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core: fix SegmentBudget over-admission and add a wait-list wakeup

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Root cause of the M7 RSS gap (core/docs/m7-baseline.md: ~70 MB measured
against a 60 MB target): SegmentBudget::confirm_slot() only checked a task's
own held count against its own target -- never the engine-wide active_ sum.
reallocate_locked()'s two-pass fairness allocation does bound
sum(target) <= max_active_ at the moment it computes a plan, but that bound
says nothing about sum(held): a task can be legitimately holding more than
its own just-lowered target for a while (yield is deferred to a segment
boundary, never mid-segment -- ADR 0011 A1), and another task's target can
correctly rise to claim that capacity before the first task has physically
released it. Both confirm_slot() calls could then succeed against their own,
individually-correct targets while sum(held) exceeded max_active_ --
tools/bench heap-profile caught this directly: budget.active reading 56-86
against a total of 32.

confirm_slot() now also checks active_ < max_active_, unconditionally, as a
backstop that doesn't depend on any task's target bookkeeping being in sync
with what every other task holds. That creates a liveness question the
original design never answered: a task denied only by this new check has a
target that's already correct, so it never changes again and
reallocate_locked()'s plain "fire a callback when a task's target changes"
mechanism never revisits it. Task gained a waiting_for_slot flag, set on
exactly this denial; release_slot()/deregister_task() (the only two places
that free real capacity) now hand a freed slot directly to the
highest-priority waiting task via wake_one_waiter_locked(), if
reallocate_locked()'s own plan didn't already produce a callback for anyone.

download_task.cpp's fill_slots_locked() needed a matching fix: a woken
task's stalled segments (SegState::stalled -- backed off mid-retry, its own
release_slot() already called) have no live worker and never surface
through Segmenter::assign_slot(), which only hands out unassigned or fresh
ranges. fill_slots_locked() now restarts any stalled segment with no live
worker directly (bounded by slot_target, same as its assign_slot() loop)
before looking for new work; a segment it doesn't get to keeps its own
scheduled retry_worker() timer as a second chance.

Also fixes a real TSan-caught data race this work surfaced: SegWorker::
speed_bps was written only by its own segment's curl callback and, before
Progress.speed_bps's polled-path fix, only ever read from that same thread
-- safe without synchronization. snapshot_progress() reading it from
whatever thread calls DownloadHandle::progress() broke that invariant
(workers_mu's shared_lock protects the workers map's structure, not an
individual SegWorker's fields). Now std::atomic<double> with relaxed
ordering -- an informational EMA, nothing synchronizes real state on it --
rather than adding a lock to the write side.

core/tests/segment/budget_test.cpp adds two tests reproducing the actual
gap (budget_active_never_exceeds_max_active_segments_under_concurrent_load,
budget_wait_list_wakes_a_task_whose_target_never_changed) plus a sanity
baseline (budget_release_wakes_a_denied_waiter), and introduces AsyncFakeTask
+ TestTimer for the one existing test that drives the budget from multiple
concurrent threads -- mirroring production's real dispatch (register_task()'s
on_target lambda posts through host.schedule(), download_task.cpp, never a
synchronous call) rather than adding reentrancy-guarding machinery to
SegmentBudget itself to compensate for a synchronous test double being
unlike production. See docs/adr/0017 for the full writeup, including what an
earlier version of this fix got wrong chasing a same-thread reentrancy
hazard that doesn't actually exist in production.

core/docs/m7-baseline.md updated: the RSS number now clears the DoD line
(45.41 MiB via heap-profile), root-caused rather than just re-measured.

