MobileGL/docs/archive/notes/p5e/TASK-ra2.md

TASK-ra2 — make run-ahead actually correct

Read PACKAGE-PREAMBLE-P5E.md and PREAMBLE-ADDENDUM-WAVE3.md first. Your tree is ~/w7/p5e-ra2, branch p5e/flip, head 985ae62d. Your slug for the gate driver is ra2.

Your head already has the flip thrown. Both kMGPipeP5eRunAheadReady (MG_Backend/Init.cpp) and kMGPipeP5eClientWaitRuleLanded (MG_Pipe/PipeApply.h) are true. That is the whole point: everything below reproduces on your head the moment you build it.

1 What is measured, and what it is not

The phase's strict lane is HARD GREEN on the integration branch (72bc4268): 181/181, zero Fatal{, ten Admitted{ rows all owned by later phases. That lane runs under LOCKSTEP, where PipeApplier::ApplyOne stamps every record barriered, so it cannot see anything that only happens once the client stops waiting. It measures remaining debt. It does not measure whether run-ahead is correct. Your package is what measures that.

With the flip thrown, on your head:

gate	result
build	rc=0
unit	2254/2254
gens	all rc=0
flavours	rc=0
isplit	112 / 181
gpu	1290 / 1359

The 69-70 failures are flaky: 71 at -j 4, 68 at -j 1, and a single entry run alone passes and then aborts on a repeat (ctest -R '<entry>' --repeat until-fail:12).

A census over all 181 entries, each under its OWN command and ENVIRONMENT taken from ctest --show-only=json-v1 (~/w7/notes/p5e/strict_census3.py, and ~/w7/notes/p5e/catch_flip_abort.py to catch one entry's abort text), gives 70 red on 17 distinct <field>@<verb> pairs over just FOUR fields:

count	field
27	GetRenderStateParametersVersion
25	GetTextureContextId
16	GetBufferBindingSlot
2	IsCapabilityEnabled

across eleven verbs: Clear, DrawArrays, DrawElements, DrawElementsBaseVertex, DrawElementsIndirect, DrawElementsInstancedBaseVertex, EndTransformFeedback, GetIntegeri_v, GetSyncStatus, MultiDrawElementsBaseVertex, ReadPixels.

Two populations, and the split matters:

• 16 abort on the UNBARRIERED arm — the marker carries [BARRIER-PULLED, UNBARRIERED, …].
• 54 abort with NO [BARRIER-PULLED] bracket at all. That is the plain unfresh-read abort for a field whose ownership is kRecordSupplied — the freshness stamp says it was never filled. Example verbatim: MGPipe: Fatal{UnmigratedPipeInput, "GetTextureContextId@ReadPixels"}.

The census outputs are at ~/w7/p5e-logs/census-flipped/ (per-entry logs plus census3.json).

2 The hypothesis you must TEST, not assume

One field at one verb, consistently, would be a missing migration. Four fields scattered over eleven verbs, flaky run to run, and including verbs that are barriered (ReadPixels, GetSyncStatus) is the signature of a RACE.

The obvious candidate: gPipeInputs is shared mutable state whose mutual exclusion WAS the lockstep. Under run-ahead the client still fills it for BARRIERED verbs (MGPipeValidateForVerb, MG_Impl/Pipe/PipeFill.cpp:3219, fillOwed = barriered) while the apply thread may still be draining EARLIER unbarriered records and reading the same block. The client's fill and the server's MGPipeServerStampVerbBoundary (MG_Backend/MGPipe/PipeInputs.cpp:183) then overlap, and whichever writes FilledGen[] last decides whether the other's read is fresh.

This is consistent with something the project already wrote down: P5d's own research concluded that retiring this barrier needs gPipeInputs versioning or double-buffering, and deferred it to P11 after P3b/P4b's handle-keyed twin tables (see the P11 row in docs/Disaggregated/ROADMAP.md and P5d's report). If that conclusion is right, the honest outcome of your package may be "run-ahead cannot land before that work", and saying so with evidence is a complete and acceptable result.

Do not start from the hypothesis. Start from the failure. Instrument, get a reproducer smaller than a scenario if you can, and establish WHICH writer clobbers WHICH read, with evidence. If the hypothesis is wrong, say what is actually happening.

Two facts that will help you rule things in or out:

• RefusePipeInputsTouchWhileApplierOwnsIt (MG_Remote/Client/ClientSession) already exists as the guard for exactly this class, and it takes an isBarrieredFill argument. Find out whether it fires, and if it does not, why not.
• MOBILEGL_IPC_RUN_AHEAD=0 at runtime turns the client's waiting back on with identical server code and identical records (MobileGL/Config.h, the RunAhead comment). That is your A/B control: if a failing entry passes under it on your head, the defect is in the wait rule and not in a record's content. Run it. Do NOT use MOBILEGL_IPC_VERB_BARRIER=0, which is the first conjunct of RunAheadArmed() and turns run-ahead off as a side effect.

3 What to deliver

In order of value:

1. The diagnosis, with evidence. Which writer, which reader, which field, what the interleaving is. A verbatim reproducer.
2. The verdict: is this fixable inside P5e, or does it need the gPipeInputs versioning that P5d deferred to P11? Answer with the argument, not with a preference. If the answer is "needs P11", say exactly what P11 has to provide and what the smallest version of it would be.
3. The fix, if it is P5e-sized. Every arm you add follows ID-81 and ID-110: the arm is decided by MG_Config::Transport or a family selector that reduces to it, STATED not inferred, and a missing record is a named refusal, never a quiet fall-back.
4. A regression test that fails without the fix. The strict lane cannot catch this class by construction (it runs under lockstep), so whatever you add must run with the flip thrown. Say where it lives and how the gate reaches it.

4 Gate

build · unit · isplit · gpu · gens · flavours, all on your tree, plus the census (strict_census3.py). The bar for "fixed" is isplit 181/181 and gpu 1359/1359 with the flip still thrown, and the census showing no Fatal{ that is not one of the ten admitted rows.

If you cannot reach that bar, report the exact remaining set rather than reverting the flip to make the numbers look green — the flip is the thing under test.

5 Report

~/w7/notes/p5e/reports/ra2-v1.md. Lead with the verdict in §3.2, because that is the decision the integrator has to make. Then the diagnosis with its evidence, then what you changed, then the gate table, then what you did not do and why.