[harshavardhana]

Stacked on #2379. This branch includes #2379's commit. Review only the last commit, 5024b442. I'll rebase onto main after #2379 merges.

What?

Let applications register GPU memory (VRAM) with the accelerated OBJ engine (accelerated=true) from any thread, not only from a thread that already has a CUDA context.

Why?

cuMemObjGetDescriptor registers device memory in the calling thread's current CUDA context. A thread that has made no CUDA calls has no current context, so the registration fails with rc=1 and registerMem returns NIXL_ERR_BACKEND:

E cuobj_client.cpp:58] cuMemObjGetDescriptor failed rc=1 ptr=0x76ca48000000 size=67108864
BUG REPRODUCED: VRAM registration from a worker thread failed: NIXL_ERR_BACKEND

This affects applications that register memory from a worker or connector thread instead of the thread that allocated the buffer. The same call succeeds from the main thread, or after the worker thread calls torch.cuda.set_device(0).

How?

• In S3AccelObjEngineImpl::registerMem, a VRAM_SEG registration selects the buffer's device (mem.devId) before calling cuObject, then restores the caller's device. The GDS and Infinia backends already call cudaSetDevice(mem.devId) before registering VRAM.
• The OBJ plugin now links cuda_dep and defines HAVE_CUDA when CUDA is found, so builds without CUDA are unchanged.
• New gtest AccelVramWorkerThreadRegistrationTest allocates VRAM on the test thread, registers it from a std::thread that makes no CUDA calls, then writes 1 MiB, reads it back over RDMA and checks the data.
• The round-trip code from OBJ: share one cuObject registration across agents registering a buffer #2379's AccelSharedBufferRegistrationTest moves into a testRoundTrip helper that both tests use.

The Dell engine (S3DellObsObjEngineImpl::registerMem) calls cuMemObjGetDescriptor directly and has the same gap. It also keeps its own cuObjClient for each engine, so it may also hit the duplicate-registration problem from #2379. I don't have a Dell ObjectScale endpoint to test against, so I left it for a follow-up.

Reproducer

vram_worker_thread_repro.py

#!/usr/bin/env python3
"""Reproducer: VRAM registration from a worker thread fails on the OBJ RDMA path.

The buffer is allocated on the main thread. Registration then runs on a new
thread that has made no CUDA calls, as an application's worker or connector
thread would, so no CUDA context is current on it.

Env: S3_ENDPOINT, S3_BUCKET, S3_ACCESS_KEY, S3_SECRET_KEY, S3_REGION, S3_SCHEME
"""

import os
import sys
import threading

import torch

from nixl._api import nixl_agent, nixl_agent_config

BACKEND_PARAMS = {
    "bucket": os.environ["S3_BUCKET"],
    "access_key": os.environ["S3_ACCESS_KEY"],
    "secret_key": os.environ["S3_SECRET_KEY"],
    "region": os.environ.get("S3_REGION", "us-east-1"),
    "endpoint_override": os.environ["S3_ENDPOINT"],
    "scheme": os.environ.get("S3_SCHEME", "http"),
    "accelerated": "true",
}

buf = torch.empty(64 << 20, dtype=torch.uint8, device="cuda:0")
result = {}


def register():
    agent = nixl_agent("worker", nixl_agent_config(backends=[]))
    agent.create_backend("OBJ", BACKEND_PARAMS)
    try:
        descs = agent.register_memory(
            [(buf.data_ptr(), buf.numel(), 0, "")], mem_type="VRAM", backends=["OBJ"]
        )
        agent.deregister_memory(descs, backends=["OBJ"])
        result["ok"] = True
    except Exception as e:
        result["error"] = e


t = threading.Thread(target=register)
t.start()
t.join()

if "error" in result:
    print(f"BUG REPRODUCED: VRAM registration from a worker thread failed: {result['error']}")
    sys.exit(1)
print("OK: VRAM registration from a worker thread succeeded")

Testing

Tested on an H200 host with ConnectX (RoCE) NICs and cuObject 13.3, against an AIStor endpoint that supports S3-over-RDMA:

Test	#2379 only	This PR
Reproducer above	fail: cuMemObjGetDescriptor failed rc=1	pass
AccelVramWorkerThreadRegistrationTest (new)	fail: same	pass
Other ObjAccelTests (9 tests)	pass	pass

I also ran a wider set of registration and transfer scenarios: shared, nested and concurrent registration, deregistering in random order, repeated register/deregister cycles, unaligned offsets, and many buffers. All 14 pass with both GPU memory and pinned host memory.

