Re: [PATCH] x86, Clean up smp_num_siblings calculation

Prarit Bhargava
2014-06-19T15:28:43Z

> On 14-06-02 07:51 AM, Prarit Bhargava wrote:
>> I have a system on which I have disabled threading in the BIOS, and I am booting
>> the kernel with the option "idle=poll".
>>
>> The kernel displays
>>
>> process: WARNING: polling idle and HT enabled, performance may degrade
>>
>> which is incorrect -- I've already disabled HT.
>>
>> This warning is issued here:
>>
>> void select_idle_routine(const struct cpuinfo_x86 *c)
>> {
>> if (boot_option_idle_override == IDLE_POLL && smp_num_siblings > 1)
>> pr_warn_once("WARNING: polling idle and HT enabled, performance may degrade\n");
>>
>> From my understanding of the other areas of kernel that use smp_num_siblings,
>> the value is supposed to be the actual number of threads per core, and
>> this value of smp_num_siblings is incorrect. In theory, it should be 1 but it
>> is reported as 2. When I looked into how smp_num_siblings is calculated I
>> found the following call sequence in the kernel:
>>
>> start_kernel ->
>> check_bugs ->
>> identify_boot_cpu ->
>> identify_cpu ->
>> c_init = init_intel
>> init_intel ->
>> detect_extended_topology
>> (sets value)
>>
>> OR
>>
>> c_init = init_amd
>> init_amd -> amd_detect_cmp
>> -> amd_get_topology
>> (sets value)
>> -> detect_ht()
>> ... (sets value)
>> detect_ht()
>> (also sets value)
>>
>> ie) it is set three times in some cases and overwritten in other cases.
>>
>> It should be noted that nothing in the identify_cpu() path or the cpu_up()
>> path requires smp_num_siblings to be set, prior to the final call to
>> detect_ht().
>>
>> For x86 boxes without X86_FEATURE_XTOPOLOGY, smp_num_siblings is set to a
>> value read in a CPUID call in detect_ht(). This value is the *factory
>> defined* value in all cases; even if HT is disabled in BIOS the value
>> still returns 2 if the CPU supports HT. AMD also reports the factory
>> defined value in all cases.
>>
>> For Intel x86 boxes with X86_FEATURE_XTOPOLOGY, smp_num_siblings is set to a
>> value read from the 0xb leaf of CPUID. This value is also the *factory
>> defined* value in all cases.
>>
>> For new-ish AMD x86 boxes, smp_num_siblings is also set to the *factory*
>> defined value.
>>
>> That is, even with threading disabled in BIOSes on these systems,
>>
>> crash> p smp_num_siblings
>> smp_num_siblings = $1 = 0x2
>>
>> smp_num_siblings should be calculated a single time on cpu 0 to determine
>> whether or not the system is multi-threaded or not. We can easily do
>> this by examining the boot cpu's cpu_sibling_mask after the mask has been
>> setup in the boot up code path.
>>
>> After the patch, on a system with HT enabled,
>>
>> crash> p smp_num_siblings
>> smp_num_siblings = $1 = 0x2
>>
>> On a system with HT disabled,
>>
>> crash> p smp_num_siblings
