From 2207def700f902f169fc237b717252c326f9e464 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Tue, 5 Jun 2018 14:00:11 +0200 Subject: [PATCH] x86/apic: Ignore secondary threads if nosmt=force MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit nosmt on the kernel command line merely prevents the onlining of the secondary SMT siblings. nosmt=force makes the APIC detection code ignore the secondary SMT siblings completely, so they even do not show up as possible CPUs. That reduces the amount of memory allocations for per cpu variables and saves other resources from being allocated too large. This is not fully equivalent to disabling SMT in the BIOS because the low level SMT enabling in the BIOS can result in partitioning of resources between the siblings, which is not undone by just ignoring them. Some CPUs can use the full resources when their sibling is not onlined, but this is depending on the CPU family and model and it's not well documented whether this applies to all partitioned resources. That means depending on the workload disabling SMT in the BIOS might result in better performance. Linus analysis of the Intel manual: The intel optimization manual is not very clear on what the partitioning rules are. I find: "In general, the buffers for staging instructions between major pipe stages are partitioned. These buffers include µop queues after the execution trace cache, the queues after the register rename stage, the reorder buffer which stages instructions for retirement, and the load and store buffers. In the case of load and store buffers, partitioning also provided an easier implementation to maintain memory ordering for each logical processor and detect memory ordering violations" but some of that partitioning may be relaxed if the HT thread is "not active": "In Intel microarchitecture code name Sandy Bridge, the micro-op queue is statically partitioned to provide 28 entries for each logical processor, irrespective of software executing in single thread or multiple threads. If one logical processor is not active in Intel microarchitecture code name Ivy Bridge, then a single thread executing on that processor core can use the 56 entries in the micro-op queue" but I do not know what "not active" means, and how dynamic it is. Some of that partitioning may be entirely static and depend on the early BIOS disabling of HT, and even if we park the cores, the resources will just be wasted. Signed-off-by: Thomas Gleixner Reviewed-by: Konrad Rzeszutek Wilk Acked-by: Ingo Molnar --- arch/x86/include/asm/apic.h | 2 ++ arch/x86/kernel/acpi/boot.c | 3 ++- arch/x86/kernel/apic/apic.c | 19 +++++++++++++++++++ 3 files changed, 23 insertions(+), 1 deletion(-) diff --git a/arch/x86/include/asm/apic.h b/arch/x86/include/asm/apic.h index 9362a3aae927..ef40e4acd7c2 100644 --- a/arch/x86/include/asm/apic.h +++ b/arch/x86/include/asm/apic.h @@ -504,8 +504,10 @@ extern int default_check_phys_apicid_present(int phys_apicid); #ifdef CONFIG_SMP bool apic_id_is_primary_thread(unsigned int id); +bool apic_id_disabled(unsigned int id); #else static inline bool apic_id_is_primary_thread(unsigned int id) { return false; } +static inline bool apic_id_disabled(unsigned int id) { return false; } #endif extern void irq_enter(void); diff --git a/arch/x86/kernel/acpi/boot.c b/arch/x86/kernel/acpi/boot.c index 3b20607d581b..db4c118a7e4d 100644 --- a/arch/x86/kernel/acpi/boot.c +++ b/arch/x86/kernel/acpi/boot.c @@ -181,7 +181,8 @@ static int acpi_register_lapic(int id, u32 acpiid, u8 enabled) } if (!enabled) { - ++disabled_cpus; + if (!apic_id_disabled(id)) + ++disabled_cpus; return -EINVAL; } diff --git a/arch/x86/kernel/apic/apic.c b/arch/x86/kernel/apic/apic.c index 8703caa9d6db..b86091add294 100644 --- a/arch/x86/kernel/apic/apic.c +++ b/arch/x86/kernel/apic/apic.c @@ -2204,6 +2204,16 @@ bool apic_id_is_primary_thread(unsigned int apicid) return !(apicid & mask); } +/** + * apic_id_disabled - Check whether APIC ID is disabled via SMT control + * @id: APIC ID to check + */ +bool apic_id_disabled(unsigned int id) +{ + return (cpu_smt_control == CPU_SMT_FORCE_DISABLED && + !apic_id_is_primary_thread(id)); +} + /* * Should use this API to allocate logical CPU IDs to keep nr_logical_cpuids * and cpuid_to_apicid[] synchronized. @@ -2299,6 +2309,15 @@ int generic_processor_info(int apicid, int version) return -EINVAL; } + /* + * If SMT is force disabled and the APIC ID belongs to + * a secondary thread, ignore it. + */ + if (apic_id_disabled(apicid)) { + pr_info_once("Ignoring secondary SMT threads\n"); + return -EINVAL; + } + if (apicid == boot_cpu_physical_apicid) { /* * x86_bios_cpu_apicid is required to have processors listed -- GitLab