macOS kernel return 27 (KERN_POLICY_LIMIT): meaning and diagnosis

Status / error code:		Category:

Constant name:
Source:		Operating System:
Description: A Mach scheduling request specifies attributes that exceed the limits imposed on the target thread.

Could be also:
Constant	Type	OS
ERROR_SECTOR_NOT_FOUND	Win32 error	Windows
ippStsOverlongString	Intel Ipp Status	Any
EFBIG	errno	Any
PFN_SHARE_COUNT	BugCheck Code	Windows

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KERN_INVALID_PROCESSOR_SET	KERN_INVALID_POLICY

KERN_POLICY_LIMIT

A limit on a valid scheduling request

KERN_POLICY_LIMIT is a Mach kern_return_t used by scheduler-policy interfaces. It means that the requested scheduling attributes are outside the limits accepted for the target thread. The policy request can be structurally valid while still asking for more than that thread is permitted to receive.

This is most relevant when code uses thread_policy_set or a related Mach interface. That call identifies a target thread, a policy flavor, a policy-data structure, and a count expressed in integer_t units. For example, THREAD_TIME_CONSTRAINT_POLICY supplies a period, computation time, constraint, and preemptibility flag; the time fields are Mach absolute-time values.

How it differs from nearby results

Status	Diagnostic meaning
`KERN_POLICY_LIMIT`	The selected attributes exceed the applicable limit for the target thread.
`KERN_INVALID_POLICY`	The requested policy is not enabled or cannot be used in the current policy context.
`KERN_INVALID_ARGUMENT`	An argument, policy flavor, structure size, or supplied value is not valid for the call.
`KERN_POLICY_STATIC`	The target thread’s policy parameters cannot be changed at that point.

Do not reduce this status to a generic “priority too high” message. The useful evidence is the complete policy request and the identity of the target thread, because the applicable limit belongs to that scheduler context rather than to a single global number.

What to record before changing the request

Capture the exact Mach routine, policy flavor, target thread port, and every field of the policy structure.
For a time-constraint request, preserve period, computation, and constraint together. The XNU definition requires the constraint to be at least the computation time, but satisfying that relationship alone does not establish that the request is within the thread’s allowed limits.
Check whether the code is trying to apply a real-time policy where a normal pthread or QoS mechanism is the intended API. Apple documents Mach policy calls for cases that need scheduler behavior beyond ordinary POSIX priority control.
Validate the effective policy after a successful retry instead of assuming that a fallback or wrapper preserved the requested attributes.

References

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