systemd driver updates CPU quota inconsitently

[hshiina]

Description

When CPU quota is updated with fractions, cpu.max is updated as specified:

$ runc update container1 --cpu-quota=123456
$ cat /sys/fs/cgroup/system.slice/runc-container1.scope/cpu.max
123456 100000

However, after another parameter is updated, cpu.max is rounded up:

$ runc update container1 --memory 100000000
$ cat /sys/fs/cgroup/system.slice/runc-container1.scope/cpu.max
124000 100000

It looks the rounded value is passed to systemd:

runc/libcontainer/cgroups/systemd/common.go

Lines 303 to 310 in 74b35d8

	// systemd converts CPUQuotaPerSecUSec (microseconds per CPU second) to CPUQuota
	// (integer percentage of CPU) internally. This means that if a fractional percent of
	// CPU is indicated by Resources.CpuQuota, we need to round up to the nearest
	// 10ms (1% of a second) such that child cgroups can set the cpu.cfs_quota_us they expect.
	cpuQuotaPerSecUSec = uint64(quota*1000000) / period
	if cpuQuotaPerSecUSec%10000 != 0 {
	cpuQuotaPerSecUSec = ((cpuQuotaPerSecUSec / 10000) + 1) * 10000
	}

However, the original value seems to be written to the file:

runc/libcontainer/cgroups/systemd/v2.go

Line 492 in 74b35d8

return m.fsMgr.Set(r)

Steps to reproduce the issue

1. Create a container with systemd driver:

   $ runc --systemd-cgroup run container1
   

2. Update the CPU quota of the container with fraction:

   $ runc update container1 --cpu-quota=123456
   

3. See cpu.max:

   $ cat /sys/fs/cgroup/system.slice/runc-container1.scope/cpu.max
   123456 100000
   

4. Update another parameter of the container:

   $ runc update container1 --memory 100000000
   

5. See cpu.max again:

   $ cat /sys/fs/cgroup/system.slice/runc-container1.scope/cpu.max
    124000 100000
   

We can see how the file is updated in the steps with tail -f:

$ tail -f  /sys/fs/cgroup/system.slice/runc-container1.scope/cpu.max
max 100000
tail: /sys/fs/cgroup/system.slice/runc-container1.scope/cpu.max: file truncated
max 100000
tail: /sys/fs/cgroup/system.slice/runc-container1.scope/cpu.max: file truncated
124000 100000
tail: /sys/fs/cgroup/system.slice/runc-container1.scope/cpu.max: file truncated
123456 100000
tail: /sys/fs/cgroup/system.slice/runc-container1.scope/cpu.max: file truncated
124000 100000

Describe the results you received and expected

The CPU quota should be configured consistently whether it is rounded up or not.

What version of runc are you using?

$ runc -v
runc version 1.2.4
commit: v1.2.4-0-g6c52b3f
spec: 1.2.0
go: go1.22.10
libseccomp: 2.5.5

Host OS information

No response

Host kernel information

No response

[rata]

@hshiina thanks for the report! I'm currently super busy with some k8s deadlines. But you got quite into the code already, wanna try to open a PR to fix this too? ;)

[hshiina]

@hshiina thanks for the report! I'm currently super busy with some k8s deadlines. But you got quite into the code already, wanna try to open a PR to fix this too? ;)

Sure. I will try to post a PR.
/assign

[rata]

Feel free to cc me in the PR :)