Date: Wed, 23 Mar 2016 12:54:10 +0000 From: bugzilla-noreply@freebsd.org To: freebsd-bugs@FreeBSD.org Subject: [Bug 208238] [Hyper-V] TSC frequency is not correctly detected: "calcru: runtime went backwards" Message-ID: <bug-208238-8@https.bugs.freebsd.org/bugzilla/>
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https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=3D208238 Bug ID: 208238 Summary: [Hyper-V] TSC frequency is not correctly detected: "calcru: runtime went backwards" Product: Base System Version: 11.0-CURRENT Hardware: Any OS: Any Status: New Severity: Affects Some People Priority: --- Component: kern Assignee: freebsd-bugs@FreeBSD.org Reporter: decui@microsoft.com I can easily get a large number of messages in the system log similar to th= is: # dmesg | grep backwards calcru: runtime went backwards from 5188 usec to 2727 usec for pid 770 (cro= n) calcru: runtime went backwards from 2944 usec to 1488 usec for pid 766 (sendmail) calcru: runtime went backwards from 2497 usec to 1262 usec for pid 766 (sendmail) It looks this is caused by the wrongly-detected TSC frequency and I made th= is patch to address this: https://github.com/dcui/freebsd/commit/07acbd8a3771adba9ac0fca807ec629ff05a= 807c " use hyper-v time counter (rather than i8254) to calibrate TSC The i8254 PIT counter emulated by Hyper-V is not reliable. In probe_tsc_freq() -> DELAY -> init_ops.early_delay() -> i8254_delay(), getit() can return these values when it is invoked 10 times: 1 pit count0 =3D 228 2 pit count0 =3D 131 3 pit count0 =3D 34 4 pit count0 =3D 65473 <-- this is a normal wrap-around. 5 pit count0 =3D 65375 6 pit count0 =3D 65278 7 pit count0 =3D 65180 8 pit count0 =3D 65388 <-- this is bad! 9 pit count0 =3D 65290 10 pit count0 =3D 65193 For the 8th time, the 'delta' in i8254_delay() is < 0 while it shouldn't, so the later "delta +=3D i8254_max_count;" and "ticks_left -=3D delta" will cause i8254_delay() to wait shorter than expected, and finally probe_tsc_freq() get a smaller 'tsc_freq'. A smaller 'tsc_freq' can cause time inaccuracy in dtrace. It can also cause warnings like calcru: runtime went backwards from 50 usec to 25 usec for pid 0 (kernel) calcru: runtime went backwards from 1471 usec to 743 usec for pid 0 (kernel) calcru: runtime went backwards from 40 usec to 20 usec for pid 0 (kernel) calcru: runtime went backwards from 18 usec to 9 usec for pid 0 (kernel) calcru: runtime went backwards from 46204978 usec to 23362331 usec for pid 0 (kernel) We use Hyper-V time counter, which is much more reliable than i8254, to calibrate TSC. " --=20 You are receiving this mail because: You are the assignee for the bug.=
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