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Allow resuming per-CPU arena selection via thread.arena (#2957)

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* 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>
This commit is contained in:
Raúl Marín
2026-07-16 23:53:25 +02:00
committed by GitHub
parent 8361239b03
commit 68c35f6557
5 changed files with 104 additions and 6 deletions
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11
test/unit/mallctl.c
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@@ -764,12 +764,17 @@ TEST_BEGIN(test_thread_arena) {
0, "Unexpected mallctl() failure");
new_arena_ind = percpu_arena_ind_limit(opt_percpu_arena) - 1;
if (old_arena_ind != new_arena_ind) {
/*
* Setting thread.arena to an index within the per-CPU
* range resumes automatic per-CPU selection rather than
* failing (see test/unit/percpu_arena_resume.c).
*/
expect_d_eq(
mallctl("thread.arena", (void *)&old_arena_ind, &sz,
(void *)&new_arena_ind, sizeof(unsigned)),
EPERM,
"thread.arena ctl "
"should not be allowed with percpu arena");
0,
"thread.arena within the per-CPU range should "
"resume per-CPU selection");
}
}
}