>> smp_num_siblings = $1 = 0x1
>>
>> Other uses of smp_num_siblings involve oprofile (used after boot), and
>> the perf code which is done well after the initial cpus are brought online.
>>
>> [v2]: After comment from Oren Twaig, rework to single patch.
>> Unfortunately there was no easy way to take into account the various
>> settings of smp_num_siblings and fix it in two patches.
>>
>> Cc: Oren Twaig <oren@scalemp.com>
>> Cc: Thomas Gleixner <tglx@linutronix.de>
>> Cc: Ingo Molnar <mingo@redhat.com>
>> Cc: "H. Peter Anvin" <hpa@zytor.com>
>> Cc: x86@kernel.org
>> Cc: Borislav Petkov <bp@suse.de>
>> Cc: Paul Gortmaker <paul.gortmaker@windriver.com>
>> Cc: Andrew Morton <akpm@linux-foundation.org>
>> Cc: Andi Kleen <ak@linux.intel.com>
>> Cc: Dave Jones <davej@redhat.com>
>> Cc: Torsten Kaiser <just.for.lkml@googlemail.com>
>> Cc: Jan Beulich <JBeulich@suse.com>
>> Cc: Jan Kiszka <jan.kiszka@siemens.com>
>> Cc: Toshi Kani <toshi.kani@hp.com>
>> Cc: Andrew Jones <drjones@redhat.com>
>> ---
>> arch/x86/kernel/cpu/amd.c | 1 -
>> arch/x86/kernel/cpu/common.c | 23 +++++++++++------------
>> arch/x86/kernel/cpu/topology.c | 2 +-
>> arch/x86/kernel/smpboot.c | 10 +++++++---
>> 4 files changed, 19 insertions(+), 17 deletions(-)
>>
>> diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
>> index ce8b8ff..6aca2b6 100644
>> --- a/arch/x86/kernel/cpu/amd.c
>> +++ b/arch/x86/kernel/cpu/amd.c
>> @@ -304,7 +304,6 @@ static void amd_get_topology(struct cpuinfo_x86 *c)
>> node_id = ecx & 7;
>>
>> /* get compute unit information */
>> - smp_num_siblings = ((ebx >> 8) & 3) + 1;
>> c->compute_unit_id = ebx & 0xff;
>> cores_per_cu += ((ebx >> 8) & 3);
>> } else if (cpu_has(c, X86_FEATURE_NODEID_MSR)) {
>> diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
>> index a135239..81a5aac 100644
>> --- a/arch/x86/kernel/cpu/common.c
>> +++ b/arch/x86/kernel/cpu/common.c
>> @@ -507,42 +507,41 @@ void detect_ht(struct cpuinfo_x86 *c)
>> u32 eax, ebx, ecx, edx;
>> int index_msb, core_bits;
>> static bool printed;
>> + int threads_per_core;
>>
>> if (!cpu_has(c, X86_FEATURE_HT))
>> return;
>>
>> - if (cpu_has(c, X86_FEATURE_CMP_LEGACY))
>> + if (cpu_has(c, X86_FEATURE_CMP_LEGACY)) {
>> + threads_per_core = 1;
>> goto out;
>> + }
>>
>> if (cpu_has(c, X86_FEATURE_XTOPOLOGY))
>> return;
>>
>> cpuid(1, &eax, &ebx, &ecx, &edx);
>>
>> - smp_num_siblings = (ebx & 0xff0000) >> 16;
>> + threads_per_core = (ebx & 0xff0000) >> 16;
>
> I wonder if this code is in need of an update? I recall reading
> this thread:
>
> http://forum.osdev.org/viewtopic.php?f=1&t=23445
>
> which suggests that we try CPUID with 0xb, and then 0x4 _before_
> relying on the EBX[23:16] of the older CPUID 0x1.
>
> AFAICT, the 0xb and 0x4 didn't exist when AP-485 was written ~2002.
>
> http://datasheets.chipdb.org/Intel/x86/CPUID/24161821.pdf
>