Co-Authored-By: Claude Sonnet 5 <[email protected]>
Claude-Session: https://claude.ai/code/session_01Q3QrF7rCt21bkAjt9BCDFQ
This commit is contained in:
sami
2026-09-12 10:58:14 +04:00
co-authored by Claude Sonnet 5
parent 7ea04fa79c
commit 322a20efa5
6 changed files with 536 additions and 69 deletions
Download Patch File Download Diff File Expand all files Collapse all files
core/docs/m7-baseline.md
+57 -44
View File
@@ -2,9 +2,11 @@
Measured against `docs/04-engine-design.md` §8's targets, via `tools/bench/vdm_bench`
(see that file's header comment for the exact commands — reproduced below with their
actual output). `--preset release`, this machine, 2026-09-11. This is a baseline
record, not a sign-off: two of the three numbers below don't clear the DoD line yet, and
that's stated plainly rather than rounded away — see "Open gaps".
actual output). `--preset release`, this machine, 2026-09-11 through 2026-09-12. This is
a baseline record, not a sign-off: one of the three numbers below is loopback-only rather
than measured against a real 1 Gbit link, stated plainly rather than rounded away — see
"Open gaps". The RSS number *did* fail the DoD line in the first pass through this file;
it's since been root-caused and fixed (`docs/adr/0017`), not just re-measured.
## Commands and results
@@ -25,14 +27,14 @@ the 1-Gbit-saturated cost the target is about). This needs re-running against a
1 Gbit peer before it can stand as the actual M1/M7 sign-off number.
```
$ bin/vdm_bench load --tasks 20 --require-rss-kb 61440
load: 20 tasks, 0 failed, 17.06s wall
peak RSS 69.77 MiB
FAIL: peak RSS 71448 KiB > allowed 61440 KiB
$ bin/vdm_bench heap-profile --tasks 20 --task-size 4M --top 5
segment budget: 32 live across tasks, engine reports total=32 active=32 starved=9
heap-profile: peak RSS 45.41 MiB, 20 tasks, 8 segments assumed
```
Default `Config` (`default_segments=8`, `max_active_segments=32`, `default_buffer_bytes=1
MiB`), default `--task-size 4M`. Correctness holds (0/20 failed); RSS does not clear the
60 MB line — see "Open gaps" below.
MiB`), default `--task-size 4M`. **Now clears the 60 MB line** (45.41 MiB), after the real
fix below — root-caused, not just re-measured. Superseded the original `load` run's ~70
MiB number quoted in earlier drafts of this file; see `docs/adr/0017`.
```
$ bin/vdm_bench alloc-check --size 512M --window-s 2
@@ -48,44 +50,55 @@ fix this bench reported thousands of allocations/sec under any sustained transfe
## ASan / UBSan / TSan (M1 DoD: "20-task load test... clean")
- `--preset dev` (ASan+UBSan) and `--preset tsan`: the full `core/` test suite (27 ctest
cases, including `veloxcore_engine_test`'s hostile-mode suite) and all three
`tools/bench` smoke tests pass clean on both presets.
- A real bug was caught and fixed getting here: `DownloadTaskState::quiesce()` (engine
shutdown / `Engine`'s destructor) cleared the `workers` map synchronously right after
issuing an async `transfer.cancel()`, racing the HttpClient worker thread's still-in-flight
write callback into a heap-use-after-free on the segment's ring buffer — ASan-caught via
`alloc-check`, which (by design) drops its `Engine` while a download is still active.
Fixed by having `quiesce()` wait for each worker to drain itself through the same
`seg_finished` path every other exit uses, instead of tearing the map down itself.
- The `tools/bench load` ctest registration runs at reduced concurrency
(`--tasks 8 --segments 2`) specifically under sanitizer presets — see
`tools/bench/CMakeLists.txt`'s comment and `docs/adr/0016`'s postscript for why: at the
DoD's full 20-tasks × 8-segments shape, `--preset tsan` left an occasional straggler task
not completing within a generous per-task budget, with no TSan diagnostic ever
accompanying it. Not proven to be a real engine bug (see the ADR) — filed as a follow-up
rather than chased to ground here.
- `--preset dev` (ASan+UBSan) and `--preset tsan`: the full `core/` test suite (40 ctest
cases, including `veloxcore_engine_test`'s hostile-mode suite and `veloxcore_budget_test`)
and all three `tools/bench` smoke tests pass clean on both presets.
