From: Len Brown <len.br...@intel.com>
Speed cpu_up() by believing CPUID's "invariant TSC" flag,
and skipping the TSC warp test on single socket systems.
On my desktop, cpu_up() duration drops from 4.4ms to 2.4ms.
That savings is per-CPU, and occurs during boot, online
and resume from S3.
Originally, this was done for all sized systems, in Feb-2009
by 83ce40092868 (x86: set X86_FEATURE_TSC_RELIABLE).
But in Dec-2009, it was disabled for all sized systems by
6c56ccecf05f (x86: Reenable TSC sync check at boot, even with NONSTOP_TSC)
due to discovery of a TSC time warp on a multi-node system.
Here we re-enable this optimization for single-socket systems.
Multi-node systems will still run the sanity test.
"tsc=reliable" is still available to tell them to not bother.
Perhaps in the future we can up the node count to 2, or perhaps 4,
using dmi_check_multi[] to workaround bugs, per the original design.
Signed-off-by: Len Brown <len.br...@intel.com>
---
arch/x86/kernel/cpu/intel.c | 21 +++++++++++++++------
arch/x86/kernel/tsc.c | 2 +-
2 files changed, 16 insertions(+), 7 deletions(-)
diff --git a/arch/x86/kernel/cpu/intel.c b/arch/x86/kernel/cpu/intel.c
index 50163fa..aa00390 100644
--- a/arch/x86/kernel/cpu/intel.c
+++ b/arch/x86/kernel/cpu/intel.c
@@ -79,17 +79,26 @@ static void early_init_intel(struct cpuinfo_x86 *c)
c->x86_phys_bits = 36;
/*
- * c->x86_power is 8000_0007 edx. Bit 8 is TSC runs at constant rate
- * with P/T states and does not stop in deep C-states.
- *
- * It is also reliable across cores and sockets. (but not across
- * cabinets - we turn it off in that case explicitly.)
+ * c->x86_power is CPUID(0x80000007).EDX
+ * Bit 8 describes the "invariant TSC" feature, where the TSC
+ * continues running at a constant rate across P/T/C states.
+ * Linux X86_FEATURE_CONSTANT_TSC describes the constant rate.
+ * Linux X86_FEATURE_NONSTOP_TSC describes not stopping in C-states.
+ * Linux X86_FEATURE_TSC_RELIABLE means that Linux should skip
+ * TSC sanity checks because we know the TSC is sufficient for
+ * clocksource. This was invented for VMM use, but can also
+ * be used to skip sanity check cost on bare metal.
+ * We set it here, but the TSC code currently tests anyway
+ * on multi-node systems, out of paranoia.
+ * dmi_check_multi[] can also be used to clear this flag.
*/
if (c->x86_power & (1 << 8)) {
set_cpu_cap(c, X86_FEATURE_CONSTANT_TSC);
set_cpu_cap(c, X86_FEATURE_NONSTOP_TSC);
- if (!check_tsc_unstable())
+ if (!check_tsc_unstable()) {
set_sched_clock_stable();
+ set_cpu_cap(c, X86_FEATURE_TSC_RELIABLE);
+ }
}
/* Penwell and Cloverview have the TSC which doesn't sleep on S3 */
diff --git a/arch/x86/kernel/tsc.c b/arch/x86/kernel/tsc.c
index 5054497..f1a25e5 100644
--- a/arch/x86/kernel/tsc.c
+++ b/arch/x86/kernel/tsc.c
@@ -1014,7 +1014,7 @@ static void __init check_system_tsc_reliable(void)
if (res_low & RTSC_SUSP)
tsc_clocksource_reliable = 1;
#endif
- if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE))
+ if (boot_cpu_has(X86_FEATURE_TSC_RELIABLE) && (nr_node_ids == 1))
tsc_clocksource_reliable = 1;
}
--
2.4.1.314.g9532ead
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