85 lines
11 KiB
Markdown
85 lines
11 KiB
Markdown
---
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id: "bug-1932"
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status: "fixed"
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status_updated: "2026-10-09"
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status_note: ""
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component: "scheduler"
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severity: "wrong-result"
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timestamp: "2026-10-09T07:23:52Z"
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last_seen: "2026-10-09T07:23:52Z"
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occurrences: 1
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files: ["src/scheduler/src/scheduler.c", "src/scheduler/src/alloc_cache.c", "src/scheduler/src/main.c"]
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tags: ["scheduler", "sim", "alloc-cache", "pend-reason"]
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related: []
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fix_commits: ["0317c7af"]
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gitea_issue: 357
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gitea_synced_status: "fixed"
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gitea_synced_hash: "0eeb39bc0f79"
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---
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# bug-1932: xce_scheduler unsatisfiable-request cache drops pend reasons on a hit and is unused in sim
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## Description
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**Observed:** in a `xce_scheduler --sim` session, array elements `[5]` and `[6]` have identical requests (`tasks 2..5`, mem 400 KB, same user/queue/project). `[5]` exhausts all 5 plans and pends; `[6]` then repeats the exact same search instead of reusing `[5]`'s verdict:
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```
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[sim] PEND jobid=0-5-1791529946-0[5]
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...
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plans: 5 generated, 5 tried, 0 discarded, 0 not tried; no plan could be placed
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outcome: PEND
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main reason: none
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host pc13v: resource slots: NO_RESOURCE
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[sim] PEND jobid=0-5-1791529946-0[6]
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... (same 5 plans, same host verdicts)
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plans: 5 generated, 5 tried, 0 discarded, 0 not tried; no plan could be placed
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```
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**Expected:** a request already proved unsatisfiable in this session pends at once, with the same pend reasons (main and per-host) the original attempt recorded, and the trace says it was skipped because of that earlier verdict. The live scheduler and the simulator should behave alike.
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**Impact:** the defect is in the normal (production) scheduler; the simulator's decision trace is what exposed it. Two defects in the session-level unsatisfiable-resreq cache (`alloc_cache.c`, scheduler-allocation Req 7):
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1. (normal scheduler, primary) see 2 below. (simulator) The simulator never registers the cache hooks (`main.c:527-528` registers them only in normal mode; `scheduler.c:247-256` treats NULL hooks as "always compute fresh"), so sim cost and sim decision traces differ from production: each identical pending element pays a full plan/host search.
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2. (normal scheduler) A cache hit reports only the stored main-reason category: `sched_pend_from_cached_reason_()` (`scheduler.c:173-184`) calls `add_job_preason(reasons, category, NULL, NULL)`, which erases host/resource reasons and sets `main_reason`. When the original attempt's main reason is 0 ("none") and its real cause is a per-host reason (as here: `pc13v: resource slots: NO_RESOURCE`), every later identical job pends with an empty reason, misleading operators (contrary to Req 7.5, "report the same reason category").
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Wrong result (missing pend reasons) plus wasted scheduling time; no crash.
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**Found by:** user review of the sim decision trace (2026-10-09); defect 2 found by code review while investigating it.
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UPDATE (2026-10-09): per the user, the fix must cover the normal scheduler, not just the simulator: a cache hit must carry the original attempt's full pend reason (main, per-host and per-resource) in production, and the simulator must use the same cache so its traces match production.
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UPDATE (2026-10-09): production evidence already existed. `sim_verify.c`'s bug-1845 comment records a real captured `.replay` in which a live cache-hit PEND was recorded as `{main_reason=0, nhost_reasons=0, nres_reasons=0}` while the full computation for the same job gave `{main_reason=0, nres_reasons=2}`: the empty-reason hit of defect 2, on a real deployment.
