The Brutal Cleanup: Killing GPU Processes to Unlock IOMMU Identity Domains

In the intricate dance of GPU-level system administration, sometimes the most profound insights are encoded in the smallest gestures. Message [msg 6387] in this opencode session is a case study: a single bash command, just three operations chained together, that reveals the entire arc of a multi-hour debugging odyssey. The message reads:

[assistant] [bash] ssh root@10.1.2.6 'pct exec 129 -- bash -c "ps aux | grep python3 | grep -v grep | awk \"{print \\\$2}\" | xargs -r kill -9" 2>/dev/null; pct exec 129 -- bash -c "fuser -k /dev/nvidia* 2>/dev/null"; sleep 2; lsmod | grep nvidia'

This is not a random command. It is the culmination of a painful discovery: that Blackwell GPUs' Firmware Security Processor (FSP) cannot initialize when IOMMU is in identity mode, and that the only remaining path to P2P DMA — dynamic reconfiguration via PCI resets — is blocked by the simple fact that containerized processes hold GPU resources hostage.

The Context: A Quest for P2P DMA

To understand why this message exists, we must trace the reasoning that led here. The session's overarching goal was to restore Peer-to-Peer (P2P) DMA between NVIDIA RTX PRO 6000 Blackwell GPUs on a Proxmox host. P2P DMA allows GPUs to communicate directly across the PCI bus without staging through host memory, which is critical for multi-GPU inference serving (e.g., tensor parallelism across four GPUs). Earlier in the session ([msg 6380]), the assistant had discovered that P2P DMA was broken under the SEV-SNP IOMMU configuration required for confidential computing, manifesting as NCCL hangs and corrupted data.

The assistant had identified a potential workaround: switching individual IOMMU groups from their default DMA-FQ (DMA with Fine-grained Queuing) translation mode to identity mode, which bypasses IOMMU translation for those devices while leaving the rest of the system protected. This approach had been tested and validated in principle — identity domains did enable P2P DMA — but a catastrophic problem emerged: Blackwell GPUs' FSP (Firmware Security Processor) fails to boot with error code 0x177 when IOMMU is in identity mode during driver initialization. The FSP apparently requires specific DMA mappings that only exist in translation mode, and identity mode breaks this initialization irrecoverably.

This was a show-stopper. As the chunk summary notes: "per-group IOMMU identity domains are fundamentally incompatible with Blackwell GPUs — the approach cannot work regardless of timing." The only remaining hope was a dynamic approach: boot the GPUs with DMA-FQ, then at runtime remove the GPU devices, set identity domains on their IOMMU groups, and re-enumerate them so the nvidia driver re-probes the devices under identity mode. This required a precise sequence of PCI hot-unplug, Secondary Bus Reset (SBR), PCI rescan, and driver reload — all while ensuring no process held GPU references.

The Partial Failure That Triggered This Message

In [msg 6385], the assistant attempted this dynamic reconfiguration but the script timed out after 120 seconds. The post-mortem in [msg 6386] revealed the system in a "partial state": GPU 0000:01:00.0 had been successfully removed (driver=none), but GPUs 0000:11:00.0, 0000:61:00.0, and 0000:71:00.0 remained bound to the nvidia driver. Critically, nvidia module had 413 users and nvidia_uvm had 44 users. The assistant correctly diagnosed the cause: "likely the LXC container started and has processes using GPUs."

The assistant then attempted to stop the SGLang service in the LXC container via systemctl stop sglang-qwen.service, which reported success. But stopping the service alone was insufficient — the GPU processes needed to be forcibly terminated.

The Reasoning Behind the Command

Message [msg 6387] is the assistant's response to this diagnosis. The command executes three operations inside LXC container 129 (the production SGLang container):

Operation 1: Kill all Python processes. The pipeline ps aux | grep python3 | grep -v grep | awk "{print \$2}" | xargs -r kill -9 finds every running Python process and sends SIGKILL. This is deliberately brutal — kill -9 cannot be caught or ignored. The -r flag on xargs ensures that if no PIDs are found, no kill command is issued (avoiding an error from an empty kill invocation).

Operation 2: Kill all processes holding NVIDIA device files. The fuser -k /dev/nvidia* command identifies every process with an open file handle on any /dev/nvidia* device (which includes /dev/nvidia0, /dev/nvidia1, /dev/nvidiactl, /dev/nvidia-uvm, etc.) and sends SIGKILL. This is a more comprehensive cleanup than just killing Python processes — it catches any process (including helper binaries, monitoring agents, or zombie processes) that might be holding GPU resources.

Operation 3: Verify module state. After a 2-second pause, lsmod | grep nvidia checks whether the nvidia driver modules are still loaded. If the kill operations were successful, the module reference counts should drop to zero, allowing subsequent rmmod commands to unload the modules.

Assumptions and Their Validity

The assistant made several assumptions in crafting this command:

Assumption 1: Killing Python processes is sufficient to release GPU resources. This is generally true for SGLang and similar inference servers, where the main process holds GPU context via CUDA. However, the assistant wisely added the fuser fallback to catch edge cases. This assumption was reasonable but incomplete — the earlier systemctl stop had already sent SIGTERM to the main process, but child processes or orphaned workers might have survived.

Assumption 2: The LXC container is the only GPU user. The assistant assumed that the 413 nvidia module users were entirely within container 129. This was a reasonable inference — the container was running the production SGLang service — but it's possible that the host itself or other containers had GPU processes. The command only targets container 129, which could leave other GPU users alive.

