The Partial Failure State: Diagnosing a Stuck Blackwell GPU Reconfiguration

Message Overview

In message [msg 6386] of this opencode session, the assistant confronts a system in a precarious partial state. After a previous command timed out during an attempt to reconfigure IOMMU identity domains for Blackwell GPUs, the assistant discovers that the reconfiguration process has stalled midway: one GPU has been successfully removed from the nvidia driver, but three others remain bound, and the nvidia kernel module itself cannot be unloaded because it has active users—specifically, the LXC container running the SGLang inference service. The message captures the critical moment of diagnosis and the necessary prerequisite action of stopping the dependent service before proceeding.

The full text of the message is:

[assistant] It's in a partial state — 01:00.0 was removed (no driver), but 11:00.0, 61:00.0, 71:00.0 are on nvidia. nvidia has 413 users (likely the LXC container started and has processes using GPUs). And nvidia_uvm has 44 users. We can't unload nvidia while the container is using it.

Let me stop the LXC SGLang service first, then do the full cycle: [bash] ssh root@10.1.230.174 'systemctl stop sglang-qwen.service 2>/dev/null; sleep 2; systemctl is-active sglang-qwen.service 2>/dev/null || echo "stopped"' inactive stopped

Context: The Quest for P2P DMA on Blackwell

To understand why this message exists, we must step back into the broader narrative. The assistant has been engaged in a multi-session effort to deploy and optimize large language model inference on a server equipped with eight NVIDIA RTX PRO 6000 Blackwell GPUs, split across two NUMA domains. A persistent challenge has been enabling Peer-to-Peer (P2P) DMA transfers between GPUs within the same NUMA domain, which is critical for the tensor-parallel communication patterns used by SGLang and vLLM.

The root cause of the P2P failure traces back to the system's IOMMU configuration. Under the default DMA-FQ (DMA with Fine-Grained Queuing) translation mode, the IOMMU remaps DMA addresses, which breaks the direct GPU-to-GPU memory access that P2P relies on. The solution appears straightforward in theory: switch the IOMMU groups containing the target GPUs to "identity" mode, where DMA addresses pass through untranslated, enabling direct peer access.

In practice, however, this has proven extraordinarily difficult. The Blackwell architecture introduces a new firmware component called the GSP (GPU Security Processor) or FSP (Firmware Security Processor), which manages GPU initialization and state. This firmware establishes a session with the nvidia driver during the initial probe at boot time, and critically, that session state appears to persist across Secondary Bus Reset (SBR) operations. Previous attempts in this session had revealed that unbinding a GPU from the nvidia driver and rebinding it—even after a full PCI remove/rescan cycle—resulted in the GSP firmware failing to reinitialize, producing errors like _kgspBootGspRm: (the GPU is likely in a bad state and may need to be reset) and RmInitAdapter: Cannot initialize GSP firmware RM.

The assistant had been iterating on a boot-time script approach: set IOMMU identity domains before the nvidia driver ever touches the GPUs. But the timing constraints are brutal. The nvidia module auto-loads during PCI rescan via the kernel's modalias mechanism, meaning it grabs the devices before any identity-domain configuration script can intervene. The previous message ([msg 6385]) had attempted a complex sequence of unbinding, removing GPUs, performing SBR, blocking nvidia auto-load, rescaming, setting identity domains, then manually loading nvidia—but that command timed out after 120 seconds.

The Diagnostic Pivot

Message [msg 6386] opens with the assistant assessing the aftermath of that timeout. The assessment is terse but reveals a sophisticated understanding of the system's state:

It's in a partial state — 01:00.0 was removed (no driver), but 11:00.0, 61:00.0, 71:00.0 are on nvidia.

This single sentence demonstrates the assistant's mental model of the PCI device topology. The four NUMA0 GPUs at BDF addresses 0000:01:00.0, 0000:11:00.0, 0000:61:00.0, and 0000:71:00.0 are being tracked individually. The timeout interrupted the reconfiguration sequence mid-stream: the first GPU (01:00.0) had been successfully removed from the PCI bus (or at least unbound from its driver), while the other three remained attached to the nvidia driver. The system is in an inconsistent state—neither fully operational nor fully reset.

The assistant then identifies the reason the nvidia module cannot be unloaded:

nvidia has 413 users (likely the LXC container started and has processes using GPUs). And nvidia_uvm has 44 users. We can't unload nvidia while the container is using it.

