Re: Kernel Benchmarking

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On 15/09/2020 21:32, Linus Torvalds wrote:>
Was that an actual UP kernel? It might be good to try that too, just
to see if it could be an SMP race in the page locking code.

I am sorry, I am not sure how to verify this. I guess it was one processor because I removed "-smp 2" option from qemu. So I guess it switched to a uniprocessor mode.

Also, when I did the test and to make sure I was using only one CPU, I also printed the output of /proc/cpuinfo:


+ cat /proc/cpuinfo processor : 0 vendor_id : AuthenticAMD cpu family : 23 model : 1 model name : AMD EPYC 7401P 24-Core Processor stepping : 2 microcode : 0x1000065
cpu MHz         : 2000.000
cache size : 512 KB physical id : 0


siblings : 1


core id : 0 cpu cores : 1 apicid : 0 initial apicid : 0 fpu : yes


fpu_exception : yes cpuid level : 13 wp : yes


flags : fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cmov pat pse36 clflush mmx fxsr sse sse2 syscall nx mmxext fxsr_opt pdpe1gb rdtscp lm rep_good nopl cpuid extd_apicid tsc_known_freq pni pclmulqdq ssse3 fma cx16 sse4_1 sse4_2 x2apic movbe popcnt tsc_deadline_timer aes xsave avx f16c rdrand hypervisor lahf_lm cmp_legacy svm cr8_legacy abm sse4a misalignsse 3dnowprefetch osvw ssbd ibpb vmmcall fsgsbase tsc_adjust bmi1 avx2 smep bmi2 rdseed adx smap clflushopt sha_ni xsaveopt xsavec xgetbv1 virt_ssbd
arat npt nrip_save arch_capabilities
bugs : fxsave_leak sysret_ss_attrs null_seg spectre_v1 spectre_v2 spec_store_bypass

bogomips        : 4000.00
TLB size        : 1024 4K pages
clflush size    : 64
cache_alignment : 64 address sizes : 40 bits physical, 48 bits virtual
power management:


Do you want me to try another qemu config?

Sorry, it is getting late for me but I also forgot to mention earlier that with 1 CPU and your new sysctl set to 1, it didn't reproduce my issue for 6 executions.

After all, one such theoretical race was one reason I started the rewrite.

And that's a good thing, thank you! :)

Cheers,
Matt
--
Tessares | Belgium | Hybrid Access Solutions
www.tessares.net



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