gh-144438: Fix false sharing between QSBR and tlbc_index by colesbury · Pull Request #144554 · python/cpython · GitHub

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.

Issue: Scaling regression in Tools/ftscalingbench/ftscalingbench.py generator #144438

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.

Co-authored-by: Kumar Aditya <kumaraditya@python.org>

…e-144438.GI_uB1LR.rst Co-authored-by: Kumar Aditya <kumaraditya@python.org>

LGTM

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

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

…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>

⚠️⚠️⚠️ Buildbot failure ⚠️⚠️⚠️

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

What do you need to do:

Don't panic.
Check the buildbot page in the devguide if you don't know what the buildbots are or how they work.
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.
Check if the failure is related to this commit (6577d87) or if it is a false positive.
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'>

…ngh-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.

…44554) (#144923) 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)

…ngh-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.

colesbury added topic-free-threading needs backport to 3.14 bugs and security fixes labels Feb 6, 2026

bedevere-app Bot mentioned this pull request Feb 6, 2026

Scaling regression in Tools/ftscalingbench/ftscalingbench.py generator #144438

Closed

Use _Py_ALIGN_UP in grow_thread_array

8662728

colesbury marked this pull request as ready for review February 6, 2026 23:03

colesbury requested a review from ericsnowcurrently as a code owner February 6, 2026 23:03

bedevere-app Bot added the awaiting core review label Feb 6, 2026

colesbury requested review from kumaraditya303 and mpage February 6, 2026 23:03

kumaraditya303 reviewed Feb 7, 2026

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Comment thread Python/qsbr.c Outdated

colesbury and others added 2 commits February 7, 2026 11:36

Update Python/qsbr.c

60daa68

Co-authored-by: Kumar Aditya <kumaraditya@python.org>

Fix indentation in qsbr.c

d29fd8e

kumaraditya303 reviewed Feb 7, 2026

View reviewed changes

Comment thread Misc/NEWS.d/next/Core_and_Builtins/2026-02-06-21-45-52.gh-issue-144438.GI_uB1LR.rst Outdated

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 approved these changes Feb 8, 2026

View reviewed changes

bedevere-app Bot added awaiting merge and removed awaiting core review labels Feb 8, 2026

colesbury merged commit 6577d87 into python:main Feb 17, 2026
89 of 91 checks passed

bedevere-app Bot removed the awaiting merge label Feb 17, 2026

colesbury deleted the gh-144438-qsbr-alignment branch February 17, 2026 16:12

miss-islington-app Bot assigned colesbury Feb 17, 2026

hugovk removed the needs backport to 3.14 bugs and security fixes label Mar 31, 2026

gh-144438: Fix false sharing between QSBR and tlbc_index by colesbury · Pull Request #144554 · python/cpython · GitHub

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

gh-144438: Fix false sharing between QSBR and tlbc_index#144554
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gh-144438: Fix false sharing between QSBR and tlbc_index by colesbury · Pull Request #144554 · python/cpython · GitHub

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

gh-144438: Fix false sharing between QSBR and tlbc_index#144554colesbury merged 5 commits intopython:mainpython/cpython:mainfrom colesbury:gh-144438-qsbr-alignmentcolesbury/cpython:gh-144438-qsbr-alignmentCopy head branch name to clipboard

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gh-144438: Fix false sharing between QSBR and tlbc_index#144554
colesbury merged 5 commits into
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colesbury:gh-144438-qsbr-alignment

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