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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: not recorded: the failing `xce_scheduler` binary is not in this checkout (no build dir); host ldd reports glibc 2.40. The defect is in scheduler logic and does not depend on libc
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- Toolchain: gcc (GCC) 15.2.0, meson 1.9.1
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- Build: profile not recorded (asan, debug or release unknown; user's sim session); the defect is profile-independent scheduler logic
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- Code: scheduler-framework @ a64d0846-dirty
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- Deployment: `xce_scheduler --sim` (sim session 23), one simulated worker pc13v (22 slots), queue xce, policy builtin_fcfs; defect 2 is in the normal-mode scheduler (any deployment with two or more pending jobs sharing a request, user, queue and project in one session)
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## Reproduction
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1. In `xce_scheduler --sim`, set up one worker with 22 slots and submit an array job of at least 6 elements with identical requests (e.g. `minTasks=2, maxTasks=5`, 400 KB mem).
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2. `trace on all`, then run one scheduling session: elements [1]-[4] start (20 slots used).
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3. Elements [5] and [6] both pend, and the trace shows a full 5-plan search for each (defect 1).
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4. Defect 2 (live, code-level): a cache hit goes through `sched_pend_from_cached_reason_()` (`src/scheduler/src/scheduler.c:173-184`) → `add_job_preason(reasons, category, NULL, NULL)` (`src/lib/preason.c:146-163`), whose main-reason branch calls `erase_job_preason()` and stores only `category`; with `category == 0` the second job's pend reason is empty. A unit test with the hooks registered and a host-only original reason shows it.
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- Frequency: deterministic
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- Evidence: decision trace quoted above (from the user's sim session); session not saved
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## Root cause
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Two separate gaps around the session-level unsatisfiable-resreq cache (`alloc_cache.c`, scheduler-allocation Req 7):
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1. **Only the main reason was cached.** `sched_make_primary_alloc_()` (`scheduler.c`) populated the cache with `ctx->reason->reasons->main_reason` (an `int`), and a hit went through `sched_pend_from_cached_reason_()`, i.e. `add_job_preason(reasons, category, NULL, NULL)`, whose main-reason branch erases the lists and sets only `main_reason`. The allocators record shortfalls as per-host/per-resource entries and leave `main_reason` at 0, so every hit pended its job with an entirely empty reason. Present since the cache was introduced (scheduler-allocation task 2.2/5.1); affects every normal-mode scheduler.
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2. **The simulator had no cache.** The table lived on `xce_scheduler_t` (`scheduler.h`), which a simulation core does not have, and the scheduler-sim-live-allocation spec made "simulation isolation" a non-goal: `sim_loop.c`/`sim.c` registered none of the hooks. So `xce_scheduler --sim` searched every identical pending job again, and its decision traces and pend reasons differed from production. This also forced the main-only `--sim-verify` PEND compare (bug-1845).
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## Fix
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- **Cache entries hold the full reason.** A cache value is now an owned entry (`cache_entry_t`, `alloc_cache.c`): a deep copy of the exhausted attempt's whole `xce_job_preason_t` (main, per-host, per-resource; strings `strdup`ed via `add_job_preason`, main reason first because that branch erases) plus the job id that proved it. The table's datakiller frees entries on clear/destroy; a failed `key_add` frees the rejected copy. `cache_lookup()` fills a caller-supplied reason; the scheduler looks up into a local and, only on a hit, moves it into `ctx->reason`, so a miss leaves `ctx->reason` exactly as before.
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- **The table lives on the core.** `xce_sched_core_t.unsatisfiable_cache` replaces `xce_scheduler_t.unsatisfiable_cache`; `cache_lookup/insert/clear` and the three hook adapters take `xce_sched_core_t*` with no cast to a driver struct (the round-1 cast is what once clobbered `alt_events_file`). `sched_init()` builds it, `cache_clear()` rebuilds it every session, `xce_sched_core_fini()` destroys it.