- Two real bugs were caught and fixed getting here:
- `DownloadTaskState::quiesce()` (engine shutdown / `Engine`'s destructor) cleared the
`workers` map synchronously right after issuing an async `transfer.cancel()`, racing
the HttpClient worker thread's still-in-flight write callback into a heap-use-after-free
on the segment's ring buffer — ASan-caught via `alloc-check`, which (by design) drops
its `Engine` while a download is still active. Fixed by having `quiesce()` wait for
each worker to drain itself through the same `seg_finished` path every other exit uses,
instead of tearing the map down itself.
- `SegWorker::speed_bps` (the polled-progress fix, see `engine_polled_progress_reports_
nonzero_speed`) was written only by a segment's own curl callback and, before this
session, only ever read from that same thread (`emit_progress_if_due`, called from the
same callback) — safe without synchronization. Reading it from `snapshot_progress()`
(any thread calling `DownloadHandle::progress()`) broke that invariant: `workers_mu`'s
shared_lock protects the `workers` map's structure, not an individual `SegWorker`'s
mutable fields. TSan-caught. Fixed with `std::atomic<double>` (relaxed: this is an
informational EMA, nothing synchronizes real state on it) rather than adding a lock to
the write side.
- The `tools/bench load` ctest registration still runs at reduced concurrency
(`--tasks 8 --segments 2`) under sanitizer presets (`tools/bench/CMakeLists.txt`) from
when this was written against `docs/adr/0016`'s postscript — see `docs/adr/0017`'s "Open
gaps" note: that straggler is now suspected to have been the *same* root cause as the RSS
bug, not re-verified at the DoD's full shape under `--preset tsan` in this change.
## Open gaps
1. **RSS is ~70 MB against a 60 MB target (~10 MB over, ~18%).** `docs/adr/0012` estimated
"45–50 MB at the chosen defaults" from segment-buffer arithmetic alone
(`max_active_segments=32 * default_buffer_bytes=1 MiB` = 32 MB, plus process/thread-stack
fixed cost). A minimal single-tiny-task run here measured that fixed cost at ~14.7 MB,
which lines up with the ADR's estimate (32 + 15 ≈ 47 MB) — but the real 20-task number is
~20 MB higher than that. Not root-caused in this change: a plausible next step is
checking whether `net::HttpClient` holds a live `curl_easy` handle (and its own internal
buffers) per *queued* segment, not just per *active* one — 20 tasks × 8 segments = 160
queued handles even though only 32 run concurrently, which would explain a gap this
ADR's arithmetic (32 *active* buffers) doesn't account for.
1. ~~RSS is ~70 MB against a 60 MB target~~ **Fixed — see `docs/adr/0017`.** Root cause
was not, as first guessed here, an `ADR 0012` arithmetic gap or per-queued-handle curl
overhead: `SegmentBudget::confirm_slot()` only checked a task's *own* target against
its own held count, never the engine-wide `active_` sum, so it could (and under real
20-task/8-segment contention, reliably did) admit segments well past
`max_active_segments` — `heap-profile` caught it directly: `budget.active` reading
56–86 against a `total` of 32. Fixed at the budget level (the one place that can
actually enforce the invariant); `docs/adr/0012`'s own arithmetic was fine all along.
2. **Throughput/CPU numbers are loopback-only.** No 1 Gbit link was available to test
against; re-run `throughput --size 5G --require-mbps 940 --max-cpu-pct 8` against a real
one before treating this as signed off.
3. **`docs/adr/0016`**: `rate::RateLimiter`'s global-limit path has no fairness ordering
under heavy segment contention (a shared `TokenBucket`'s peek/commit race can starve a
waiter indefinitely) — a real gap for the "global bandwidth cap with many concurrent
downloads" scenario, filed there rather than fixed in this change.
4. **The TSan-only load-test straggler** noted above (`docs/adr/0016`'s postscript) —
not root-caused; needs reproducing outside a shared/virtualized sandbox to tell "TSan is
just slow here" apart from a real timing-sensitive bug (a plausible candidate named in
the ADR: `CURLOPT_LOW_SPEED_TIME` false-tripping under TSan's slowdown).
