BIOS禁用超线程的Broadwell CPU上perf stat无法统计cycles:u的原因
2026-7-29
问题：Broadwell CPU下perf stat统计cycles:u异常及PMC4-7缺陷疑问
环境说明
BIOS中禁用超线程的Broadwell CPU，执行lscpu输出如下：
[root@ny4srv03 ~]# lscpu Architecture: x86_64 CPU op-mode(s): 32-bit, 64-bit Address sizes: 46 bits physical, 48 bits virtual Byte Order: Little Endian CPU(s): 44 On-line CPU(s) list: 0-43 Vendor ID: GenuineIntel BIOS Vendor ID: Intel Model name: Intel(R) Xeon(R) CPU E5-2696 v4 @ 2.20GHz BIOS Model name: Intel(R) Xeon(R) CPU E5-2696 v4 @ 2.20GHz CPU family: 6 Model: 79 Thread(s) per core: 1 Core(s) per socket: 22 Socket(s): 2 Stepping: 1 CPU max MHz: 3700.0000 CPU min MHz: 1200.0000 BogoMIPS: 4399.69 Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush dts acpi mmx fxsr sse sse2 ss ht tm pbe syscall nx pdpe1gb rdtscp lm constant_tsc arch_perfmon pebs bts rep_good nopl xtopology nonstop_tsc cpuid aper fmperf pni pclmulqdq dtes64 monitor ds_cpl vmx smx est tm2 ssse3 sdbg fma cx16 xtpr pdcm pcid dca sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand lahf_lm abm 3dnowprefetch cpuid_fault epb cat_l3 cdp_ l3 invpcid_single intel_ppin tpr_shadow vnmi flexpriority ept vpid ept_ad fsgsbase tsc_adjust bmi1 hle avx2 smep bmi2 erms invpcid rtm cqm rdt_a rdseed adx smap intel_pt xsaveopt cqm_llc cqm_occup_llc cqm_mbm_total cqm_mbm_local d therm ida arat pln pts Virtualization features: Virtualization: VT-x Caches (sum of all): L1d: 1.4 MiB (44 instances) L1i: 1.4 MiB (44 instances) L2: 11 MiB (44 instances) L3: 110 MiB (2 instances) NUMA: NUMA node(s): 2 NUMA node0 CPU(s): 0-21 NUMA node1 CPU(s): 22-43 Vulnerabilities: Itlb multihit: KVM: Mitigation: VMX disabled L1tf: Mitigation; PTE Inversion; VMX vulnerable, SMT disabled Mds: Vulnerable; SMT disabled Meltdown: Vulnerable Mmio stale data: Vulnerable Retbleed: Not affected Spec store bypass: Vulnerable Spectre v1: Vulnerable: __user pointer sanitization and usercopy barriers only; no swapgs barriers Spectre v2: Vulnerable, STIBP: disabled, PBRSB-eIBRS: Not affected Srbds: Not affected Tsx async abort: Vulnerable
根据《Intel 64和IA-32架构软件开发手册》：
如果处理器核心未被两个逻辑处理器共享，则最多可见8个通用计数器...
cpuid验证该CPU拥有8个性能计数器：
[root@ny4srv03 ~]# cpuid -1 | grep counters number of counters per logical processor = 0x8 (8) number of contiguous fixed counters = 0x3 (3) bit width of fixed counters = 0x30 (48)
异常现象
以root用户（kernel.perf_event_paranoid设为-1）执行如下perf stat命令时，cycles:u始终为0：
[root@ny4srv03 ~]# perf stat \ -r 100 \ -e cycles:u \ -e instructions:u \ -e branches:u \ -e branch-misses:u \ -e cache-references:u \ -e cache-misses:u \ -e faults:u \ ls>/dev/null Performance counter stats for 'ls' (100 runs): 0 cycles:u 668753 instructions:u ( +- 0.01% ) 131991 branches:u ( +- 0.00% ) 6936 branch-misses:u # 5.25% of all branches ( +- 0.33% ) 11105 cache-references:u ( +- 0.13% ) 6 cache-misses:u # 0.055 % of all cache refs ( +- 5.86% ) 103 faults:u ( +- 0.19% ) 0.00100211 +- 0.00000487 seconds time elapsed ( +- 0.49% )
移除branches:u、branch-misses:u、cache-references:u、cache-misses:u中任意一个事件后，cycles:u统计恢复正常：
