fix(adaptive): treat io_uring capped to none as not viable; correct stale io_uring env docs (celeris#679) - #694
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Failing-first: on 9f4d89b ioUringViable ignores the probed tier, so CELERIS_MAX_IOURING_TIER=none on a 6.10+ kernel (GitHub's runners are on 6.17) leaves the conns-per-worker up-switch enabled and a high-concurrency hint picks an io_uring start that iouring.New then refuses. The fix follows in the next commit.
FumingPower3925
added
bug
…tale io_uring env docs (celeris#679)
ioUringViable ignored the probed io_uring tier. CELERIS_MAX_IOURING_TIER=none
clears the tier and every feature flag but not the kernel version, so on a
6.10+ kernel the "bundles era" branch alone called io_uring viable. The
same holds when io_uring_setup is refused on such a kernel (a default
container seccomp profile, kernel.io_uring_disabled), because the probe
then reports tier None too. With io_uring deemed viable:
- the conns-per-worker up-switch stayed enabled, and each promotion
built an io_uring engine that iouring.New refuses ("io_uring not
available on this system") and backed off with a WARN;
- a high-concurrency hint chose an io_uring start that fell back to
epoll with a WARN.
ioUringViable now checks IOUringTier.Available() first, which is the
predicate iouring.New uses, so the two cannot disagree.
The test-only commit before this one failed on GitHub's 6.17 runner, in
CI run 36241380107 (Adaptive job: 103 PASS, 4 FAIL, 0 SKIP). It failed
TestIOUringViable_TierUnavailable and both subtests of
TestNew_TierCapNoneNeverPromotes, with the up-switch enabled and New
logging "io_uring start engine unavailable, falling back to epoll start".
Comment corrections, from #679:
1. engine/iouring/doc.go CELERIS_IOURING_PBUF_COUNT: a value that is not a
power of 2 is rounded up to one and clamped to [1024, 32768]. It is not
clamped to [16, 32768], and it does not fail ring registration. The
envPbufCount comment in worker.go carried the same claim.
2. doc.go and probe.go (resolveSendZCPolicy) CELERIS_IOURING_SEND_ZC: they
no longer name the closed #465. Whether "auto" should keep enabling
SEND_ZC is open and owned by celeris#585, so nothing claims it is
decided.
3. The adaptive.Engine comment said CELERIS_ADAPTIVE_START "disables the
runtime switch". It only chooses the start engine, and determinism
comes from Config.Engine.
4. doc.go and README: CELERIS_MAX_IOURING_TIER is not used by CI. The
README now also says what `none` does to Adaptive.
…ning condition (celeris#679) - probe.Probe's godoc still said the tier cap "allows CI to exercise every tier's code path". No workflow sets CELERIS_MAX_IOURING_TIER. It now says what the cap does and what it leaves alone: any other non-empty value counts as none, and the kernel version stays as detected (the fact behind #679's bug). - engine/iouring/doc.go introduced its knobs as being "for CI matrix testing". No workflow sets any of them; tests do, with t.Setenv. - An unrecognized CELERIS_IOURING_SEND_ZC value is logged only when the probed profile has SEND_ZC (optional tier) and the functional probe passed: engine.go reads the variable inside `if profile.SendZC`, and resolveSendZCPolicy returns (false, true) before looking at it when the probe failed. doc.go, the README row and resolveSendZCPolicy's comment said, or implied, that it is always logged.