Assumption 3: A 2-second sleep is sufficient for process cleanup. After sending SIGKILL, the kernel needs time to reap the processes and release their resources (file descriptors, memory mappings, GPU contexts). Two seconds is usually enough, but on a heavily loaded system with many GPU contexts to tear down, it might be insufficient. The assistant's follow-up check via lsmod would catch this.

Assumption 4: fuser -k will successfully kill all GPU-holding processes. The fuser command can fail if processes are owned by different users or if the process is in an unkillable state (e.g., D-state waiting on I/O). The 2>/dev/null redirection suggests the assistant anticipated potential failures.

Input Knowledge Required

To fully understand this message, the reader needs knowledge of:

LXC container management: pct exec 129 executes a command inside Proxmox LXC container 129. Understanding Proxmox's container tooling is essential.
NVIDIA GPU device files: /dev/nvidia* encompasses the control device (nvidiactl), GPU devices (nvidia0, nvidia1, etc.), and the UVM (Unified Virtual Memory) device (nvidia-uvm). These are the kernel-level interfaces for CUDA.
Linux kernel module reference counting: lsmod shows module usage counts. A non-zero "Used by" count means the module cannot be unloaded. Each process using CUDA increments the nvidia module's reference count.
The PCI hotplug and IOMMU architecture: The broader context involves PCI device removal (/sys/bus/pci/devices/*/remove), Secondary Bus Reset (SBR), IOMMU group type switching (/sys/kernel/iommu_groups/*/type), and the nvidia driver's probe path. Without this context, the command seems like random process killing.

Output Knowledge Created

This message produced several pieces of actionable information:

Confirmation of process cleanup: The lsmod output would reveal whether the kill operations succeeded in reducing nvidia module reference counts to zero. If counts remained high, the assistant would know that other processes (perhaps on the host or in other containers) were still holding GPU resources.
Validation of the cleanup strategy: Success would validate that forcible process termination is a viable prerequisite for GPU driver reload. Failure would force the assistant to investigate other sources of GPU usage.
A reusable cleanup pattern: The combination of pct exec with ps | grep | kill and fuser -k establishes a template for cleaning up GPU-holding processes in containerized environments — a pattern that could be extracted into a script for future use.

The Thinking Process Visible

The reasoning in this message is compressed but visible. The assistant had just discovered ([msg 6386]) that the system was in a "partial state" with nvidia having 413 users. The immediate conclusion — "We can't unload nvidia while the container is using it" — shows correct diagnosis. The assistant then tried the polite approach (systemctl stop), which reported success but clearly didn't fully clean up (the service was "inactive" but GPU processes remained).

The shift from polite service stop to forcible process kill represents a critical reasoning step: the assistant recognized that systemctl stop only terminates the main service process, not necessarily all child processes or processes that had inherited GPU file descriptors. The fuser -k approach is more thorough because it operates at the file descriptor level rather than the process name level.

The 2>/dev/null on both commands is also telling — it shows the assistant anticipated that some of these operations might fail (e.g., if no Python processes were found, or if fuser couldn't kill certain processes), and the assistant wanted clean output for the subsequent lsmod check.

Mistakes and Incorrect Assumptions

The most significant potential mistake is the assumption that all GPU users are within container 129. The host system itself might have processes using CUDA (e.g., monitoring agents, the SGLang host process if any part runs outside the container). The 413 module users could include kernel threads, UVM handles, or processes from other containers. If lsmod still shows non-zero counts after the kill, the assistant would need to broaden the search.

Another subtle issue: fuser -k /dev/nvidia* will fail silently (due to 2>/dev/null) if the glob expands to no files — for instance, if the nvidia device files were already removed during the earlier PCI removal step. The assistant's earlier script had removed GPU 0000:01:00.0 from the PCI bus, which could have affected device file availability.

The Broader Significance

This message, for all its apparent brutality, represents a key moment of learning in the session. The assistant had been operating under the assumption that GPU driver reconfiguration could be done cleanly — stop the service, unload the module, reconfigure, reload. The discovery that service stop is insufficient, and that forcible process termination is required, reflects a deeper understanding of how NVIDIA's driver stack interacts with the Linux process model.

The CUDA driver creates per-process state that persists beyond the main service process. Child processes, forked workers, or processes that have called cudaSetDevice() maintain references that increment the module's usage count. A simple systemctl stop may terminate the parent but leave orphaned children or processes with inherited GPU contexts. The fuser -k approach, operating at the VFS level, catches all processes with open file handles on GPU devices regardless of their parentage.

This insight — that GPU resource cleanup requires VFS-level rather than process-tree-level intervention — is a valuable piece of systems knowledge that emerges from this debugging session. It's the kind of hard-won understanding that distinguishes engineers who have wrestled with real GPU infrastructure from those who have only worked in cleanroom environments.

Conclusion

Message [msg 6387] is a masterclass in concise, targeted system administration. In three piped commands, it encapsulates the assistant's diagnosis of the previous failure, its understanding of the Linux process model and GPU driver architecture, and its strategy for proceeding. The command is not elegant — it uses kill -9 and fuser -k, the bluntest instruments available — but it is appropriate for the situation. When you need to reconfigure GPU IOMMU domains at runtime, and Blackwell's FSP refuses to initialize under identity mode, there is no room for gentleness. You kill the processes, you check the module counts, and you try again.