On Thu, 21 Mar 2019, Bruce Evans wrote:
On Wed, 20 Mar 2019, Mark Johnston wrote:
On Wed, Mar 20, 2019 at 08:20:18PM -0700, Rodney W. Grimes wrote:
Author: markj
Date: Thu Mar 21 02:52:22 2019
New Revision: 345359
URL: https://svnweb.freebsd.org/changeset/base/345359
Log:
Don't attempt to measure TSC skew when running as a VM guest.
It simply doesn't work in general since VCPUs may migrate between
physical cores. The approach used to measure skew also doesn't
make much sense in a VM.
I think it simply doesn't work.
[... lots]
I don't normally use dtrace, but built a kernel with it to test this, and
got the expected garbage for skew values on SandyBridge and Haswell systems.
First, the build was broken on at least i386:
- i386 NOTES only has KTRACE_HOOKS. All other dtrace options (devices) are
commented out. This defeats the reason for existence of NOTES. It is
not to supply documenation, but to test everything except negative options
using LINT.
- since dtrace was not tested in LINT, of course it doesn't compile
- "device dtrace" doesn't compile, since it depends on the "optional"
"module" "device fbt" for 1 symbol
- "device fbt" doesn't compile, since kern.pre.mk has an invalid -I path
for it, and at least gcc considers invalid -I paths as errors.
Fix for the -I path:
XX Index: kern.pre.mk
XX ===================================================================
XX --- kern.pre.mk (revision 345315)
XX +++ kern.pre.mk (working copy)
XX @@ -202,9 +202,13 @@
XX DTRACE_S= ${CC} -c ${DTRACE_ASM_CFLAGS} ${WERROR} ${.IMPSRC}
XX
XX # Special flags for managing the compat compiles for DTrace/FBT
XX -FBT_CFLAGS= -DBUILDING_DTRACE -nostdinc
-I$S/cddl/dev/fbt/${MACHINE_CPUARCH} -I$S/cddl/dev/fbt
-I$S/cddl/compat/opensolaris -I$S/cddl/contrib/opensolaris/uts/common -I$S
${CDDL_CFLAGS}
XX +FBT_CFLAGS= -DBUILDING_DTRACE -nostdinc -I$S/cddl/dev/fbt \
XX + -I$S/cddl/compat/opensolaris \
XX + -I$S/cddl/contrib/opensolaris/uts/common -I$S ${CDDL_CFLAGS}
XX .if ${MACHINE_CPUARCH} == "amd64" || ${MACHINE_CPUARCH} == "i386"
XX FBT_CFLAGS+= -I$S/cddl/dev/fbt/x86
XX +.else
XX +FBT_CFLAGS+= -I$S/cddl/dev/fbt/${MACHINE_CPUARCH}
XX .endif
XX FBT_C= ${CC} -c ${FBT_CFLAGS} ${WERROR} ${PROF}
${.IMPSRC}
XX
This also fixes some style bugs (a too-long line). kern.pre.mk has about 12
other too-long lines, with about half of these for cddl.
Perhaps the order of the -I paths matters, but this worked for me.
I once eliminated almost all -I paths in kernel Makefiles. There are about
100 times as many now as before I removed some :-(. At least the -I maze
for FBT doesn't affect other subsystems.
Results of calculating skews:
On SandyBridge i5-2560M 2 cores * 2 threads:
boot1 boot2 boot3 boot4
----- ----- ----- -----
tsc_skew[0] = 0 0 0 0
tsc_skew[1] = 0x40 0x58 0x5c 0x8c
tsc_skew[2] = 0x68 0x68 0x7c 0x7c
tsc_skew[3] = 0x118 0x98 0x104 0x104
On Haswell i7-4790K 4 cores * 2 threads:
boot1 boot2 boot3 boot4
----- ----- ----- -----
tsc_skew[0] = 0 0 0 0
tsc_skew[1] = 0x40 0x40 0x40 0x30
tsc_skew[2] = 0x40 -0x40 0x54 0x5c
tsc_skew[3] = 0xf0 0xf4 0x74 0x58
tsc_skew[4] = 0xdc 0x60 0x6c 0x138
tsc_skew[5] = 0x38 0x6c 0x38 0x38
tsc_skew[6] = 0x4c 0x38 0xe0 0xb0
tsc_skew[7] = 0x3c 0x3c -0x38 0x38
This looks like IPC delays with jitter from sloppy measurements. Nothing
to do with skews, except it approximates 0.
Bruce
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