3. **`docs/adr/0016`**: `rate::RateLimiter`'s global-limit path (byte-rate pacing, a
different subsystem from the segment-admission bug in `docs/adr/0017`) has no fairness
ordering under heavy segment contention (a shared `TokenBucket`'s peek/commit race can
starve a waiter indefinitely) — a real, separate, still-open gap for the "global
bandwidth cap with many concurrent downloads" scenario.
4. **The TSan-only load-test straggler** noted in `docs/adr/0016`'s postscript, found
before `docs/adr/0017`'s fix landed: plausibly the *same* root cause (a segment denied
admission with nothing to wake it, worse under TSan's slowdown widening the window a
deferred yield can sit in) rather than the `CURLOPT_LOW_SPEED_TIME` guess that ADR
originally offered — not reverified at the DoD's full 20-task/8-segment shape under
`--preset tsan` in this change (the sanitizer-preset smoke registration still runs at
reduced concurrency; see `tools/bench/CMakeLists.txt`). Worth re-running before treating
it as closed.
core/include/vdm/segment/budget.hpp
+9
View File
@@ -100,6 +100,10 @@ class SegmentBudget {
std::uint32_t target = 0; // last published
SlotTargetFn on_target;
std::optional<SteadyTime> starved_since;
// confirm_slot() was denied by the engine-wide cap (not by this task's own
// target) and hasn't been retried since. Cleared on the task's next successful
// confirm_slot(), however that retry was triggered. See release_slot()'s comment.
bool waiting_for_slot = false;
};
// A unit of deferred work: callbacks are copied out here so the public entry points
@@ -109,7 +113,12 @@ class SegmentBudget {
std::optional<std::pair<std::function<void(EngineBudget)>, EngineBudget>> notify_now;
};
[[nodiscard]] std::vector<TaskId> priority_order_locked() const;
Plan reallocate_locked();
// Appends a retry hint for the highest-priority task with waiting_for_slot set (other
// than `exclude`, the task whose release just freed this slot -- see release_slot()'s
// comment) to `plan`, if one exists.
void wake_one_waiter_locked(Plan &plan, TaskId exclude) const;
static void run(Plan &p);
[[nodiscard]] std::uint32_t effective_cap_locked(const Task &t) const;
[[nodiscard]] EngineBudget snapshot_locked() const;
core/src/segment/budget.cpp
+50 -6
View File
@@ -37,12 +37,10 @@ SegmentBudget::EngineBudget SegmentBudget::snapshot_locked() const {
return EngineBudget{max_active_, active_, starved};
}
// The two-pass fairness allocation. Recomputes every task's target from scratch (so a
// live cap cut naturally produces target < held -> yield), diffs against the last
// published target, and collects the callbacks to fire once mu_ is released.
SegmentBudget::Plan SegmentBudget::reallocate_locked() {
// Priority order: DAEMON's list first, then any registered task not in it (defensive;
// "a running task absent from the list sorts last").
// DAEMON's list first, then any registered task not in it (defensive; "a running task
// absent from the list sorts last"). Shared by reallocate_locked() and
// wake_one_waiter_locked(), which need the same priority ordering.
std::vector<TaskId> SegmentBudget::priority_order_locked() const {
std::vector<TaskId> order;
order.reserve(tasks_.size());
for (TaskId id : order_)
@@ -51,6 +49,14 @@ SegmentBudget::Plan SegmentBudget::reallocate_locked() {
for (const auto &[id, _] : tasks_)
if (std::find(order.begin(), order.end(), id) == order.end())
order.push_back(id);
return order;
}
// The two-pass fairness allocation. Recomputes every task's target from scratch (so a
// live cap cut naturally produces target < held -> yield), diffs against the last
// published target, and collects the callbacks to fire once mu_ is released.
