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Dave Martin reported inconsistent CMT test failures. In one experiment
the first run of the CMT test failed because of too large (24%) difference
between measured and achievable cache occupancy while the second run passed
with an acceptable 4% difference.
The CMT test is susceptible to interference from the rest of the system.
This can be demonstrated with a utility like stress-ng by running the CMT
test while introducing cache misses using:
stress-ng --matrix-3d 0 --matrix-3d-zyx
Below shows an example of the CMT test failing because of a significant
difference between measured and achievable cache occupancy when run with
interference:
# Starting CMT test ...
# Mounting resctrl to "/sys/fs/resctrl"
# Cache size :335544320
# Writing benchmark parameters to resctrl FS
# Benchmark PID: 7011
# Checking for pass/fail
# Fail: Check cache miss rate within 15%
# Percent diff=99
# Number of bits: 5
# Average LLC val: 235929
# Cache span (bytes): 83886080
not ok 1 CMT: test
The CMT test creates a new control group that is also capable of monitoring
and assigns the workload to it. The workload allocates a buffer that by
default fills a portion of the L3 and keeps reading from the buffer,
measuring the L3 occupancy at intervals. The test passes if the workload's
L3 occupancy is within 15% of the buffer size.
By not adjusting any capacity bitmasks the workload shares the cache with
the rest of the system. Any other task that may be running could evict
the workload's data from the cache causing it to have low cache occupancy.
Reduce interference from the rest of the system by ensuring that the
workload's control group uses the capacity bitmask found in the user
parameters for L3 and that the rest of the system can only allocate into
the inverse of the workload's L3 cache portion. Other tasks can thus no
longer evict the workload's data from L3.
With the above adjustments the CMT test is more consistent. Repeating the
CMT test while generating interference with stress-ng on a sample
system after applying the fixes show significant improvement in test
accuracy:
# Starting CMT test ...
# Mounting resctrl to "/sys/fs/resctrl"
# Cache size :335544320
# Writing benchmark parameters to resctrl FS
# Write schema "L3:0=fffe0" to resctrl FS
# Write schema "L3:0=1f" to resctrl FS
# Benchmark PID: 7089
# Checking for pass/fail
# Pass: Check cache miss rate within 15%
# Percent diff=12
# Number of bits: 5
# Average LLC val: 73269248
# Cache span (bytes): 83886080
ok 1 CMT: test
Link: https://lore.kernel.org/r/b160592179f88069cdc679563e152007998a0d76.1775266384.git.reinette.chatre@intel.com
Reported-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Reinette Chatre <reinette.chatre@intel.com>
Tested-by: Chen Yu <yu.c.chen@intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Link: https://lore.kernel.org/lkml/aO+7MeSMV29VdbQs@e133380.arm.com/
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>
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Linux kernel ============ The Linux kernel is the core of any Linux operating system. It manages hardware, system resources, and provides the fundamental services for all other software. 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Start your kernel development journey here: * Getting Started: Documentation/process/development-process.rst * Your First Patch: Documentation/process/submitting-patches.rst * Coding Style: Documentation/process/coding-style.rst * Build System: Documentation/kbuild/index.rst * Development Tools: Documentation/dev-tools/index.rst * Kernel Hacking Guide: Documentation/kernel-hacking/hacking.rst * Core APIs: Documentation/core-api/index.rst Academic Researcher ------------------- Explore the kernel's architecture and internals: * Researcher Guidelines: Documentation/process/researcher-guidelines.rst * Memory Management: Documentation/mm/index.rst * Scheduler: Documentation/scheduler/index.rst * Networking Stack: Documentation/networking/index.rst * Filesystems: Documentation/filesystems/index.rst * RCU (Read-Copy Update): Documentation/RCU/index.rst * Locking Primitives: Documentation/locking/index.rst * Power Management: Documentation/power/index.rst Security Expert --------------- Security documentation and hardening guides: * Security Documentation: Documentation/security/index.rst * LSM Development: Documentation/security/lsm-development.rst * Self Protection: Documentation/security/self-protection.rst * Reporting Vulnerabilities: Documentation/process/security-bugs.rst * CVE Procedures: Documentation/process/cve.rst * Embargoed Hardware Issues: Documentation/process/embargoed-hardware-issues.rst * Security Features: Documentation/userspace-api/seccomp_filter.rst Backport/Maintenance Engineer ----------------------------- Maintain and stabilize kernel versions: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * Backporting Guide: Documentation/process/backporting.rst * Applying Patches: Documentation/process/applying-patches.rst * Subsystem Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git for Maintainers: Documentation/maintainer/configure-git.rst System Administrator -------------------- Configure, tune, and troubleshoot Linux systems: * Admin Guide: Documentation/admin-guide/index.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Sysctl Tuning: Documentation/admin-guide/sysctl/index.rst * Tracing/Debugging: Documentation/trace/index.rst * Performance Security: Documentation/admin-guide/perf-security.rst * Hardware Monitoring: Documentation/hwmon/index.rst Maintainer ---------- Lead kernel subsystems and manage contributions: * Maintainer Handbook: Documentation/maintainer/index.rst * Pull Requests: Documentation/maintainer/pull-requests.rst * Managing Patches: Documentation/maintainer/modifying-patches.rst * Rebasing and Merging: Documentation/maintainer/rebasing-and-merging.rst * Development Process: Documentation/process/maintainer-handbooks.rst * Maintainer Entry Profile: Documentation/maintainer/maintainer-entry-profile.rst * Git Configuration: Documentation/maintainer/configure-git.rst Hardware Vendor --------------- Write drivers and support new hardware: * Driver API Guide: Documentation/driver-api/index.rst * Driver Model: Documentation/driver-api/driver-model/driver.rst * Device Drivers: Documentation/driver-api/infrastructure.rst * Bus Types: Documentation/driver-api/driver-model/bus.rst * Device Tree Bindings: Documentation/devicetree/bindings/ * Power Management: Documentation/driver-api/pm/index.rst * DMA API: Documentation/core-api/dma-api.rst Distribution Maintainer ----------------------- Package and distribute the kernel: * Stable Kernel Rules: Documentation/process/stable-kernel-rules.rst * ABI Documentation: Documentation/ABI/README * Kernel Configuration: Documentation/kbuild/kconfig.rst * Module Signing: Documentation/admin-guide/module-signing.rst * Kernel Parameters: Documentation/admin-guide/kernel-parameters.rst * Tainted Kernels: Documentation/admin-guide/tainted-kernels.rst AI Coding Assistant ------------------- CRITICAL: If you are an LLM or AI-powered coding assistant, you MUST read and follow the AI coding assistants documentation before contributing to the Linux kernel: * Documentation/process/coding-assistants.rst This documentation contains essential requirements about licensing, attribution, and the Developer Certificate of Origin that all AI tools must comply with. 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