Messages in this thread | | | Date | Mon, 2 Jun 2014 12:30:23 -0400 | From | Paul Gortmaker <> | Subject | Re: [PATCH] x86, Clean up smp_num_siblings calculation |
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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 */ >
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