Like the other accel tests, the new test is skipped in CI because CI has no RDMA fabric.

Summary by CodeRabbit

• Bug Fixes
  • Improved CUDA handling during accelerated OBJ VRAM registration. Registration now selects the device associated with the buffer and restores the calling thread’s previous CUDA context afterward.
  • VRAM registration reports an error if the CUDA context cannot be read or the device cannot be selected. DRAM registration continues without selecting a CUDA device.
  • Added coverage for VRAM registration from a worker thread without an existing CUDA context and for preserving an existing context.

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📝 Walkthrough

The OBJ accelerated build now enables CUDA support when CUDA is found. VRAM registration selects the buffer’s device and restores the calling thread’s saved context when available. CUDA-gated tests cover registration from worker threads and preservation of an existing context.

Changes: OBJ CUDA device-aware registration

Layer / File(s)	Summary
CUDA build support and context guard src/plugins/obj/meson.build, src/plugins/obj/s3_accel/engine_impl.cpp	When CUObjClient and CUDA are available, the build adds CUDA dependencies and defines HAVE_CUDA. The CUDA-only guard records context or device-selection errors and restores the saved context when available.
Device-aware VRAM registration src/plugins/obj/s3_accel/engine_impl.cpp, test/gtest/plugins/obj_cuobj_plugin.cpp	RDMA-ready registration selects mem.devId for VRAM and returns NIXL_ERR_BACKEND if selection fails. CUDA-gated tests cover worker-thread registration, a VRAM round trip, and preservation of an existing context.

Priority: ➖ Normal

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix

Sequence Diagram(s)

sequenceDiagram
  participant Registration as RDMA-ready registration
  participant Guard as scopedCudaDevice
  participant CUDA as CUDA driver/runtime
  participant Buffer as RDMA buffer registration
  Registration->>Guard: select mem.devId for VRAM
  Guard->>CUDA: read current context and select device
  Registration->>Buffer: register buffer when selection succeeds
  Guard->>CUDA: restore saved context on destruction

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Merge Risk: ⚪ Minimal · up to 20171

No confirmed issue prevents merging. The RDMA-backed tests were not run in this review and still require validation on suitable hardware.

Pre-merge checks · 4 · 1

Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 26.92% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 26 functions across 4 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks (4 passed)

Check name	Status	Explanation
Description check		The description includes complete What, Why, and How sections. It explains the CUDA context issue, implementation, testing, CI limitations, and related follow-up scope.
Title check		The title clearly and concisely describes the main change: selecting the buffer's CUDA device during VRAM registration for RDMA.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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[maheshrbapatu]

One context-restoration issue below, reproduced with the guard from this PR.

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Actionable comments posted: 2

---

• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @src/plugins/obj/s3_accel/engine_impl.cpp:
- Around line 34-36: Update the context-capture flow around cuCtxGetCurrent to
report capture failures and stop registration before device selection when
capture fails; preserve successful captures of a null context without treating
them as errors.
- Around line 27-30: Change the comment documenting scopedCudaDevice to Doxygen
style, preserving its explanation of CUDA context selection and restoration.

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[svc-nixl]

🤖 CI Triage Agent — PR Size Check · commit d98e704b

TL;DR: The PR Size Check workflow failed because PR #2380 adds 580 lines to existing (modified) files, exceeding the repo's hard 500-line policy limit; the fix is to split the PR, not to change CI.

