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gh-144438: Fix false sharing between QSBR and tlbc_index#144554

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colesbury merged 5 commits intopython:mainfrom
colesbury:gh-144438-qsbr-alignment
Feb 17, 2026
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gh-144438: Fix false sharing between QSBR and tlbc_index#144554
colesbury merged 5 commits intopython:mainfrom
colesbury:gh-144438-qsbr-alignment

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@colesbury colesbury commented Feb 6, 2026 •
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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.

@colesbury
pythongh-144438: Fix false sharing between QSBR and tlbc_index
ded8366 
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.
@bedevere-app bedevere-app bot mentioned this pull request Feb 6, 2026
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@colesbury colesbury marked this pull request as ready for review February 6, 2026 23:03
kumaraditya303
kumaraditya303 reviewed Feb 7, 2026
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colesbury and others added 2 commits February 7, 2026 11:36
@colesbury @kumaraditya303
Update Python/qsbr.c
60daa68 
Co-authored-by: Kumar Aditya <kumaraditya@python.org>
@colesbury
Fix indentation in qsbr.c
d29fd8e
@colesbury @kumaraditya303
Update Misc/NEWS.d/next/Core_and_Builtins/2026-02-06-21-45-52.gh-issu…
db5f73e 
…e-144438.GI_uB1LR.rst

Co-authored-by: Kumar Aditya <kumaraditya@python.org>
kumaraditya303
kumaraditya303 approved these changes Feb 8, 2026
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LGTM

@colesbury colesbury merged commit 6577d87 into python:main Feb 17, 2026
89 of 91 checks passed
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miss-islington-app bot commented Feb 17, 2026

Thanks @colesbury for the PR 🌮🎉.. I'm working now to backport this PR to: 3.14.
🐍🍒⛏🤖

@colesbury colesbury deleted the gh-144438-qsbr-alignment branch February 17, 2026 16:12
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miss-islington-app bot commented Feb 17, 2026

Sorry, @colesbury, I could not cleanly backport this to 3.14 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker 6577d870b0cb82baf540f4bcf49c01d68204e468 3.14

colesbury added a commit to colesbury/cpython that referenced this pull request Feb 17, 2026
@colesbury
[3.14] pythongh-144438: Fix false sharing between QSBR and tlbc_index (…
5e2993f 
…pythongh-144554)

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 6577d87)

Co-authored-by: Sam Gross <colesbury@gmail.com>
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bedevere-bot commented Feb 17, 2026

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

Hi! The buildbot AMD64 Ubuntu Shared 3.x (tier-1) has failed when building commit 6577d87.

What do you need to do:

  1. Don't panic.
  2. Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
  3. Go to the page of the buildbot that failed (https://buildbot.python.org/#/builders/506/builds/12364) and take a look at the build logs.
  4. Check if the failure is related to this commit (6577d87) or if it is a false positive.
  5. If the failure is related to this commit, please, reflect that on the issue and make a new Pull Request with a fix.

You can take a look at the buildbot page here:

https://buildbot.python.org/#/builders/506/builds/12364

Failed tests:

  • test_pyrepl

Failed subtests:

  • test_repl_eio - test.test_pyrepl.test_unix_console.TestUnixConsoleEIOHandling.test_repl_eio

Summary of the results of the build (if available):

==

Click to see traceback logs 
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/test/test_pyrepl/test_unix_console.py", line 390, in test_repl_eio
    _, err = proc.communicate(timeout=support.LONG_TIMEOUT)
             ~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/subprocess.py", line 1273, in communicate
    stdout, stderr = self._communicate(input, endtime, timeout)
                     ~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/subprocess.py", line 2293, in _communicate
    self._check_timeout(endtime, orig_timeout, stdout, stderr)
    ~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/subprocess.py", line 1320, in _check_timeout
    raise TimeoutExpired(
    ...<2 lines>...
            stderr=b''.join(stderr_seq) if stderr_seq else None)
subprocess.TimeoutExpired: Command '['/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/python', '-E', '-S', '/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/test/test_pyrepl/eio_test_script.py']' timed out after 300.0 seconds


Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/subprocess.py", line 1191, in __del__
    _warn("subprocess %s is still running" % self.pid,
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
          ResourceWarning, source=self)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ResourceWarning: subprocess 2201962 is still running
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name=10 mode='rb' closefd=True>:
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/unittest/case.py", line 666, in run
    with outcome.testPartExecutor(self):
         ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
ResourceWarning: unclosed file <_io.TextIOWrapper name=10 encoding='utf-8'>
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name=8 mode='rb' closefd=True>:
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/unittest/case.py", line 666, in run
    with outcome.testPartExecutor(self):
         ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
ResourceWarning: unclosed file <_io.TextIOWrapper name=8 encoding='utf-8'>


Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/subprocess.py", line 1191, in __del__
    _warn("subprocess %s is still running" % self.pid,
    ~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
          ResourceWarning, source=self)
          ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
ResourceWarning: subprocess 2236824 is still running
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name=10 mode='rb' closefd=True>:
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/unittest/case.py", line 666, in run
    with outcome.testPartExecutor(self):
         ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
ResourceWarning: unclosed file <_io.TextIOWrapper name=10 encoding='utf-8'>
Warning -- Unraisable exception
Exception ignored while finalizing file <_io.FileIO name=8 mode='rb' closefd=True>:
Traceback (most recent call last):
  File "/srv/buildbot/buildarea/3.x.bolen-ubuntu/build/Lib/unittest/case.py", line 666, in run
    with outcome.testPartExecutor(self):
         ~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^
ResourceWarning: unclosed file <_io.TextIOWrapper name=8 encoding='utf-8'>

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@kumaraditya303 kumaraditya303 kumaraditya303 approved these changes

@ericsnowcurrently ericsnowcurrently Awaiting requested review from ericsnowcurrently ericsnowcurrently is a code owner

@mpage mpage Awaiting requested review from mpage

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