Re: [PATCH] x86, Clean up smp_num_siblings calculation

Prarit Bhargava
2014-06-02T17:21:41Z

> 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.

I think the first case (0xb leaf) is done when cpu_has(c, X86_FEATURE_XTOPOLOGY)
is true. I don't think we've been doing the latter though and it could be
something introduced in a new patch?

>
> 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

Yeah -- I was actually debating about masking it off when smp_num_siblings == 1,
but wasn't sure how we felt about that these days. The problem with mucking
around with /proc/cpuinfo is that we're never clear if the values are the values
read from the hardware, or the interpreted software values.

I can certainly retest the ht flag masking if one of the x86@kernel.org people
give me an ack to do so.

hpa? tglx? Ingo?

P.