sched_ext: Don't warn on core-sched forced idle in put_prev_task_scx()

Overview

In the Linux kernel, the following vulnerability has been resolved:

sched_ext: Don't warn on core-sched forced idle in put_prev_task_scx()

put_prev_task_scx() warns when a runnable task drops to a lower sched_class without SCX_OPS_ENQ_LAST, on the assumption that balance_one() would have kept it running. Core scheduling breaks that: a forced-idle SMT sibling reschedules through the core_pick fast path in pick_next_task(), which skips pick_task_scx() and thus balance_one(), so a runnable task can drop to idle with ENQ_LAST unset.

Gate the warning on sched_cpu_cookie_match(): a cookie mismatch means core scheduling forced the idle, while a match (or core scheduling off) still catches a genuine missing-ENQ_LAST drop.

Affected

• Linux >= 7c65ae81ea86a6ed8086e1a5651acd766187f19b < 6267ebee9f2f3e26f4968bf6dc29d350620fa320
• Linux >= 7c65ae81ea86a6ed8086e1a5651acd766187f19b < 2907e9d0f05b506dfd58aec589a23042a56ef36b
• Linux >= 7c65ae81ea86a6ed8086e1a5651acd766187f19b < e2f188cdbf8312289532c36eb4e9eb1c9544d43a
• Linux >= 7c65ae81ea86a6ed8086e1a5651acd766187f19b < b7d9c359e5cf867f7eb23df3bb1c6b9e58af24da
• Linux 6.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.