From: Len Brown <len.brown@xxxxxxxxx>
Linux-4.9 added INTEL_FAM6_SKYLAKE_X to native_calibrate_tsc(),
allowing the kernel to calculate the TSC frequency,
instead of calibrating:
commit 6baf3d61821f
("x86/tsc: Add additional Intel CPU models to the crystal quirk list")
There are several problems with doing this.
The first is that while SKX servers use a 25 MHz crystal,
SKX workstations (with same model #) use a 24 MHz crystal.
This results in a -4.0% time drift rate on SKX workstations.
But even SKX servers have a problem.
SKX subjects the crystal to an EMI reduction circuit that
reduces its actual value by (approximately) -0.25%.
This results in -1 second per 10 minute time drift
as compared to network time on SKX servers.
This also results in an even more subtle symptom on systems
that use the LAPIC timer (versus the TSC deadline timer).
Clock ticks scheduled with the LAPIC timer arrive a few usec
before the time they are expected (according to the slow TSC).
This causes Linux to poll-idle, when it should be in an idle
power saving state. The idle and clock code do not graciously
recover from this error, sometimes resulting in significant polling
and significant power impact.
So stop using native_calibrate_tsc() for INTEL_FAM6_SKYLAKE_X.
native_calibrate_tsc() will return 0, boot will run with
tsc_khz = cpu_khz, and the TSC refined calibration will
update tsc_khz to correct for the difference.
Yes, a future patch can detect 24 vs 25 MHz crystals,
and it can estimate a correction for the EMI circuit impact.
However, that would effect boot-time only, as we'd probably
choose to keep the refined TSC calibration, because the EMI
correction estimate may not be precise.
Signed-off-by: Len Brown <len.brown@xxxxxxxxx>
Cc: Prarit Bhargava <prarit@xxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx> # v4.9+
---
arch/x86/kernel/tsc.c | 1 -
1 file changed, 1 deletion(-)
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 49d772672367..19ca0004caf6 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -602,7 +602,6 @@ unsigned long native_calibrate_tsc(void)
case INTEL_FAM6_KABYLAKE_DESKTOP:
crystal_khz = 24000; /* 24.0 MHz */
break;
- case INTEL_FAM6_SKYLAKE_X:
case INTEL_FAM6_ATOM_DENVERTON:
crystal_khz = 25000; /* 25.0 MHz */
break;
--
2.14.0-rc0