83 lines
5 KiB
Markdown
83 lines
5 KiB
Markdown
---
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id: "bug-1893"
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status: "fixed"
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status_updated: "2026-09-30"
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status_note: ""
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component: "scheduler"
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severity: "wrong-result"
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timestamp: "2026-09-30T03:58:52Z"
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last_seen: "2026-09-30T03:58:52Z"
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occurrences: 1
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files: ["src/scheduler/src/plugins/seqjob_allocator.c", "src/scheduler/src/plugins/parjob_allocator.c"]
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tags: ["scheduler", "allocation", "preason", "code-review"]
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related: ["bug-1892"]
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fix_commits: ["d9ef9830", "239bcf95", "65679285"]
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---
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# bug-1893: xce_scheduler allocators never record shared-resource pend reasons (vget rc check inverted)
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## Description
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**Observed:** found by code reading, so there is no failing run yet. When the shared-resource check
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fails, both allocators add the pend reasons with
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```c
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for (i = 0; i < ctx->alloc_req.shared.count; i++) {
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rc = vget(core->resmgr, ((int*)ctx->unmeet.idxs.data)[i], (void**)&r);
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if (rc <= 0) continue;
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add_job_preason(reason->reasons, PREASON_NO_RESOURCE, NULL, r->name);
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}
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```
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(`seqjob_allocator.c` `my_alloc`, `parjob_allocator.c` `my_alloc`). The loop
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bound is the number of *requested* shared resources, but `ctx->unmeet` holds
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only the *unmet* ones, packed at `[0, unmeet.count)` by `unsatisfy_()`
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(`resource.c:152-165`). Its arrays were `calloc`ed with capacity
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`shared.count` (`resource.c:280`), so entries `[unmeet.count, shared.count)`
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are 0.
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**Expected:** iterate `i < ctx->unmeet.count` (as the host path does,
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`resource.c:566`), giving one `NO_RESOURCE` reason per unmet shared resource.
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**Impact:** wrong pend reason, noise level. When a job requests N shared
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resources and only M < N are unmet, the loop adds N - M extra
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`PREASON_NO_RESOURCE` reasons naming whatever resource has index 0 (if one
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exists), which that job may never have requested. Main reason and allocation
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decision are unaffected. No out-of-bounds memory access (the tail is inside the
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allocation).
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**Found by:** review of the `scheduler-allocator-runner` requirements
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(2026-09-29); the loop moves into the framework runner in that spec.
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UPDATE (2026-09-30): Found in scheduler-allocator-runner task 2.1 (characterization test test_seqjob_pending_on_short_shared_no_reason_today): the observable effect is not an extra resource-0 reason. vget() (xzarray_get_vec, src/lib/zarray.c) returns 0 on success, and the loop skips on rc <= 0, so it skips every successful lookup and NO shared NO_RESOURCE reason is ever recorded; the over-read tail entries (idx 0) are skipped the same way. Real impact: a job pending on a short shared resource carries no resource reason naming it (main_reason stays 0). The host path (resource.c) correctly skips only rc < 0.
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## Environment
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- OS: no /etc/os-release; Docker container; NixOS kernel; kernel Linux 6.18.33 x86_64
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- libc: glibc 2.42-47 (interpreter /nix/store/wb6rhpznjfczwlwx23zmdrrw74bayxw4-glibc-2.42-47/lib/ld-linux-x86-64.so.2)
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- Toolchain: gcc (GCC) 15.2.0, meson 1.9.1
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- Build: asan (xce.build)
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- Code: scheduler-framework @ a370d724-dirty
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- Deployment: n/a: found by code reading; any deployment where a job requests 2+ shared resources and only some are short
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## Reproduction
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1. `git checkout a370d724`; read the loop in
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`src/scheduler/src/plugins/seqjob_allocator.c` / `parjob_allocator.c`
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`my_alloc()` (after `sched_check_shared_satisfy`), and
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`src/scheduler/src/resource.c:130-169,266-316`.
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2. Unit-test sketch: resource idx 0 = `slots`; two shared resources A (idx 7,
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plenty) and B (idx 8, total 0); a seqjob primary requesting 1 of each.
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Expected: exactly one host-less reason (`NO_RESOURCE`, res "B"); today a
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second reason names `slots`.
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- Frequency: deterministic (by code reading; no test exercises it yet)
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- Evidence: not kept (code reading only)
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## Root cause
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Both allocators' `my_alloc()` shared-reason loop skipped on `vget() <= 0`, but `vget()` (`xzarray_get_vec`, src/lib/zarray.c) returns 0 on success, so every successful lookup was skipped and no shared `PREASON_NO_RESOURCE` reason was ever added. The loop also ran to `ctx->alloc_req.shared.count` instead of `ctx->unmeet.count` (the tail entries were zero-filled and skipped the same way).
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## Fix
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Allocation moved into the framework runner (spec scheduler-allocator-runner). `sched_alloc_prepare_shared_()` in src/scheduler/src/alloc_runner.c loops over `ctx->unmeet.count` and skips only on `vget() < 0`, adding one host-less NO_RESOURCE reason per unmet shared resource. parjob (task 4.1) and seqjob (task 4.2) now allocate through it; both private loops are deleted.
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## Verification
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- Test: `test_shared_short_one_reason_per_unmet_entry` (testsuite/test_alloc_runner.c) RED under an `rc <= 0` skip ("Expected 1 Was 0"), GREEN after; characterization `test_parjob_pending_on_short_shared_names_short_one` and `test_seqjob_pending_on_short_shared_names_it` RED against the old plugins ("Expected 1 Was 0"), GREEN after the switch.
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- Suites: scheduler ASAN suite 96/96 (`bash src/tools/xce-build.sh scheduler asan build.alloc --jobs 12`), 2026-09-30.
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- Live: n/a (pend-reason content only; main_reason, decisions, unsatisfiable cache and sim parity unaffected).
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## References
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- Related: none
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