Full analysis

Summary: GitHub Actions job check-pr-size (run 38216219248) exited 1 at the "Check PR size" step with ❌ PR size check failed! This PR adds 580 lines (excluding subprojects), which exceeds the maximum of 500 lines.

Root cause: Not a defect or infra problem — this is an intentional policy gate firing. .github/workflows/pr-size-check.yml computes added lines on modified files via git show --numstat --pretty="" --diff-filter=M -- . ':(exclude)subprojects/*' against the merge commit bf11456 (merge of d98e704 into e3e1700) and fails when the sum exceeds 500. The sum for this PR is 580, so exit 1 was taken. The log shows a clean checkout and the step completing in ~3 ms — no timeout, no runner issue, no flakiness.

Worth noting about the check's semantics: --diff-filter=M counts only modified files, so lines in newly added files are not counted at all. The 580 therefore comes purely from edits to pre-existing files on the obj-rdma-vram-thread-device branch — the real change is at least that large.

Implicated commit: [REDACTED:Hex High Entropy String] (PR #2380 head, branch obj-rdma-vram-thread-device) — the oversized change. The gate itself was introduced in 16dcb7db, "[CI] Add new CI test to detect large PRs (#627)", Daniel Pressler.

File: .github/workflows/pr-size-check.yml:21-26 (threshold at line 23)

Suggested fix: Split PR #2380 into reviewable pieces under 500 modified-file lines each — e.g. separate the VRAM/thread-device object-RDMA core change from test and example updates, and stack them as dependent PRs (the repo already uses this "part N of M" pattern, as in #2283/#2327/#2328). Alternatives, in order of preference:

1. Move net-new code into new files where practical — --diff-filter=M excludes added files, so genuinely new modules don't count against the budget.
2. If the change is truly atomic and cannot be decomposed, request a maintainer override rather than editing the workflow; raising the 500 limit in pr-size-check.yml for one PR would weaken the gate repo-wide and should not be done as part of this PR.

Related: PR #2380; gate added by PR #627. Similar policy-driven splits: #2283, #2327, #2328.

🛡️ This comment had 1 potential secret(s) redacted (Hex High Entropy String). See request_id 010dff5e-de93-4aa3-b803-a1995e858e86 in the triage console for the audit trail.

[svc-nixl]

🤖 CI Triage Agent — PR Size Check · commit 0ad54668

TL;DR: Nothing is broken in CI — the PR Size Check gate failed because PR #2380 modifies 608 lines in existing files, above the hard 500-line limit; the fix is to split the PR (or add an override label to the workflow), not to debug the build.

Full analysis

Summary: The check-pr-size job in .github/workflows/pr-size-check.yml exited 1 with "This PR adds 608 lines (excluding subprojects), which exceeds the maximum of 500 lines."

Root cause: A deliberate repository policy gate, not a defect or infra problem. The single-step job computes added lines across modified files only and fails hard above 500:

LINES_CHANGED=$(git show --numstat --pretty="" --diff-filter=M -- . ':(exclude)subprojects/*' | awk '{s+=$1} END {print s}')
if [[ "$LINES_CHANGED" -gt 500 ]]; then ... exit 1

The log shows the whole job ran in ~2 seconds (19:09:01 checkout → 19:09:03 failure) with no gaps, no compile step, and no test execution — checkout succeeded cleanly at 6babfd5 (merge of 0ad5466 into 86d9626), and the only error is the size assertion. The measured value was 608, i.e. 108 lines over the threshold. There is no exemption path in the workflow: no label check, no continue-on-error, no maintainer override, so any PR over the limit blocks unconditionally.

Two properties of the check are worth knowing when deciding how to split: --diff-filter=M counts only lines added to pre-existing files (brand-new files are invisible to the gate), and it counts additions only, not net change — so moving code into a new file, rather than growing an existing one, reduces the measured size.

Implicated commit: The gate itself is 16dcb7db ("[CI] Add new CI test to detect large PRs (#627)", Daniel Pressler, 2025-08-14) — unchanged since. The oversized change is PR #2380 head [REDACTED:Hex High Entropy String] on obj-rdma-vram-thread-device.

