Integrated: 8351500: G1: NUMA migrations cause crashes in region allocation

[Thomas Stuefe]

On Tue, 11 Mar 2025 14:01:11 GMT, Thomas Stuefe <stuefe at openjdk.org> wrote:

> For details, please see JBS issue.
> 
> _Please note that this bug only shows symptoms in JDK 21 and JDK 17! Due to code shuffling done as part of G1 region-local pinning work, the error does not show in JDKs 22 and later._
> 
> I originally planned to fix this just for JDK 21 and 17 (see https://github.com/openjdk/jdk21u-dev/pull/1460). However, I would rather have it fixed in the mainline, even though it is symptom-free. It is a lingering issue that may surface later if the code is ever changed. Plus, this prevents the fix from being accidentally overwritten in JDK 21 if we backport.
> 
> ----
> 
> The fix is simple: we fix (hah) the NUMA association for the full duration of a heap allocation in G1. That way, regardless of the OS scheduler moving the thread to a different NUMA node, we always use the same `G1AllocRegion` object, and changes in the control flow that rely on that won't break on NUMA.
> 
> This has the disadvantage of allocating memory from a node we are potentially moving away from. However, I argue that this is exceedingly rare, and if it happens, the OS will cope by eventually migrating the memory to the correct node.
> 
> ---
> 
> Testing:
> 
> Testing is difficult. See remark in JBS issue. 
> 
> I tested a modified version of this patch on JDK 21, where the error does cause crashes. I tested with an additional patch mimicking tons of NUMA node migrations. As I wrote in JBS, I plan to contribute that "FakeNUMA" mode eventually, but lack the time to polish it up for now. I hope the fix is simple and uncontested enough to go in quickly, since I would like to fix JDK 21 soon via backporting this patch.

This pull request has now been integrated.

Changeset: 37ec7962
Author:    Thomas Stuefe <stuefe at openjdk.org>
URL:       https://git.openjdk.org/jdk/commit/37ec796255ae857588a5c7e0d572407dd81cbec9
Stats:     53 lines in 5 files changed: 17 ins; 11 del; 25 mod

8351500: G1: NUMA migrations cause crashes in region allocation

Reviewed-by: rkennke, sjohanss, tschatzl

-------------

PR: https://git.openjdk.org/jdk/pull/23984