TPU Connection Failure and VBAR OOM Troubleshooting

Use this skill to systematically diagnose and prevent vbar_control_agent segfaults and Out-Of-Memory (OOM) errors on TPU v6e nodes.

⚠️ Prerequisites

• Cloud Logging must be enabled for the project.
• Access to the project and cluster via gcloud or equivalent tool.

🔍 Diagnostic Workflow

Step 0: Context Acquisition & Time Window Definition

Independently gather required context using available GCP/GKE tools or use the provided {variable} placeholders:

• {project_id}: The GCP Project ID (e.g., customer-ai-project-123).
• {cluster_name}: The GKE Cluster Name (e.g., tpu-cluster-prod).
• {node_name}: The Node Name or Instance ID (e.g., tpu-node-1).
• {workload_name}: The Workload Name / JobSet Name (e.g., my-training-job-456).
• {namespace}: The Workload Namespace.
• {issue_time}: The timestamp of the issue (e.g., 2026-04-14T20:00:00Z).

Time Handling & Execution Rules

1. Window Calculation: If an issue timestamp {issue_time} is provided, calculate the query time window as [{issue_time} - 30m] to [{issue_time} + 30m].
   • Let {start_time} = {issue_time} - 30m
   • Let {end_time} = {issue_time} + 30m
2. Informational vs. Live Execution: If the user request is informational or query-formulation (e.g. "How can I check...", "How do I determine..."), or if live GCP project resources are not actively targetable, directly output the calculated time window, log names, and Cloud Logging filter templates without attempting live log execution commands.

Step 1: Check for vbar_control_agent OOMs

Look for specific out of memory messages from vbar_control_agent in serial console logs (serialconsole.googleapis.com%2fserial_port_1_output).

• Tool to use: query_logs (for live diagnostics)
• Filter Templates:

Serial Console Logs (OOMs):

logName="projects/{project_id}/logs/serialconsole.googleapis.com%2fserial_port_1_output"
AND labels."compute.googleapis.com/resource_name"="{node_name}"
AND SEARCH(text_payload, "Memory cgroup out of memory: Killed process .* (vbar_control_ag)")
AND timestamp >= "{start_time}"
AND timestamp <= "{end_time}"

• Logic: Presence of Memory cgroup out of memory messages related to vbar_control_agent. Stack traces pointing to libtpu::tpunetd::VBARControlHelper::MetricsReadFromVBAR are a strong indicator.
• Automation: Proceed to next step automatically after reporting findings.
• Reference: See references/failure_signatures.md for example log patterns.

Step 2: Investigate tpu-device-plugin Metrics Fetch Failures [Low Risk]

Check if tpu-device-plugin is reporting metric fetch failures.

• Tool to use: query_logs
• Filter Template:

resource.type="k8s_container"
AND resource.labels.project_id="{project_id}"
AND resource.labels.cluster_name="{cluster_name}"
AND resource.labels.container_name="tpu-device-plugin"
AND severity=ERROR
AND textPayload:"metrics fetch failed for .* deviceID and .* device path with error: checksum didn't match with the metrics data. Corrupt data found"
AND timestamp >= "{start_time}"
AND timestamp <= "{end_time}"

• Logic: Errors indicating "metrics fetch failed" with "checksum didn't match" suggest vBAR memory corruption.
• Automation: Proceed to next step automatically after reporting findings.

Step 3: Check for Custom Metrics Collection Usage [Low Risk]

Inspect cluster configurations, workloads, or container specs to determine if custom TPU metrics collection mechanisms are deployed.

• Action: Check if custom scripts or agents (e.g., using libtpu.sdk.tpumonitoring) are deployed that frequently query GetHostMetrics from vBAR Control Agent.

• Verification Commands:

  • Kubectl Search (Inspect workload env/specs):

  kubectl get pods -A -o jsonpath='{range .items[*]}{.metadata.namespace}{"/"}{.metadata.name}{"\t"}{.spec.containers[*].image}{"\n"}{end}'
  

  • Log Search Filter (query_logs):

  resource.type="k8s_container"
  AND resource.labels.project_id="{project_id}"
  AND resource.labels.cluster_name="{cluster_name}"
  AND textPayload:"libtpu.sdk.tpumonitoring"
  AND timestamp >= "{start_time}"
  AND timestamp <= "{end_time}"
  

• Logic: Confirmation of custom metrics collection helps confirm the race condition hypothesis.

🛠️ Resolution Workflow

Resolution 1: Temporarily Disable Custom Metrics Collection [High Risk]

If a custom metrics collection agent is identified, recommend disabling it.

• Action: Recommend disabling the custom metrics collector.
• Justification: Prevents reads from vBAR during device resets, stopping crashes and OOMs.

Resolution 2: Await vbar_control_agent Resiliency Update [Low Risk]

Advise that a permanent fix will be available in a future GKE version.

• Action: Recommend upgrading GKE when the fix is available.
• Justification: The updated agent will be resilient to memory corruption and gracefully handle reads from unbound vBARs.

📋 copypaste checklist

• Acquire context and compute [{start_time}, {end_time}] window.
• Check for vbar_control_agent segfaults and OOMs using query_logs.
• Investigate tpu-device-plugin failures using query_logs.
• Inspect for custom metrics collection usage.
• Advise disabling custom metrics collection if applicable.
• Advise awaiting resiliency update.