fix: Parallelism CPU calculation inside Kubernetes and Docker with limits

[daniel-sampliner]

The value of /sys/fs/cgroup/cpu/cpu.cfs_quota_us is not in milliseconds and cannot be simply divided by 1000 to determine the CPU limit. As per kernel documentation¹, the cpu limit can be determined by dividing that value by /sys/fs/cgroup/cpu/cpu.cfs_period_us.

Put an x into the box if that apply:

• This PR introduces breaking change.
• This PR fixes a bug.
• This PR adds new functionality.
• This PR enhances existing functionality.

Description of your changes

Fixes the max CPU calculation when run inside kubernetes.

How can we test changes

1. Run pre-commit inside kubernetes pod with CPU limit N.
2. Without specifying --parallelism-limit, confirm that pre-commit is only spawning N-1 concurrent processes.

Summary by CodeRabbit

• Bug Fixes
  • Improved CPU core detection mechanism for more accurate resource allocation.
  • Enhanced compatibility with cgroup-based environments.
  • Updated logic for calculating CPU cores based on new variables for quota and period.

Footnotes

1. https://docs.kernel.org/scheduler/sched-bwc.html ↩

[coderabbitai]

Walkthrough

The changes modify the common::get_cpu_num function in the hooks/_common.sh script to enhance the logic for detecting CPU cores. The function now utilizes cpu_quota and cpu_period instead of the previous millicpu variable. The calculation for determining the number of CPU cores has been updated to use the formula cpu_quota / cpu_period, with a minimum return value of 1. Comments have been added to clarify the purpose of the CPU quota calculation in relation to Kubernetes and Docker.

Changes

File	Change Summary
hooks/_common.sh	- Introduced cpu_quota and cpu_period local variables - Updated CPU core calculation logic - Modified return value calculation to use cpu_quota / cpu_period - Ensured minimum return value of 1 - Added comments for clarification

Sequence Diagram

sequenceDiagram
    participant Script
    participant get_cpu_num Function
    participant cgroup
    
    Script->>get_cpu_num Function: Call function
    get_cpu_num Function->>cgroup: Read cpu_quota
    get_cpu_num Function->>cgroup: Read cpu_period
    get_cpu_num Function->>get_cpu_num Function: Calculate cpu_num = cpu_quota / cpu_period
    alt cpu_num < 1
        get_cpu_num Function-->>Script: Return 1
    else cpu_num >= 1
        get_cpu_num Function-->>Script: Return cpu_num
    end

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• hooks/_common.sh

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[coderabbitai]

Actionable comments posted: 0

🧹 Nitpick comments (1)

hooks/_common.sh (1)

Line range hint 200-243: Consider adding documentation about cgroup versions.

The implementation looks good, but consider adding comments to explain:

1. The kernel documentation reference for CPU quota calculation
2. The differences between cgroup v1 (/sys/fs/cgroup/cpu/) and v2 (/sys/fs/cgroup/) paths

 function common::get_cpu_num {
   local -r parallelism_ci_cpu_cores=$1
 
+  # According to kernel documentation, CPU quota should be calculated as cpu.cfs_quota_us/cpu.cfs_period_us
+  # See: https://www.kernel.org/doc/html/latest/scheduler/sched-bwc.html#cpu-bandwidth-control
   local cpu_quota cpu_period cpu_num
   local millicpu
 
+  # Check for cgroup v1 paths and exclude WSL which has misleading cgroup values
   if [[ -f /sys/fs/cgroup/cpu/cpu.cfs_quota_us &&
     ! -f /proc/sys/fs/binfmt_misc/WSLInterop ]]; then

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🔇 Additional comments (2)

hooks/_common.sh (2)

200-206: LGTM! Clear variable declarations and proper WSL detection.

The code correctly handles edge cases by checking for WSL environment, which could provide misleading cgroup values.

---

Line range hint 208-243: Correct implementation of CPU quota calculation.

The fix properly implements the CPU core calculation by using cpu.cfs_period_us as per kernel documentation. This resolves the bug where CPU limits were incorrectly calculated in Kubernetes environments.

Key improvements:

1. Uses cpu_quota/cpu_period instead of the incorrect millicpu/1000
2. Properly handles edge cases:
   • No CPU limits (cpu_quota = -1)
   • Values less than 1
   • Custom CPU core specifications via parallelism_ci_cpu_cores

[yermulnik]

@daniel-sampliner Do you think that there's no need for echo $((millicpu / 1000)) as e.g. a fallback if /sys/fs/cgroup/cpu/cpu.cfs_period_us does not exist (in case it may not exist at all)? 🤔

[daniel-sampliner]

Good question. I don't believe it's possible for cpu.cfs_period_us to not exist if cpu.cfs_quota_us does exist and is > 0. But I also don't know for certain.