This is a critical insight. The lsmod output (visible in the previous message's context) shows nvidia with 413 module users and nvidia_uvm with 44 users. These "users" are processes that have opened the nvidia device files or are holding references to the kernel module. The assistant correctly infers that the LXC container—which runs the SGLang inference service on the host at 10.1.230.174—has started and its processes are holding these references. The container was presumably started automatically by the system's service manager after the previous reboot, or it survived the partial reconfiguration attempt.

This inference is not obvious from the raw numbers alone. The assistant must combine knowledge of the system architecture (that an LXC container exists on a separate host, that it runs SGLang, that SGLang opens CUDA contexts which pin the nvidia module) with the observed module usage count. The leap from "413 users" to "the LXC container started" is a synthesis of system topology and process behavior.

The Decision to Stop the Service

Having diagnosed the blocker, the assistant makes a pragmatic decision: stop the LXC SGLang service first, then proceed with the full reconfiguration cycle. This is captured in the second half of the message:

Let me stop the LXC SGLang service first, then do the full cycle: [bash] ssh root@10.1.230.174 'systemctl stop sglang-qwen.service 2>/dev/null; sleep 2; systemctl is-active sglang-qwen.service 2>/dev/null || echo "stopped"' inactive stopped

The assistant executes a remote SSH command to the LXC host (10.1.230.174), stopping the sglang-qwen.service systemd unit. The 2>/dev/null redirections suppress error output (in case the service doesn't exist or is already stopped), and the sleep 2 ensures the stop command has time to complete before the verification. The systemctl is-active command returns "inactive" and the fallback echo "stopped" confirms the service is no longer running.

This decision reveals several assumptions:

That stopping the service will release the nvidia module references. This is a reasonable assumption—SGLang processes hold CUDA contexts, which hold GPU file descriptors, which pin the nvidia and nvidia_uvm modules. Terminating the processes should release these references, allowing the module to be unloaded.
That the service can be safely stopped. The assistant does not check whether there are active inference requests or whether stopping the service will cause data loss. This is acceptable in a development/tuning context, but it's an assumption worth noting.
That stopping the service is sufficient. There may be other processes holding nvidia references—the 413 users might include system processes, monitoring agents, or other containers. The assistant implicitly assumes the LXC container is the primary consumer.
That the remote host is accessible and the SSH credentials are valid. The command runs without visible authentication issues.

Input Knowledge Required

To fully understand this message, a reader needs knowledge of several domains:

Linux kernel module mechanics: The concept of "module users" as tracked by lsmod, and the fact that a module cannot be unloaded (rmmod) while processes hold references to it. The nvidia driver stack consists of multiple interdependent modules (nvidia, nvidia_modeset, nvidia_uvm, nvidia_drm), which must be unloaded in reverse dependency order.

PCI device topology and driver binding: The BDF (Bus:Device.Function) addressing scheme (0000:01:00.0), the concept of driver binding via /sys/bus/pci/drivers/, and the PCI remove/rescan cycle. The assistant tracks four specific GPUs by their BDF addresses and knows their individual states.

IOMMU groups and domain types: The distinction between DMA-FQ (translation mode) and identity (passthrough mode) IOMMU domains, and the implications for P2P DMA. The /sys/kernel/iommu_groups/ interface for querying and setting domain types.

Blackwell GPU architecture: The GSP/FSP firmware component, its initialization sequence during driver probe, and its sensitivity to state resets. The error codes (0x177, _kgspBootGspRm) that indicate GSP initialization failures.

Systemd service management: The systemctl stop, systemctl is-active commands, and the convention of checking service state after stopping.

LXC containerization: The concept of an LXC container running on a separate host (10.1.230.174) that shares GPU resources with the host system, and the implications for module reference counting when GPU devices are passed through to containers.

Output Knowledge Created

This message produces several pieces of actionable knowledge:

The system is in a partial state. The timeout left the reconfiguration incomplete, with one GPU detached and three still bound. This is a fragile state—the detached GPU cannot be used for inference, and the remaining GPUs are operating under the wrong IOMMU mode.
The LXC container is the blocker for module unloading. The 413 module users are attributed to the SGLang service running in the container. This is a specific, testable hypothesis that the assistant immediately acts on.
The service can be stopped remotely. The SSH command succeeds, confirming network connectivity and service manageability. The service reports "inactive" and "stopped," indicating a clean shutdown.
The reconfiguration sequence must be retried from scratch. The assistant's stated plan ("then do the full cycle") implies that the previous attempt's partial state must be cleaned up and the entire unbind-remove-SBR-rescan-identity-reload sequence must be re-executed from the beginning.

