Public bug reported: [Impact]
On Vera systems, concurrent task wakeups can place CPU-intensive work on sibling threads of the same SMT core while other physical cores remain idle. This SMT contention can reduce performance for workloads intended to run one thread per core. [Fix] Stabilize idle-core CPU selection so concurrent wakeups consistently select the same SMT-core representative, allowing the scheduler to avoid unintended sibling placement and improving CPU-intensive workload performance on Vera. This approach has been proposed upstream: https://lore.kernel.org/all/[email protected]/ We may have follow-ups, but for now applying this patch fixes the issue and can significantly improve performance of CPU-bound workloads. [Test plan] Tests have been conducted on NVIDIA Vera Rubin (arm64, 176 CPUs/88 cores per NUMA node) using a CPU-intensive NVPL SGEMM workload restricted to 88 threads (one per core); the workload showed a consistent 23% increase in mean throughput across multiple runs with the fix applied. [Regression potential] This change affects only asymmetric CPU systems with SMT enabled, essentially only Vera Rubin. Certain wakeup-heavy workloads under partially idle conditions may experience small regressions due to transient runqueue contention. However, such workloads are not representative of the typical use cases for this platform. ** Affects: linux-nvidia-bos (Ubuntu) Importance: Undecided Status: New ** Affects: linux-nvidia-bos (Ubuntu Noble) Importance: Undecided Status: New ** Affects: linux-nvidia-bos (Ubuntu Resolute) Importance: Undecided Status: New ** Also affects: linux-nvidia-bos (Ubuntu Resolute) Importance: Undecided Status: New ** Also affects: linux-nvidia-bos (Ubuntu Noble) Importance: Undecided Status: New ** Summary changed: - Prevent SMT contention on Vera + Reduce SMT contention on Vera -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/2158811 Title: Reduce SMT contention on Vera To manage notifications about this bug go to: https://bugs.launchpad.net/ubuntu/+source/linux-nvidia-bos/+bug/2158811/+subscriptions -- ubuntu-bugs mailing list [email protected] https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