Paul,

Sorry I didn't get back to this as soon as I wanted to.

If the processor has X86_FEATURE_XTOPOLOGY, then we look at the extended leaf
0xb in detect_extended_topology(). Otherwise the code looks at the older CPUID
read below, so I think we're okay as is.

> Also, there was a discussion of masking the "ht" flag in /proc/cpuinfo
> for when it is "off" -- since the common sense interpretation of it
> doesn't match the implementation in the specification:

I'm implementing and testing this now.

P.

>
> http://codemonkey.org.uk/2009/11/10/common-hyperthreading-misconception/
> https://lkml.org/lkml/2009/11/13/33
>
> ...but I don't think that ever happened, even though Ingo thought it
> would probably be OK if there was no obvious fallout.
>
> Paul.
> --
>
>>
>> - if (smp_num_siblings == 1) {
>> - printk_once(KERN_INFO "CPU0: Hyper-Threading is disabled\n");
>> + if (threads_per_core <= 1) {
>> + pr_info_once("CPU: Hyper-Threading is unsupported on this processor.\n");
>> goto out;
>> }
>>
>> - if (smp_num_siblings <= 1)
>> - goto out;
>> -
>> - index_msb = get_count_order(smp_num_siblings);
>> + index_msb = get_count_order(threads_per_core);
>> c->phys_proc_id = apic->phys_pkg_id(c->initial_apicid, index_msb);
>>
>> - smp_num_siblings = smp_num_siblings / c->x86_max_cores;
>> + threads_per_core = threads_per_core / c->x86_max_cores;
>>
>> - index_msb = get_count_order(smp_num_siblings);
>> + index_msb = get_count_order(threads_per_core);
>>
>> core_bits = get_count_order(c->x86_max_cores);
>>
>> c->cpu_core_id = apic->phys_pkg_id(c->initial_apicid, index_msb) &
>> ((1 << core_bits) - 1);
>> -
>> out:
>> - if (!printed && (c->x86_max_cores * smp_num_siblings) > 1) {
>> + if (!printed && (c->x86_max_cores * threads_per_core) > 1) {
>> printk(KERN_INFO "CPU: Physical Processor ID: %d\n",
>> c->phys_proc_id);
>> printk(KERN_INFO "CPU: Processor Core ID: %d\n",
>> diff --git a/arch/x86/kernel/cpu/topology.c b/arch/x86/kernel/cpu/topology.c
>> index 4c60eaf..a9b837e 100644
>> --- a/arch/x86/kernel/cpu/topology.c
>> +++ b/arch/x86/kernel/cpu/topology.c
>> @@ -55,7 +55,7 @@ void detect_extended_topology(struct cpuinfo_x86 *c)
>> /*
>> * Populate HT related information from sub-leaf level 0.
>> */
>> - core_level_siblings = smp_num_siblings = LEVEL_MAX_SIBLINGS(ebx);
>> + core_level_siblings = LEVEL_MAX_SIBLINGS(ebx);
>> core_plus_mask_width = ht_mask_width = BITS_SHIFT_NEXT_LEVEL(eax);
>>
>> sub_index = 1;
>> diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
>> index 3482693..9eb96d2 100644
>> --- a/arch/x86/kernel/smpboot.c
>> +++ b/arch/x86/kernel/smpboot.c
>> @@ -351,8 +351,7 @@ static bool match_mc(struct cpuinfo_x86 *c, struct cpuinfo_x86 *o)
>>
>> void set_cpu_sibling_map(int cpu)
>> {
>> - bool has_smt = smp_num_siblings > 1;
>> - bool has_mp = has_smt || boot_cpu_data.x86_max_cores > 1;
>> + bool has_mp = boot_cpu_data.x86_max_cores > 1;
>> struct cpuinfo_x86 *c = &cpu_data(cpu);
>> struct cpuinfo_x86 *o;
>> int i;
>> @@ -364,13 +363,14 @@ void set_cpu_sibling_map(int cpu)
>> cpumask_set_cpu(cpu, cpu_llc_shared_mask(cpu));
>> cpumask_set_cpu(cpu, cpu_core_mask(cpu));
>> c->booted_cores = 1;
>> + smp_num_siblings = 1;
>> return;
>> }
>>
>> for_each_cpu(i, cpu_sibling_setup_mask) {
>> o = &cpu_data(i);
>>
>> - if ((i == cpu) || (has_smt && match_smt(c, o)))
>> + if ((i == cpu) || match_smt(c, o))
>> link_mask(sibling, cpu, i);
>>
>> if ((i == cpu) || (has_mp && match_llc(c, o)))
>> @@ -408,6 +408,10 @@ void set_cpu_sibling_map(int cpu)
>> c->booted_cores = cpu_data(i).booted_cores;
>> }
>> }
>> +
>> + /* Only need to check this on the boot cpu, o/w it is disabled */
>> + if (cpu == 0)
>> + smp_num_siblings = cpumask_weight(cpu_sibling_mask(cpu));
>> }
>>
>> /* maps the cpu to the sched domain representing multi-core */
>>