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X-OriginatorOrg: amd.com X-MS-Exchange-CrossTenant-OriginalArrivalTime: 12 Nov 2025 06:07:04.8323 (UTC) X-MS-Exchange-CrossTenant-Network-Message-Id: c81ede63-8244-4bf5-82e2-08de21b1b437 X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d X-MS-Exchange-CrossTenant-OriginalAttributedTenantConnectingIp: TenantId=3dd8961f-e488-4e60-8e11-a82d994e183d;Ip=[165.204.84.17];Helo=[satlexmb08.amd.com] X-MS-Exchange-CrossTenant-AuthSource: SJ5PEPF00000204.namprd05.prod.outlook.com X-MS-Exchange-CrossTenant-AuthAs: Anonymous X-MS-Exchange-CrossTenant-FromEntityHeader: HybridOnPrem X-MS-Exchange-Transport-CrossTenantHeadersStamped: CH3PR12MB7500 Hello Wanpeng, On 11/12/2025 10:24 AM, Wanpeng Li wrote: >>> Problem Statement >>> ----------------- >>> >>> In overcommitted virtualization scenarios, vCPUs frequently spin on locks >>> held by other vCPUs that are not currently running. The kernel's >>> paravirtual spinlock support detects these situations and calls yield_to() >>> to boost the lock holder, allowing it to run and release the lock. >>> >>> However, the current implementation has two critical limitations: >>> >>> 1. Scheduler-side limitation: >>> >>> yield_to_task_fair() relies solely on set_next_buddy() to provide >>> preference to the target vCPU. This buddy mechanism only offers >>> immediate, transient preference. Once the buddy hint expires (typically >>> after one scheduling decision), the yielding vCPU may preempt the target >>> again, especially in nested cgroup hierarchies where vruntime domains >>> differ. >> >> So what you are saying is there are configurations out there where vCPUs >> of same guest are put in different cgroups? Why? Does the use case >> warrant enabling the cpu controller for the subtree? Are you running > > You're right to question this. The problematic scenario occurs with > nested cgroup hierarchies, which is common when VMs are deployed with > cgroup-based resource management. Even when all vCPUs of a single > guest are in the same leaf cgroup, that leaf sits under parent cgroups > with their own vruntime domains. > > The issue manifests when: > - set_next_buddy() provides preference at the leaf level > - But vruntime competition happens at parent levels If that is the case, then NEXT_BUDDY is in-eligible as a result of its vruntime being higher that the weighted averages of other entity. Won't this break fairness? Let me go look at the series and come back. > - The buddy hint gets "diluted" when pick_task_fair() walks up the hierarchy > -- Thanks and Regards, Prateek