Messages in this thread Patch in this message | | | From | Len Brown <> | Subject | [PATCH 1/1] x86 TSC: set X86_FEATURE_TSC_RELIABLE, per CPUID | Date | Sun, 31 May 2015 01:44:42 -0400 |
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From: Len Brown <len.brown@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.brown@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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