Using Linux Perf Tool For Nuclei RISC-V Processor

[fanghuaqi]

Linux Perf Tool is an outstanding tool that serves the significant purpose of measuring CPU performance and offers valuable assistance to developers in optimizing software.

Linux Kernel For RISC-V improved perf support for RISC-V using SBI PMU extension and Sscofpmf extension, which are both supported by Nuclei RISC-V Processor now.

• SBI PMU extension is an interface for supervisor-mode to configure and use the RISC-V hardware performance counters with assistance from the machine-mode (or hypervisor-mode)
• Sscofpmf risc-v extension is used to add an counter overflow interrupt and Privilege Mode counter Filtering

Note

Please manually enable kernel PERF feature by CONFIG_PERF_EVENTS=y in conf/evalsoc/linux_rv64*_defconfig

By default, now in dev_nuclei_6.1_v2/dev_nuclei_6.6_v2/dev_nuclei_6.9_v2 branches, you need to manually enable CONFIG_PERF_EVENTS=y for Linux Kernel and build linux kernel, then PMU support will be enabled in Linux Kernel, if your CPU is configured with PMU, you will be able to see following output during boot up, see fd3dc31 commit for changes we made to support PMU v1.

And now we support PMU v2 in dev_nuclei_6.6_v3, see commit 4ca5808 for changes made.

We tested Linux Kernel 6.9 with PMU.

# In OpenSBI
Boot HART ISA Extensions  : sscofpmf,time,sstc
... ...
Boot HART MHPM Count      : 4
# In Linux Kernel
[   17.066711] riscv-pmu-sbi: SBI PMU extension is available
[   17.071960] riscv-pmu-sbi: 16 firmware and 6 hardware counters

But when CONFIG_PERF_EVENTS=y enabled, you will not be able to read cycle/instret/time csr, you need to execute echo 2 > /proc/sys/kernel/perf_user_access to allow user mode access these counters, and now the cycle/instret counter by default is not counted, it is controlled by PMU now, you need to use linux perf tool to enable it.

Here is a sample usage for coremark program ported by us using perf tool.

# echo 2 > /proc/sys/kernel/perf_user_access
# perf stat -e cycles -e instructions -e cache-misses -e branches -e branch-misses coremark_1core
Start to run CoreMark
2K performance run parameters for coremark.
CoreMark Size    : 666
Total ticks      : 14788
Total time (secs): 14.788000
Iterations/Sec   : 270.489586
Iterations       : 4000
Compiler version : GCC10.2.0
Compiler flags   : -Ofast -mbranch-cost=1 -mstrict-align -funroll-all-loops -finline-limit=1000 -ftree-dominator-opts -fselective-scheduling -funroll-loops -finline-functions -falign-functions=4 -falign-jumps=4 -falign-loops=4 -fipa-pta -fno-code-hoisting -fno-common -fno-if-conversion -fno-if-conversion2 -fno-tree-loop-distribute-patterns -fno-tree-vectorize -fno-tree-loop-ivcanon -fno-tree-vrp -fgcse-las --param=max-loop-header-insns=4 --param loop-max-datarefs-for-datadeps=0 --param=unroll-jam-min-percent=0 --param=max-goto-duplication-insns=0 -DMULTITHREAD=1 -DUSE_PTHREAD  -lrt -lpthread -march=rv64imafdc -mabi=lp64d
Memory location  : Please put data memory location here
                        (e.g. code in flash, data on heap etc)
seedcrc          : 0xe9f5
[0]crclist       : 0xe714
[0]crcmatrix     : 0x1fd7
[0]crcstate      : 0x8e3a
[0]crcfinal      : 0x65c5
Correct operation validated. See README.md for run and reporting rules.
CoreMark 1.0 : 270.489586 / GCC10.2.0 -Ofast -mbranch-cost=1 -mstrict-align -funroll-all-loops -finline-limit=1000 -ftree-dominator-opts -fselective-scheduling -funroll-loops -finline-functions -falign-functions=4 -falign-jumps=4 -falign-loops=4 -fipa-pta -fno-code-hoisting -fno-common -fno-if-conversion -fno-if-conversion2 -fno-tree-loop-distribute-patterns -fno-tree-vectorize -fno-tree-loop-ivcanon -fno-tree-vrp -fgcse-las --param=max-loop-header-insns=4 --param loop-max-datarefs-for-datadeps=0 --param=unroll-jam-min-percent=0 --param=max-goto-duplication-insns=0 -DMULTITHREAD=1 -DUSE_PTHREAD  -lrt -lpthread -march=rv64imafdc -mabi=lp64d / Heap
Begin_Cycle 9223372037062820250, End_Cycle 9223372037805837188, User_Cycle 743016938 cycles
Begin_Instret 9223372037119765118, End_Instret 9223372038073768277, User_Instret 954003159 Instrets
CoreMark/MHz calc via cycle: 5.383457 CoreMark/MHz
IPC:  1.284

