Kirill Kolyshkin (Gerrit)

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Dec 5, 2025, 9:14:20 PM (11 days ago) Dec 5

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Kirill Kolyshkin has uploaded the change for review

Commit message

unix: add CPUSetInterface for variable-size CPU affinity masks

The existing CPUSet type is a fixed-size array limited to 1024 CPUs,
which makes it problematic to use for large systems (see e.g.
https://github.com/opencontainers/runc/issues/5023).

This change introduces CPUSetInterface in an attempt to allow both
the existing CPUSet as well as the variable-size implementation
(see the following commit) to work functions currently accepting CPUSet.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <nor...@anthropic.com>

Change-Id: I3e96c430f18cbb18edf9ca25418c363aad92d9fe

Change diff

diff --git a/unix/affinity_linux.go b/unix/affinity_linux.go
index 3ea4703..7ef8c95 100644
--- a/unix/affinity_linux.go
+++ b/unix/affinity_linux.go
@@ -13,11 +13,25 @@
 
 const cpuSetSize = _CPU_SETSIZE / _NCPUBITS
 
+// CPUSetInterface is an interface for CPU affinity masks.
+// It allows both fixed-size and variable-size CPU sets to be used
+// with affinity functions.
+type CPUSetInterface interface {
+	Zero()
+	Fill()
+	Set(cpu int)
+	Clear(cpu int)
+	IsSet(cpu int) bool
+	Count() int
+	Size() int
+	Pointer() unsafe.Pointer
+}
+
 // CPUSet represents a CPU affinity mask.
 type CPUSet [cpuSetSize]cpuMask
 
-func schedAffinity(trap uintptr, pid int, set *CPUSet) error {
-	_, _, e := RawSyscall(trap, uintptr(pid), uintptr(unsafe.Sizeof(*set)), uintptr(unsafe.Pointer(set)))
+func schedAffinity(trap uintptr, pid int, set CPUSetInterface) error {
+	_, _, e := RawSyscall(trap, uintptr(pid), uintptr(set.Size()), uintptr(set.Pointer()))
 	if e != 0 {
 		return errnoErr(e)
 	}
@@ -26,13 +40,13 @@
 
 // SchedGetaffinity gets the CPU affinity mask of the thread specified by pid.
 // If pid is 0 the calling thread is used.
-func SchedGetaffinity(pid int, set *CPUSet) error {
+func SchedGetaffinity(pid int, set CPUSetInterface) error {
 	return schedAffinity(SYS_SCHED_GETAFFINITY, pid, set)
 }
 
 // SchedSetaffinity sets the CPU affinity mask of the thread specified by pid.
 // If pid is 0 the calling thread is used.
-func SchedSetaffinity(pid int, set *CPUSet) error {
+func SchedSetaffinity(pid int, set CPUSetInterface) error {
 	return schedAffinity(SYS_SCHED_SETAFFINITY, pid, set)
 }
 
@@ -91,3 +105,13 @@
 	}
 	return c
 }
+
+// Size returns the size of the CPU set in bytes.
+func (s *CPUSet) Size() int {
+	return int(unsafe.Sizeof(*s))
+}
+
+// Pointer returns an unsafe.Pointer to the CPU set.
+func (s *CPUSet) Pointer() unsafe.Pointer {
+	return unsafe.Pointer(s)
+}
diff --git a/unix/syscall_linux.go b/unix/syscall_linux.go
index 06c0eea..497d1da 100644
--- a/unix/syscall_linux.go
+++ b/unix/syscall_linux.go
@@ -2644,8 +2644,8 @@
 //sys	Cachestat(fd uint, crange *CachestatRange, cstat *Cachestat_t, flags uint) (err error)
 //sys	Mseal(b []byte, flags uint) (err error)
 
-//sys	setMemPolicy(mode int, mask *CPUSet, size int) (err error) = SYS_SET_MEMPOLICY
+//sys	setMemPolicy(mode int, mask unsafe.Pointer, size int) (err error) = SYS_SET_MEMPOLICY
 
-func SetMemPolicy(mode int, mask *CPUSet) error {
-	return setMemPolicy(mode, mask, _CPU_SETSIZE)
+func SetMemPolicy(mode int, mask CPUSetInterface) error {
+	return setMemPolicy(mode, mask.Pointer(), mask.Size())
 }
diff --git a/unix/zsyscall_linux.go b/unix/zsyscall_linux.go
index 8935d10..e378761 100644
--- a/unix/zsyscall_linux.go
+++ b/unix/zsyscall_linux.go
@@ -2241,8 +2241,8 @@
 
 // THIS FILE IS GENERATED BY THE COMMAND AT THE TOP; DO NOT EDIT
 
-func setMemPolicy(mode int, mask *CPUSet, size int) (err error) {
-	_, _, e1 := Syscall(SYS_SET_MEMPOLICY, uintptr(mode), uintptr(unsafe.Pointer(mask)), uintptr(size))
+func setMemPolicy(mode int, mask unsafe.Pointer, size int) (err error) {
+	_, _, e1 := Syscall(SYS_SET_MEMPOLICY, uintptr(mode), uintptr(mask), uintptr(size))
 	if e1 != 0 {
 		err = errnoErr(e1)
 	}

Change information

Files:

M unix/affinity_linux.go
M unix/syscall_linux.go
M unix/zsyscall_linux.go

Change size: S

Delta: 3 files changed, 33 insertions(+), 9 deletions(-)

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Kirill Kolyshkin (Gerrit)

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Dec 5, 2025, 9:16:04 PM (11 days ago) Dec 5

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Michael Pratt (Gerrit)

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Dec 8, 2025, 1:54:22 PM (9 days ago) Dec 8

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Commit Message

Line 17, Patchset 1 (Latest):🤖 Generated with [Claude Code](https://claude.com/claude-code)

Michael Pratt . unresolved

No markdown in commit messages (though I appreciate the code generation disclaimer).

File unix/affinity_linux.go

Line 35, Patchset 1 (Parent):func SchedSetaffinity(pid int, set *CPUSet) error {

Michael Pratt . unresolved

Unfortunately changing the type of function arguments is not a backwards compatible change. It breaks code using the function as a function value. e.g., https://go.dev/play/p/StqvRpFMU1-

So this would need to be a new API, at which point (IMO) it may as well use CPUSetDynamic directly rather than an interface.

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Kirill Kolyshkin (Gerrit)

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Dec 16, 2025, 7:32:17 PM (7 hours ago) Dec 16

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File unix/affinity_linux.go

Line 35, Patchset 1 (Parent):func SchedSetaffinity(pid int, set *CPUSet) error {

Michael Pratt . unresolved

Unfortunately changing the type of function arguments is not a backwards compatible change. It breaks code using the function as a function value. e.g., https://go.dev/play/p/StqvRpFMU1-
So this would need to be a new API, at which point (IMO) it may as well use CPUSetDynamic directly rather than an interface.

Kirill Kolyshkin

How would you name new `Sched[GS]etaffinity` and `SetMemPolicy`? Add the `Dynamic` suffix to all of them?

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[sys] unix: add CPUSetInterface for variable-size CPU affinity masks

Kirill Kolyshkin (Gerrit)

Kirill Kolyshkin has uploaded the change for review

Commit message

Change diff

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Kirill Kolyshkin (Gerrit)

Kirill Kolyshkin voted Commit-Queue+1

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Michael Pratt (Gerrit)

Michael Pratt added 2 comments

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Kirill Kolyshkin (Gerrit)

Kirill Kolyshkin added 1 comment

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