The Thinking Process

The reasoning visible in this message is a textbook example of diagnostic inference under partial information. The assistant starts with an observed fact: the previous command timed out. It then probes the system state (via the context of [msg 6385], which shows the nvidia-smi and lsmod output) and discovers an inconsistency—only three of four GPUs are visible to nvidia-smi, and the nvidia module is still loaded.

The key inference step is connecting the module user count to the LXC container. The assistant does not have direct evidence that the container is running—it infers this from the high user count (413) and the knowledge that the container is the primary GPU workload on this system. This is a Bayesian-style inference: given the system architecture and the observed module usage, the most likely explanation is that the container's processes are holding the references.

The assistant also demonstrates an understanding of causality and ordering constraints. It recognizes that "we can't unload nvidia while the container is using it" and therefore the container must be stopped before attempting the module unload. This ordering constraint is fundamental to the reconfiguration sequence—you cannot skip ahead to the identity-domain setup without first clearing the nvidia module.

The decision to stop the service remotely (via SSH to 10.1.230.174) rather than locally (on the host at 10.1.2.6) is also telling. The assistant understands the system topology: the LXC container is a separate logical host, and its services must be managed through its own systemd instance. The SSH command is the correct interface for this remote management.

Mistakes and Incorrect Assumptions

While the diagnostic reasoning is sound, several assumptions embedded in this message deserve scrutiny:

The assumption that stopping the service is sufficient to release all module references. In practice, the nvidia module's user count includes not just process file descriptors but also internal kernel references (e.g., from the DRM subsystem, from render nodes, from the nvidia-persistenced daemon). The 413 users might not all be attributable to the SGLang container. If the nvidia-persistenced service is running, it will hold a persistent reference to the nvidia module even after all CUDA processes terminate. The assistant does not check for this.

The assumption that the full cycle will succeed this time. The previous attempts have all failed at various points—the GSP firmware state survives SBR, the nvidia module auto-loads during rescan, the identity domain setting races with driver binding. The assistant's confidence ("then do the full cycle") may be premature given the accumulated evidence that the Blackwell GSP is fundamentally incompatible with post-boot IOMMU reconfiguration.

The assumption that the timeout was caused by the module unload hanging. The timeout could have occurred at any point in the 10-step sequence. The assistant implicitly assumes it was the rmmod step that hung (because the module had active users), but the SBR step or the rescan step could also have timed out under certain conditions.

The assumption that the LXC container is at 10.1.230.174. This is consistent with the system topology established in earlier messages, but it's worth noting that the assistant is operating with a mental map of the network that may not be verified in this message.

Broader Significance

This message, while brief, captures a pivotal moment in the reconfiguration effort. It represents the transition from a failed attempt (the timeout) to a new attempt with a corrected prerequisite (stopping the container). The assistant demonstrates the essential skill of reading system state, forming hypotheses about root causes, and taking corrective action.

The partial-state diagnosis is particularly instructive. In complex systems engineering, "partial failure" states are among the most dangerous—the system appears to be running (three GPUs are working), but it is in an inconsistent configuration that will produce mysterious failures later. The assistant's ability to detect this inconsistency and refuse to proceed until the state is cleaned up is a mark of disciplined operations.

The message also highlights the tension between the assistant's iterative, experimental approach and the underlying hardware constraints. Each attempt to reconfigure the IOMMU domains risks leaving the system in a degraded state (as happened here), and the recovery path requires careful sequencing. The Blackwell GSP's resistance to post-boot reconfiguration is a hard constraint that no amount of scripting can circumvent—a lesson that will become definitive later in this segment when the assistant ultimately concludes that "per-group IOMMU identity domains are fundamentally incompatible with Blackwell GPUs."

Conclusion

Message [msg 6386] is a compact but rich example of diagnostic reasoning in a complex systems context. It demonstrates how an experienced operator reads system state from multiple sources (nvidia-smi, lsmod, PCI device files), synthesizes those observations into a coherent model of the system's partial failure, identifies the root cause blocker (the LXC container holding module references), and takes the appropriate corrective action (stopping the service). The message also reveals the assistant's mental model of the system topology, its understanding of kernel module dependency ordering, and its ability to execute remote management commands across the network boundary.

The partial-state diagnosis and the decision to stop the container before proceeding are the key contributions of this message. They set the stage for the next attempt at the reconfiguration sequence—an attempt that will ultimately fail for deeper architectural reasons, but that failure will itself be informative, leading to the final conclusion that Blackwell's GSP firmware is fundamentally incompatible with IOMMU identity domain reconfiguration after boot.