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use clone3 for exec process creation to reduce cgroup lock contention #4782

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use clone3 for exec process creation to reduce cgroup lock contention #4782

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29 changes: 28 additions & 1 deletion libcontainer/process_linux.go
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@ import (
"runtime"
"strconv"
"sync"
"syscall"
"time"

"github.com/opencontainers/runtime-spec/specs-go"
Expand Down Expand Up @@ -203,6 +204,28 @@ func (p *setnsProcess) start() (retErr error) {

// Get the "before" value of oom kill count.
oom, _ := p.manager.OOMKillCount()
useClone3 := false
if cgroups.IsCgroup2UnifiedMode() && p.initProcessPid != 0 {
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start() is already complex, let's move this to a function with a clear name, so it's more readable.

initProcCgroupFile := fmt.Sprintf("/proc/%d/cgroup", p.initProcessPid)
initCg, initCgErr := cgroups.ParseCgroupFile(initProcCgroupFile)
if initCgErr == nil {
if initCgPath, ok := initCg[""]; ok {
useClone3 = true
initCgDirpath := filepath.Join(fs2.UnifiedMountpoint, initCgPath)
fd, err := os.Open(initCgDirpath)
if err != nil {
return fmt.Errorf("error opening cgroup dir %q: %w", initCgDirpath, err)
}
defer fd.Close()
if p.cmd.SysProcAttr == nil {
p.cmd.SysProcAttr = &syscall.SysProcAttr{}
}
p.cmd.SysProcAttr.UseCgroupFD = true
p.cmd.SysProcAttr.CgroupFD = int(fd.Fd())
Comment on lines +223 to +224
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man clone3 says this is available since linux 5.7. You are setting useClone3 to true, but I don't think that is detecting CLONE_INTO_CGROUP is supported in this kernel, right? We will need to improve the detection. Not sure about the golang wrapper, but IIRC @kolyshkin wrote that for Go. He might have tips :)

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In case clone3 is not supported by the kernel or denied by the security policy, we'll get an error from exec. In this case, I guess, we should retry without UseCgroupFD.

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Or, we can rely on a kernel version (and have it in mind that it can still fail if e.g. clone3 is denied by the security policy).

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Usually a "dummy" exec of the syscall (with some nil params or whatever) is enough to know if it's supported. I guess something similar should be possible for this clone_into_cgroup param.

Usually that is recommended because people might backport it to old kernels, and if you just check the version you might not use it when it is available.

}
}
}

err := p.startWithCPUAffinity()
// Close the child-side of the pipes (controlled by child).
p.comm.closeChild()
Expand Down Expand Up @@ -232,7 +255,11 @@ func (p *setnsProcess) start() (retErr error) {
return fmt.Errorf("error executing setns process: %w", err)
}
for _, path := range p.cgroupPaths {
if err := cgroups.WriteCgroupProc(path, p.pid()); err != nil && !p.rootlessCgroups {
procPid := p.pid()
if useClone3 {
procPid = -1
}
if err := cgroups.WriteCgroupProc(path, procPid); err != nil && !p.rootlessCgroups {
Comment on lines +258 to +262
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I guess if it's -1 it's not written? Let's just useClone3 for the condition, explaining that if we are using clone3, then it's already set in the cgroup.

// On cgroup v2 + nesting + domain controllers, WriteCgroupProc may fail with EBUSY.
// https://github.com/opencontainers/runc/issues/2356#issuecomment-621277643
// Try to join the cgroup of InitProcessPid.
Expand Down
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