[root@ny4srv03 ~]# perf stat \ -r 100 \ -e cycles:u \ -e instructions:u \ -e branches:u \ -e branch-misses:u \ -e cache-references:u \ -e faults:u \ ls>/dev/null Performance counter stats for 'ls' (100 runs): 614142 cycles:u ( +- 0.06% ) 668790 instructions:u # 1.09 insn per cycle ( +- 0.00% ) 132052 branches:u ( +- 0.00% ) 6874 branch-misses:u # 5.21% of all branches ( +- 0.11% ) 10735 cache-references:u ( +- 0.05% ) 101 faults:u ( +- 0.06% ) 0.00095650 +- 0.00000108 seconds time elapsed ( +- 0.11% )
正常场景
- 统计无修饰符或带
:k修饰符的cycles事件时，结果正常：
[root@ny4srv03 ~]# perf stat \ -r 100 \ -e cycles \ -e instructions:u \ -e branches:u \ -e branch-misses:u \ -e cache-references:u \ -e cache-misses:u \ -e faults:u \ ls>/dev/null Performance counter stats for 'ls' (100 runs): 1841276 cycles ( +- 0.79% ) 668400 instructions:u ( +- 0.00% ) 131966 branches:u ( +- 0.00% ) 6121 branch-misses:u # 4.64% of all branches ( +- 0.40% ) 10987 cache-references:u ( +- 0.16% ) 0 cache-misses:u # 0.000 % of all cache refs 102 faults:u ( +- 0.18% ) 0.00102359 +- 0.00000649 seconds time elapsed ( +- 0.63% )
- BIOS启用超线程后通过
nosmt内核参数禁用时，cpuid显示4个计数器，perf stat可正常统计所有7个事件：
[root@ny4srv03 ~]# perf stat \ -r 100 \ -e cycles:u \ -e instructions:u \ -e branches:u \ -e branch-misses:u \ -e cache-references:u \ -e cache-misses:u \ -e faults:u \ ls>/dev/null Performance counter stats for 'ls' (100 runs): 618443 cycles:u ( +- 0.39% ) 668466 instructions:u # 1.05 insn per cycle ( +- 0.00% ) 131968 branches:u ( +- 0.00% ) 6529 branch-misses:u # 4.95% of all branches ( +- 0.34% ) 11096 cache-references:u ( +- 0.47% ) 1 cache-misses:u # 0.010 % of all cache refs ( +- 53.16% ) 107 faults:u ( +- 0.18% ) 0.00097825 +- 0.00000554 seconds time elapsed ( +- 0.57% )
cpuid输出：
[root@ny4srv03 ~]# cpuid -1 | grep counters number of counters per logical processor = 0x4 (4) number of contiguous fixed counters = 0x3 (3) bit width of fixed counters = 0x30 (48)
排查过程
Update 1
执行perf -d发现NMI watchdog已启用：
[root@ny4srv03 likwid]# perf stat \ -e cycles:u \ -e instructions:u \ -e branches:u \ -e branch-misses:u \ -e cache-references:u \ -e cache-misses:u \ -e faults:u \ -d \ ls>/dev/null Performance counter stats for 'ls': 0 cycles:u 709098 instructions:u 140131 branches:u 6826 branch-misses:u # 4.87% of all branches 11287 cache-references:u 0 cache-misses:u # 0.000 % of all cache refs 104 faults:u 593753 L1-dcache-loads 32677 L1-dcache-load-misses # 5.50% of all L1-dcache accesses 8679 LLC-loads <not counted> LLC-load-misses (0.00%) 0.001102213 seconds time elapsed 0.000000000 seconds user 0.001134000 seconds sys Some events weren't counted. Try disabling the NMI watchdog: echo 0 > /proc/sys/kernel/nmi_watchdog perf stat ... echo 1 > /proc/sys/kernel/nmi_watchdog
禁用NMI watchdog后，cycles:u统计恢复正常，但dmesg显示其仅占用1个硬件计数器，无法解释原现象。
Update 2
在likwid工具源码中发现注释：
请注意，Intel Broadwell的PMC4-7计数器存在缺陷。当应用用户态或内核态过滤时，这些计数器不会递增。LIKWID默认使用用户态过滤，因此会自动为PMC4-7添加内核态过滤，返回的计数可能会高得多。
该注释可解释现象，现寻求该信息的官方来源。
问题咨询
- 上述现象是perf工具bug还是系统配置问题？
- 请提供Intel Broadwell CPU的PMC4-7计数器缺陷的官方来源信息。
内容的提问来源于stack exchange，提问作者szhem
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