed Jan 18 2017 - 07:47:05 EST



On 18/01/2017 13:24, Marcelo Tosatti wrote:
> 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:
>>>>> 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.
>>
>> 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.

It's just that it's not obvious why you get better results with biased
host timestamps. What makes the biased host timestamp more precise?

I'd rather use PTP_SYS_OFFSET_PRECISE instead, but unfortunately chrony
does not support it---but I would still prefer you to support
PTP_SYS_OFFSET_PRECISE as well.

Paolo