Re: [PATCH] x86, Clean up smp_num_siblings calculation

Paul Gortmaker
2014-06-02T16:31:07Z

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

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:

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 */
>