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- **Both drivers register the hooks.** `main.c` registers them on the live core as before and passes the same three adapters to the simulator through new `sched_sim_args_t` fields; `sched_sim_driver_start()` sets them on the simulation core (plain field writes, since white-box tests `#include` `sim.c` without `core_session.c`/`alloc_cache.c` and leave them NULL).
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- **Hook signatures** (`core.h`): lookup returns the reason and the source job id; populate takes the reason and job id. `sched_allocator()` (`sched_election.c`) sets `core->alloc_jobid` around `sched_make_allocation()` so the populate knows the job.
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- **Decision trace:** a hit adds a `NO_PLAN` entry with the new reason `XCE_TRACE_NOPLAN_KNOWN_UNSAT` and the source job (`sched_trace_known_unsat()`), rendered as `no plan generated: skipped, same request as <job> proved unsatisfiable earlier this session`; the outcome lines then show the restored reasons. `trace.h` stays at 796 lines.
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- `sim_verify.c` keeps its main-only PEND compare (old recordings still carry main-only hits, and a hit carries the proving job's reasons); its comment, `preason.h`'s, `core.h`'s and the scheduler-sim-live-allocation design.md (superseded note) were updated.
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## Verification
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- Test: `test_standalone_primary_cache_hit_skips_allocator` in `src/scheduler/testsuite/test_scheduler_cache_dispatch.c` (live scheduler, real `sched_init()`/allocator thread): RED before the fix, run against the unmodified scheduler -- `test_scheduler_cache_dispatch.c:456 ... FAIL: bug-1932: a cache hit must restore the original attempt's FULL pend reason (main, per-host and per-resource), not only its main reason` (job 1's reason had one per-resource entry, job 2's hit had none); GREEN after. Same assertion added to `test_scheduler_cache_dispatch_compound.c` (GREEN).
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- Test: `test_known_unsatisfiable_request_skipped_with_full_reasons` in `src/scheduler/testsuite/test_repl_trace_e2e.c` (real `xce_scheduler --sim -i`, new jobs 204/205 in the extra replay): RED before -- `missing (or out of order): no plan generated: skipped, same request as 0-0-0-204 proved unsatisfiable earlier this session` (204's full search assertions passed, 205 searched again); GREEN after, with 205's outcome lines byte-equal to 204's (`host w01: resource mem: NO_RESOURCE`, `host w02: ...`).
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- Test: `test_alloc_cache.c` (18 tests: deep copy independent of the original and of earlier lookups, NULL reason stored empty, clear frees entries, duplicate insert frees the rejected copy -- ASAN clean), `test_core_session_cache_clear.c`, `test_sim_cache_clear_isolation.c` (new: a sim core with the registered clear hook builds the core table and leaves `alt_events_file` alone), `test_trace_render.c` (`test_known_unsatisfiable_names_source_job`), `test_maxtasks_wiring_ingest.c` (ported).
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- Suites: `xce-build.sh scheduler` (asan, fresh `xce.build`) 118/118 OK; after the last edit, `meson test --suite scheduler` 118/118 OK; libxce `libxce test` 748 tests + `libxce alloc OOM test` OK (`preason.h` comment changed).
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- Live: `xce_scheduler --sim <replay> --sim-verify` (debug profile) on the two real captured replays in `data/` (`scheduler-replay-20260925-133708.replay`, `scheduler-replay-20260928-064842.replay`), pre-fix binary (temporary worktree at a64d0846) vs fixed binary: identical mismatch sets (35 and 4848 lines, compared line by line without log prefixes) and rc 0 both -- those mismatches are pre-existing and unchanged. No HA soak: the change is confined to the scheduler's allocation path and simulator.
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## References
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- Related: bug-1931 (same trace; parjob ignores minTasks), bug-1927 (pend reasons recorded once per job)
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- Docs: .kiro/specs/scheduler-allocation/requirements.md (Req 7), .kiro/specs/scheduler-sim-live-allocation/design.md (task 2.2 cache callback seam; superseded-non-goals note added by this fix)
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- Fix commit: 0317c7af
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