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WalkthroughAdaptive now rejects unavailable io_uring tiers when checking viability. Tests cover tier caps and epoll selection. Documentation describes tier-cap, SEND_ZC, and PBUF_COUNT rules. ChangesAdaptive io_uring tier handling
Priority: ➖ Normal Estimated code review effort: 3 (Moderate) | ~20 minutes Change: Bug fix · Severity of issue fixed: Low Merge Risk: ⚪ Minimal · up to Adaptive starts on epoll when io_uring is unavailable, including when io_uring was explicitly requested. No actionable merge-blocking risk remains. Security Architecture ReviewSecurity architecture risk: 🔵 Low · up to The new availability check prevents Adaptive from automatically choosing an io_uring engine that the system reports as unavailable. No introduced security issue was identified, but live startup and switching under this configuration were not exercised by the changed test. Retained concerns Security review detailsSecurity Blast Radius
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FumingPower3925
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to goceleris/docs
that referenced
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Sep 26, 2026
… and parser claims (celeris#679, celeris#424) - engines.md, CELERIS_MAX_IOURING_TIER: at `none` Adaptive neither starts on io_uring nor switches to it (goceleris/celeris#694, celeris#679), as the celeris README row now says. - engines.md, CELERIS_IOURING_SEND_ZC: an unrecognized value is logged only where the startup probe finds SEND_ZC working; elsewhere the variable has no effect (engine/iouring/engine.go reads it only inside `if profile.SendZC`, and resolveSendZCPolicy ignores it when the functional probe failed). - index.astro, the parser card: claim what the HTTP/1.1 parser does (its slices alias the bytes it parses), not that every request is parsed in place in the read buffer. epoll and io_uring serve a request that spans reads, has a chunked body, or runs on an async handler from a per-connection buffer.
This was referenced Sep 26, 2026
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Inline comments:
In @README.md:
- Line 339: Qualify the tier `none` behavior: in `README.md` lines 339-339,
limit the claim that Adaptive will not start io_uring to automatic selection and
state that explicitly setting `CELERIS_ADAPTIVE_START=iouring` selects io_uring
and fails in `iouring.New` rather than falling back to epoll. Apply the same
qualification in `engine/iouring/doc.go` lines 28-29.
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September 27, 2026 13:47
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Summary
This PR fixes the one behaviour bug #679 reports, and corrects the stale comments it lists.
The bug.
ioUringViable(adaptive/engine.go:136-147) ignored the probed io_uring tier. WithCELERIS_MAX_IOURING_TIER=noneon a 6.10+ kernel:iouring.Newrefuses ("io_uring not available on this system"), and backed off with a WARN.Fixes #679
Mechanism
probe.capIOUringTier(p, None)clearsIOUringTierand every feature flag. It does not touchKernelMajor/KernelMinor.ioUringViable's "bundles era" branch (kernel ≥ 6.10) therefore still returned true.ProbeIOUringfails (probe/probe.go:119-121 at 450d538: the tier is set only when it returns no error). A 6.10+ host that refusesio_uring_setuptherefore meets the same condition without any env var.kernel.io_uring_disabled: from reading only, not measured.Changes
ioUringViablenow checksp.IOUringTier.Available()first. This is the predicateiouring.Newuses, so the two cannot disagree.engine/iouring/doc.go,CELERIS_IOURING_PBUF_COUNT, and theenvPbufCountcomment inworker.go: a value that is not a power of 2 is rounded up to one and clamped to [1024, 32768]. It is not clamped to [16, 32768], and it does not fail ring registration.doc.go:12-13andprobe.go:294-295,CELERIS_IOURING_SEND_ZC: these no longer point at io_uring: probeSendZC can never detect copy-fallback — SEND_ZC enabled on every host regardless of NIC #465, which is already closed. Whetherautoshould keep enabling SEND_ZC is an open measurement owned by Measure #465: SEND_ZC on/off A/B on the real fabric with per-scenario CPU, both arches #585. Nothing now says the default is decided.adaptive.Enginecomment:CELERIS_ADAPTIVE_STARTonly chooses the start engine (chooseStartEngineis its only reader). Deterministic behaviour comes fromConfig.Engine.doc.goand the README row forCELERIS_MAX_IOURING_TIER: CI does not use it. Atnone, Adaptive neither starts on io_uring nor switches to it.probe.Probe's godoc still said the cap "allows CI to exercise every tier's code path". It now says what the cap does: any other non-empty value counts as none, and the kernel version stays as detected.engine/iouring/doc.go's knob list said "for ... CI matrix testing". No workflow sets any of the four knobs; tests set them witht.Setenv.CELERIS_IOURING_SEND_ZCvalue is logged only when the probed profile has SEND_ZC (optional tier) and the functional probe passed.engine.go:144-148reads the variable only insideif profile.SendZC(:125), andresolveSendZCPolicyreturns(false, true)before looking at it when the probe failed.doc.go, the README row andresolveSendZCPolicy's comment said, or implied, that it is always logged; they now give the exact condition.viableProfile/oldProfilenow carry realistic tiers (Optional / Base). The old-kernel case is therefore still rejected by the kernel test, not by the new availability test.Test Plan
Evidence:
evidence/celeris-673-679-653-424/lane-20260926/679/(per-finding index for review round 2:ROUND2.md).Failing first, on GitHub's runner. Commit b3c8ee9 is the tests alone on 9f4d89b's code. CI run 36241380107, Adaptive job (kernel
6.17.0-1022-azure,discriminating=true): 103 PASS, 4 FAIL, 0 SKIP (--- PASS/FAIL/SKIP: Testlines).TestIOUringViable_TierUnavailable: "io_uring tier None on a 6.12 kernel is viable".TestNew_TierCapNoneNeverPromotes/no-hintand/high-concurrency-hint: "the conns-per-worker UP switch is enabled with io_uring capped to none". The hint case also loggedWARN msg="io_uring start engine unavailable, falling back to epoll start".Fixed.
