macOS kernel return 28 (KERN_INVALID_POLICY): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: A requested Mach scheduling policy is unavailable for the target thread's current context.

Could be also:
Constant	Type	OS
ERROR_OUT_OF_PAPER	Win32 error	Windows
ippStsAffineQuadChanged	Intel Ipp Status	Any
ENOSPC	errno	Any
PFN_REFERENCE_COUNT	BugCheck Code	Windows

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KERN_POLICY_LIMIT	KERN_INVALID_OBJECT

KERN_INVALID_POLICY

The policy is not available in this context

KERN_INVALID_POLICY is a Mach scheduler result, not a statement that the policy-data pointer is malformed. The declaration in kern_return.h describes a scheduling policy that is not currently enabled for the processor-set context. In current XNU code, the same result is also used when a thread is not allowed to enter a requested real-time mode or when affinity sets are disabled.

That makes the policy flavor essential diagnostic data. A request that is valid on one system, thread class, or workload context can be unavailable on another without any change to the structure layout.

Two concrete policy contexts

THREAD_TIME_CONSTRAINT_POLICY requests real-time scheduling parameters. Current XNU rejects that request with this result when the thread belongs to a work interval with a workload ID that does not permit real-time operation.
THREAD_AFFINITY_POLICY is marked experimental in the XNU header. It expresses an affinity relationship between threads; matching tags are only a scheduler hint to share an L2 cache when possible. It is not a request to pin a thread to a particular processor, and XNU returns this result when affinity sets are disabled.

Separate availability from bad input

Result	What it points to
This result	The policy cannot be applied in the target’s current scheduling context.
`KERN_POLICY_LIMIT`	The requested attributes exceed a limit even though the policy is applicable.
`KERN_INVALID_ARGUMENT`	The call contract is wrong: for example, an invalid target, flavor, count, or field value.
`KERN_NOT_SUPPORTED`	The selected facility is not supported by the current platform or implementation.

Useful investigation data

Log the policy flavor and its complete structure, rather than only the numeric return value.
Identify whether the target is the calling thread or another thread, and whether it has joined a work interval or has an existing QoS policy.
For affinity, verify the intended outcome. Cache-affinity tags are advisory relationships, not a portable CPU-binding mechanism.
For real-time requests, review the timing model before retrying. The policy data specifies period, computation, and constraint in absolute-time units, and the constraint must not be smaller than computation.

References

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