* Allow resuming per-CPU arena selection via thread.arena
With percpu_arena enabled, thread.arena control is one-directional. A
thread can be bound to a manually created arena (an index at or above the
per-CPU auto range) to route a bounded region of work to a dedicated,
long-lived arena, but there is no way back: thread_arena_ctl returns EPERM
for any index within the auto range, and arena_choose_impl only re-selects
a per-CPU arena for threads whose current arena is already in that range.
So once a thread is bound to a manual arena it stays pinned there forever,
and its later allocations land there instead of following the CPU.
Treat setting thread.arena to an index within the per-CPU range as a
request to resume automatic per-CPU selection: hand the thread back to
percpu management (rebinding it to the current CPU's arena via
percpu_arena_update) and return 0 instead of EPERM. Binding to a manual
arena is unchanged. The requested index is advisory; under percpu the
thread is governed by its current CPU, so it resumes on the current CPU's
arena regardless of the value passed.
Add test/unit/percpu_arena_resume covering the manual-arena to resume
round trip with allocs & dallocs happening meanwhile, and update
test_thread_arena, which asserted the old EPERM.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