File: .github/workflows/pr-size-check.yml:21-26 (threshold at line 23)

Suggested fix: Primary action is on the PR author: split #2380 into reviewable pieces under 500 added lines each — the same incremental pattern already used elsewhere in this repo (e.g. the "CPU Proxy part N" series #2283/#2327/#2328, and the sibling OBJ change #2382). Since the gate ignores new files, factoring the new VRAM/CUDA-device selection logic and its tests into new source/test files rather than expanding existing ones will both shrink the diff and lower the measured count.

Separately, if maintainers want an escape hatch for legitimately large-but-atomic changes, the workflow needs one added — it currently has none. A minimal version is to skip the failure when a label is present:

- name: Check PR size
  if: ${{ !contains(github.event.pull_request.labels.*.name, 'size-override') }}

Also consider having the step print the per-file --numstat breakdown before failing; right now it emits only the total, so an author has to reproduce the command locally to see what is driving the number.

Related: PR #2380 (this PR), #2382 (sibling OBJ change), #627 / commit 16dcb7db (introduced the gate)

🛡️ This comment had 1 potential secret(s) redacted (Hex High Entropy String). See request_id c8cb767e-b419-4d3d-ab9e-3dd948084432 in the triage console for the audit trail.

[svc-nixl]

🤖 CI Triage Agent — PR Size Check · commit 9473657a

TL;DR: The PR Size Check workflow failed because PR #2380 modifies 611 lines in existing files, over the hard 500-line cap enforced by .github/workflows/pr-size-check.yml. This is a working policy gate, not a broken build — the fix is to split the PR (or get the limit/exemption adjusted), not to retry CI.

Full analysis

Summary: Job check-pr-size exited 1 at the "Check PR size" step with ❌ PR size check failed! This PR adds 611 lines (excluding subprojects), which exceeds the maximum of 500 lines.

Root cause: Policy violation, not a defect. The workflow runs git show --numstat --pretty="" --diff-filter=M -- . ':(exclude)subprojects/*' against the PR merge commit (e73119a, merging 9473657 into 9fbfcbb) and sums the added-lines column. The result, 611, is compared with -gt 500 and the step exits 1. The whole job ran in ~2 seconds (20:22:54 → 20:22:56) with no gaps, no timeouts, and no infrastructure involvement — the gate simply fired as designed. Note the metric is additions to modified files only (--diff-filter=M), so brand-new files aren't counted; the 611 lines are all edits to pre-existing files.

Implicated commit: [REDACTED:Hex High Entropy String] (head of obj-rdma-vram-thread-device, PR #2380) is what tripped the gate. The gate itself was added in 16dcb7d, Daniel Pressler, "[CI] Add new CI test to detect large PRs (#627)".

File: .github/workflows/pr-size-check.yml:21-26 (threshold at line 23)

Suggested fix: Split PR #2380 into smaller PRs each under 500 modified-file additions — the natural seam for "OBJ: select the buffer's CUDA device when registering VRAM for RDMA" is to land the device-selection helper/plumbing separately from the test and call-site changes. Two secondary options if splitting isn't practical: (a) move net-new code into new files, since --diff-filter=M excludes additions, though that games the metric rather than satisfying its intent; (b) ask the CI owners to add an explicit opt-out, e.g. a size-exempt label checked in the if: of the step, as there is currently no override mechanism at all. Don't re-run the job — it is deterministic on this diff and will keep failing until the diff shrinks.

Related: PR #2380 (#2380). Other open PRs likely hitting the same gate: #2283, #2327, #2328 — if this is a recurring blocker across a multi-part series, raising the limit or adding a label-based exemption is worth discussing with the workflow owner rather than fixing per-PR.

🛡️ This comment had 1 potential secret(s) redacted (Hex High Entropy String). See request_id 5272aa89-aa4b-4249-afea-e2d2a39cb422 in the triage console for the audit trail.

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