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Subject: Re: Kernel Benchmarking
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From:   Matthieu Baerts <matthieu.baerts@tessares.net>
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Date:   Tue, 15 Sep 2020 21:56:41 +0200
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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
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