discriminating=trueon 6.17 (ci-36254798298-adaptive-450d538.log).TestNew_TierCapNoneNeverPromotesgoes through the realprobe.Probe()andNew. It discriminates only on 6.10+ kernels and logs which case it is in.Mutants (Docker golang:1.27, under the laptop slot lock;
docker-mutants.sh; kernel 7.0.12, discriminating=true):IOUringTier < High)No CI claim left (
ci-claim-grep.sh→ci-claim-grep.txt). The same queries run on db3cc3b and on 450d538.git grep -n CELERIS_MAX_IOURING_TIER -- .githubhas no hit on either. The positive control for that pathspec:CELERIS_REQUIRE_IOURING_WORKERS, which CI does set, is found 4 times in.github/workflows/ci.yml.probe/probe.go:13("CI to exercise every tier's code path"), which is the positive control that it finds the claim. On 450d538 it finds nothing.probe/probe.goandengine/iouring/doc.go: on db3cc3b it findsdoc.go:7andprobe.go:13. On 450d538 it finds nothing.Round-2 local checks (
round2-native-v.log):go vetof./probe/ ./engine/iouring/ ./adaptive/passes for linux/amd64 and linux/arm64../probenatively: 24 PASS, 0 FAIL, 12 SKIP. The skips are "tier detection requires linux GOOS"../engine/iouringand./adaptivehave no buildable files on darwin, so their verdict is the Linux CI above.Default seccomp profile (review round 2)
Commit db3cc3b's message states as fact that "a default container seccomp profile, kernel.io_uring_disabled" triggers the same bug. The round-1 body of this PR called that claim "from reading only, not measured". Published history is not rewritten.
Measured for Docker's default seccomp profile (
docker-seccomp-probe.sh→docker-seccomp-probe.log). Setup: golang:1.27 on Docker Desktop's VM, kernel7.0.12-linuxkit, Docker's default seccomp profile, and noCELERIS_MAX_IOURING_TIER.probe.Probe()reportstier=none available=falseon both trees.TestNew_TierCapNoneNeverPromoteswas run with itst.Setenv("CELERIS_MAX_IOURING_TIER", "none")changed to"", so that it measures the host as it is.discriminating=true.Newlogs at WARN. The failure message still says "capped to none", its round-1 wording; here the cap comes from the seccomp profile.Control (
docker-seccomp-control.sh→docker-seccomp-control.log): the same probe program, image, VM and kernel (7.0.12-linuxkit), run on 450d538 twice.tier=none available=false.--security-opt seccomp=unconfined:tier=optional available=true.So the
tier=noneabove is the seccomp profile's doing, not the kernel's or the VM's.kernel.io_uring_disabled: not measured. That half is from reading: the sysctl makesio_uring_setupfail, and any failure ofProbeIOUringleaves the tier at None. The commit message therefore overstates it. The squash message will be corrected at merge time to say that the seccomp half was measured and the sysctl half was read.No hot-path change:
ioUringViableruns atNew()only.