Re: [signal] 4bad58ebc8: will-it-scale.per_thread_ops -3.3% regression

[Thomas Gleixner <tglx@linutronix.de>]

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On Tue, Apr 20 2021 at 11:08, kernel test robot wrote:
> FYI, we noticed a -3.3% regression of will-it-scale.per_thread_ops due to commit:
>
> commit: 4bad58ebc8bc4f20d89cff95417c9b4674769709 ("signal: Allow tasks to cache one sigqueue struct")
> https://git.kernel.org/cgit/linux/kernel/git/tip/tip.git sched/core
>
> in testcase: will-it-scale
> on test machine: 192 threads Intel(R) Xeon(R) Platinum 9242 CPU @ 2.30GHz with 192G memory
> with following parameters:
>
> 	nr_task: 100%
> 	mode: thread
> 	test: futex3
> 	cpufreq_governor: performance
> 	ucode: 0x5003006
>
> test-description: Will It Scale takes a testcase and runs it from 1 through to n parallel copies to see if the testcase will scale. It builds both a process and threads based test in order to see any differences between the two.
> test-url: https://github.com/antonblanchard/will-it-scale
> commit: 
>   69995ebbb9 ("signal: Hand SIGQUEUE_PREALLOC flag to __sigqueue_alloc()")
>   4bad58ebc8 ("signal: Allow tasks to cache one sigqueue struct")
>
> 69995ebbb9d37173 4bad58ebc8bc4f20d89cff95417 
> ---------------- --------------------------- 
>          %stddev     %change         %stddev
>              \          |                \  
>  1.273e+09            -3.3%  1.231e+09        will-it-scale.192.threads
>    6630224            -3.3%    6409738        will-it-scale.per_thread_ops
>  1.273e+09            -3.3%  1.231e+09        will-it-scale.workload
>       1638 ±  3%      -7.8%       1510 ±  5%  sched_debug.cfs_rq:/.runnable_avg.max
>     297.83 ± 68%   +1747.6%       5502 ±152%  interrupts.33:PCI-MSI.524291-edge.eth0-TxRx-2
>     297.83 ± 68%   +1747.6%       5502 ±152%  interrupts.CPU12.33:PCI-MSI.524291-edge.eth0-TxRx-2

This change is definitely not causing more network traffic

>       8200           -33.4%       5459 ± 35%  interrupts.CPU27.NMI:Non-maskable_interrupts
>       8200           -33.4%       5459 ± 35%  interrupts.CPU27.PMI:Performance_monitoring_interrupts
>       8199           -33.4%       5459 ± 35%  interrupts.CPU28.NMI:Non-maskable_interrupts
>       8199           -33.4%       5459 ± 35%  interrupts.CPU28.PMI:Performance_monitoring_interrupts
>       6148 ± 33%     -11.2%       5459 ± 35%  interrupts.CPU29.NMI:Non-maskable_interrupts
>       6148 ± 33%     -11.2%       5459 ± 35%  interrupts.CPU29.PMI:Performance_monitoring_interrupts
>       4287 ±  8%     +33.6%       5730 ± 15%  interrupts.CPU49.CAL:Function_call_interrupts
>       6356 ± 19%     +49.6%       9509 ± 19%  interrupts.CPU97.CAL:Function_call_interrupts

Neither does it increase the number of function calls

>     407730 ±  8%     +37.5%     560565 ±  7%  perf-stat.i.dTLB-load-misses
>     415959 ±  8%     +40.4%     583928 ±  7%  perf-stat.ps.dTLB-load-misses

And this massive increase does not make sense either.

Confused.

Thanks,

        tglx