 Performance counter stats for 'coremark_1core':

         951534830      cycles                                                  
        1219293100      instructions                     #    1.28  insn per cycle
             13957      cache-misses                                            
         264065366      branches                                                
           6903336      branch-misses                    #    2.61% of all branches

      18.942352295 seconds time elapsed

      18.884026000 seconds user
       0.039923000 seconds sys

About Linux Perf Tool, you need to cross build it using Linux Kernel, you will need at least to cross build zlib, elfutils and libtraceevent and then build linux perf tool

• zlib 1.3.1: https://github.com/madler/zlib/tree/v1.3.1
• elfutils 0.191: https://sourceware.org/git/elfutils.git
• libtraceevent 1.8.3: https://git.kernel.org/pub/scm/libs/libtrace/libtraceevent.git

We provide sample script to cross build perf tool for linux kernel 6.9 for rv64imafdc and rv32imafdc:

Please use dev_nuclei_6.9.y-perf linux kernel branch to build linux perf tool.

• https://github.com/Nuclei-Software/linux/blob/dev_nuclei_6.9.y-perf/tools/perf/pre_perf.sh : build perf required library
• https://github.com/Nuclei-Software/linux/blob/dev_nuclei_6.9.y-perf/tools/perf/do_perf.sh: build perf tool itself

Tools directories must be like this:

├── linux
├── elfutils
├── libtraceevent
└── zlib

Download prebuilt Nuclei RISC-V Toolchain ( Linux/Glibc )(gcc14) from https://nucleisys.com/download.php

Warning

If you are using 2025.10 glibc toolchain or shellcheck installed in your linux pc, please take a look at this note
#26 (comment)

# Clone correct dev_nuclei_6.9.y-perf linux kernel branch, clone less code using --depth 1
git clone -b dev_nuclei_6.9.y-perf --depth 1 https://github.com/Nuclei-Software/linux
# Make sure above directory include source code are setup
# Make sure toolchain PATH is already setup
$ which riscv64-unknown-linux-gnu-gcc
/path/to/your/gcc/bin/riscv64-unknown-linux-gnu-gcc
$ cd linux/tools/perf
$ ls *_perf.sh
do_perf.sh  pre_perf.sh
# Please dont place install in current directory, so we placed it into ../install
# Build perf required third party libraries
# If you want to build for rv32imafdc, please change
# rv64imafdc lp64d -> rv32imafdc ilp32d 
./pre_perf.sh riscv64-unknown-linux-gnu rv64imafdc lp64d ../install
# Build perf tools
./do_perf.sh riscv64-unknown-linux-gnu rv64imafdc lp64d ../install
 
# Then you will be able to see perf tools in `../install` folder
$ ls ../install/
bin/  env.sh  etc/  include/  lib/  libexec/  share/

# WARNING: Below is in a riscv linux environment, not x86 linux environment
# You just need to copy these install folder in your riscv linux environment and execute following command
# you will be able to use perf in your riscv linux
source /path/to/install/env.sh
perf --version

[fanghuaqi]

In order to make it easier to run linux benchmark such as coremark, dhrystone with cycle/instret, we disabled PERF feature 6a1f4dc by default now, you can enable it by adding CONFIG_PERF_EVENTS=y in conf/evalsoc/linux_rv64*_defconfig

[fanghuaqi]

We added nuclei pmu v2 support in branch dev_nuclei_6.6_v3 and also support linux perf for rv32 and rv64, see https://github.com/Nuclei-Software/linux/tree/dev_nuclei_6.9.y-perf branch of linux kernel code, you can also use branch https://github.com/Nuclei-Software/linux/tree/dev_nuclei_6.16.y-perf

[fanghuaqi]

Fixed linux perf tool build when shellcheck installed in your pc, you need to pass NO_SHELLCHECK when build perf tools, changes have been pushed to https://github.com/Nuclei-Software/linux/tree/dev_nuclei_6.9.y-perf and https://github.com/Nuclei-Software/linux/tree/dev_nuclei_6.16.y-perf

Please pull latest changes of above branches

And if you are building linux perf tool using 2025.10 glibc toolchain, it will show a segmentation fault issue like below

    GEN     pmu-events/test-empty-pmu-events.c
  Segmentation fault (core dumped)
  make[4]: *** [/Local/hqfang/workspace/software/linuxbench/benchmark/linux/tools/build/Makefile.build:148: arch/perf-test-in.o] Error 139
  make[4]: *** Deleting file 'arch/perf-test-in.o'
  make[3]: *** [/Local/hqfang/workspace/software/linuxbench/benchmark/linux/tools/build/Makefile.build:142: arch] Error 2

This is caused by 2025.10 introduced support for zcmt, but it didn't handle empty archive file input, which cause segmentation fault for linux perf tool build.

To workaround it, just replace riscv64-unknown-linux-gnu-ld and riscv64-unknown-linux-gnu-ld.bfd in gcc/bin folder using the ld tools of 2025.02 version.