It definitely can't hurt to fall back to something if cpu.cfs_period_us doesn't exist, but I don't think millicpu / 1000 would be the right value. Perhaps like this to default to 1 CPU when cpu.cfs_period_us doesn't exist

Suggested change

	cpu_period=$(< /sys/fs/cgroup/cpu/cpu.cfs_period_us)
	cpu_num=$((cpu_quota / cpu_period))
	if ((cpu_num < 1)); then
	echo 1
	else
	echo $cpu_num
	fi
	cpu_period=$(< /sys/fs/cgroup/cpu/cpu.cfs_period_us || echo -1)
	cpu_num=$((cpu_quota / cpu_period))
	[[ $cpu_num -lt 1 ]] && echo 1 || echo $cpu_num

[yermulnik]

Yep, the above commit suggestion makes sense to me.
@MaxymVlasov WDYT?

[coderabbitai]

Actionable comments posted: 0

🧹 Nitpick comments (1)

hooks/_common.sh (1)

238-239: Consider adding a comment explaining the CPU calculation formula.

While the implementation is correct, adding a brief comment explaining why we divide cpu_quota by cpu_period would help future maintainers understand the logic.

+    # Calculate CPU cores by dividing quota by period as per cgroup v1 documentation
     cpu_num=$((cpu_quota / cpu_period))
     [[ $cpu_num -lt 1 ]] && echo 1 || echo $cpu_num

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🔇 Additional comments (3)

hooks/_common.sh (3)

200-207: LGTM! Improved CPU calculation logic.

The changes correctly implement the CPU calculation by using cpu.cfs_quota_us and cpu.cfs_period_us values, which aligns with the kernel documentation for accurate CPU limit calculations in Kubernetes environments.

---

Line range hint 209-237: Comprehensive error handling for edge cases.

The implementation properly handles various scenarios:

1. When no CPU limits are set (cpu_quota = -1)
2. When cpu_period is invalid (< 1)
3. When running in DinD without limits
4. When parallelism_ci_cpu_cores is provided but invalid

The warning messages are clear and provide actionable guidance to users.

---

Line range hint 200-241: Verify the impact on different container runtimes.

The changes handle Kubernetes and Docker environments differently. Let's verify that the CPU calculation works correctly across different container runtimes.

[yermulnik]

LGTM. @MaxymVlasov WDYT?

[MaxymVlasov]

Regarding thread above about fallbacking to millicpu
In docker we have next picture:
https://docs.docker.com/engine/containers/resource_constraints/#configure-the-default-cfs-scheduler

➜ docker run -ti --cpus="1.5" alpine sh
/ # grep ^ /dev/null /sys/fs/cgroup/cpu*
/sys/fs/cgroup/cpu.idle:0
/sys/fs/cgroup/cpu.max:150000 100000

Where 150000 is cpu-quota and 100000 is cpu-period

150000 / 1000 (millicpu) != 1.5, so, I'll fix it in this PR too

[MaxymVlasov]

Great findings

[coderabbitai]

Actionable comments posted: 0

🧹 Nitpick comments (1)

hooks/_common.sh (1)

205-211: Accurate Retrieval of CPU Quota and Period Values
The updates correctly read the cpu_quota from /sys/fs/cgroup/cpu/cpu.cfs_quota_us and attempt to retrieve cpu_period from /sys/fs/cgroup/cpu/cpu.cfs_period_us. This ensures that the calculation later uses the proper values from cgroup settings. For consistency and clarity, consider using the same file input redirection style for both values.

-    cpu_period=$(cat /sys/fs/cgroup/cpu/cpu.cfs_period_us 2> /dev/null || echo "$cpu_quota")
+    cpu_period=$(< /sys/fs/cgroup/cpu/cpu.cfs_period_us 2> /dev/null || echo "$cpu_quota")

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• hooks/_common.sh (3 hunks)

🔇 Additional comments (3)

hooks/_common.sh (3)

Line range hint 213-243: Robust Fallback and Validation for Kubernetes CPU Limits
The conditional block properly checks if either cpu_quota is -1 or cpu_period is less than 1. This branch then either uses the provided parallelism_ci_cpu_cores (after validating it as a positive integer) or falls back gracefully by echoing 1 with an informative warning. This robust handling is well aligned with the PR objective of accurate CPU calculation in Kubernetes environments.

---

249-259: Proper Handling of cgroup v2 Environments
The new block that handles the /sys/fs/cgroup/cpu.max file correctly addresses scenarios (typically in cgroup v2) where the quota might be expressed differently. The check for a quota value of "max" and the accompanying condition on cpu_period ensure that the function falls back to using nproc when limits are not defined. This implementation cleanly extends support to environments using cgroup v2.

---

Line range hint 191-259: Overall, CPU Limit Calculation Fix Is Correct and Well-Documented
The modifications in common::get_cpu_num update the CPU calculation to use the formula cpu_quota / cpu_period, which now aligns with the kernel documentation and addresses the previously incorrect division by 1000. The function properly distinguishes between cgroup v1 and v2 while offering robust fallbacks and informative warnings. This fix effectively meets the PR objectives of accurate CPU limit detection in Kubernetes.

[antonbabenko]

This PR is included in version 1.97.1 🎉