[3.14] gh-144438: Fix false sharing between QSBR and tlbc_index (gh-144554) (#144923)

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Align the QSBR thread state array to a 64-byte cache line boundary
and add padding at the end of _PyThreadStateImpl. Depending on heap
layout, the QSBR array could end up sharing a cache line with a
thread's tlbc_index, causing QSBR quiescent state updates to contend
with reads of tlbc_index in RESUME_CHECK. This is sensitive to
earlier allocations during interpreter init and can appear or
disappear with seemingly unrelated changes.

Either change alone is sufficient to fix the specific issue, but both
are worthwhile to avoid similar problems in the future.

(cherry picked from commit 6577d870b0)

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Sam Gross 2026-03-31 15:20:24 -04:00 • committed by GitHub

parent 25b48b84b8

commit 6ea4f842fb

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