[REGRESSION]: mmap performance regression starting with k-6.1

[Chun Ng]

Hi,

Recently I observed there is performance regression on system call mmap(..). I tried both vanilla kernels and Raspberry Pi kernels on a Raspberry Pi 4 box and the results are pretty consistent among them.

Bisection showed that the regression starts from k-6.1, and the latest vanilla k-6.7 is still showing the same regression.

The test program calls mmap/munmap for a 4K page with MAP_ANON and MAP_PRIVATE flags, and ftrace is used to measure the time spent on the do_mmap(..) call.  Measured time of a sample run with different vanilla kernel versions are:
k-5.10 and k-6.0: ~157us
k-6.1: ~194us
k-6.7: ~214us
Results are pretty consistent across multiple runs with a small percentage variance.  Ftrace shows that latency of mmap_region(...) has increased since k-6.1.  An application that makes frequent mmap(..) calls the accumulated extra latency is very noticeable. 

Please find the ftrace results and kernel config files in this folder:
https://drive.google.com/drive/folders/1qy8YTBqxu8Gdbs7IigYbSd4FXldId5sd?usp=drive_link

The test program can be found in here:
https://drive.google.com/file/d/1tG6_BbQMCHwfKebvAIAg_xqbM_lpPcuM/view?usp=sharing

Info on the testing environment:
cpufreq_governor: performance
Test machine: Raspberry Pi 4, 8GB DDR
SCHED_FIFO with priority 99 for running the test program

Vanilla kernels are not tainted. However on k-6.0 and k-6.7, I have to patch the drivers/clk/bcm/clk-raspberrypi.c file with the version in Raspberry Pi kernel tree for the CPU frequency governor to work.

Best,
Chun
[nvpublic]