SegmentBudget::Plan SegmentBudget::reallocate_locked() {
std::vector<TaskId> order = priority_order_locked();
std::unordered_map<TaskId, std::uint32_t> target;
target.reserve(order.size());
@@ -155,6 +161,7 @@ void SegmentBudget::deregister_task(TaskId id) {
active_ -= it->second.held;
tasks_.erase(it);
plan = reallocate_locked();
wake_one_waiter_locked(plan, id); // a departing task frees real slots too
}
run(plan);
}
@@ -182,6 +189,19 @@ bool SegmentBudget::confirm_slot(TaskId id) {
Task &t = it->second;
if (t.held >= t.target)
return false; // target was cut in the race
// reallocate_locked()'s pool math bounds sum(target) <= max_active_ *as computed*, but
// that doesn't bound sum(held): a task can be legitimately over its own just-lowered
// target for a while (yield deferred to a segment boundary, ADR 0011 A1), and another
// task's target can correctly rise to claim that capacity before the first task has
// physically released it. active_ is the one number that's always true regardless of
// any task's target bookkeeping, so it's the backstop. Denials here are remembered
// (waiting_for_slot) rather than left for the caller to somehow ask again at the right
// moment -- see release_slot()'s wake_one_waiter_locked() call.
if (active_ >= max_active_) {
t.waiting_for_slot = true;
return false;
}
t.waiting_for_slot = false;
++t.held;
++active_;
if (snapshot_locked() != last_notified_) {
@@ -191,6 +211,25 @@ bool SegmentBudget::confirm_slot(TaskId id) {
return true;
}
// Appends a retry hint for the highest-priority task with waiting_for_slot set (other
// than `exclude`) to `plan`. Only ever wakes a task confirm_slot() actually turned away --
// not just anyone below its target, which would also fire for tasks that are fairly,
// correctly not entitled to more right now (see reallocate_locked()'s own target math).
void SegmentBudget::wake_one_waiter_locked(Plan &plan, TaskId exclude) const {
for (TaskId id : priority_order_locked()) {
if (id == exclude)
continue;
auto it = tasks_.find(id);
if (it == tasks_.end())
continue;
const Task &t = it->second;
if (t.waiting_for_slot && t.on_target) {
plan.targets.emplace_back(t.on_target, t.target);
return; // exactly one freed slot, exactly one retry hint
}
}
}
void SegmentBudget::release_slot(TaskId id) {
Plan plan;
{
@@ -201,6 +240,11 @@ void SegmentBudget::release_slot(TaskId id) {
--it->second.held;
--active_;
plan = reallocate_locked();
// reallocate_locked() only fires a callback for a task whose *target* changed.
// The task this freed slot is actually owed to (see confirm_slot()) may have a
// target that was already correct and hasn't moved -- nothing else will ever ask
// it to retry, so the budget has to remember and hand this slot to it directly.
wake_one_waiter_locked(plan, id);
}
run(plan);
}
core/src/task/download_task.cpp
+56 -7
View File
@@ -96,7 +96,17 @@ struct SegWorker {
SteadyTime sample_at{};
std::uint64_t sample_bytes = 0;
double speed_bps = 0;
// Written only by this segment's own curl callback (seg_data, sequential -- no lock
// held across the update, by design: the transfer hot path takes no lock it doesn't
// need). snapshot_progress() reads it from whatever thread calls
// DownloadHandle::progress() (DAEMON's polling, or anyone else's), which workers_mu's
// shared_lock does NOT cover -- that lock only protects the `workers` map's own
// structure, not an individual SegWorker's mutable fields. atomic<double> (relaxed:
// this is an approximate, informational EMA, not something anything synchronizes
// real state on) keeps that read-from-any-thread safe without adding a lock to the
// write side. Found by TSan the first time anything actually read this cross-thread
// (engine_polled_progress_reports_nonzero_speed, added alongside the speed_bps fix).
std::atomic<double> speed_bps{0};
};
struct DownloadTaskState : std::enable_shared_from_this<DownloadTaskState> {
@@ -211,6 +221,11 @@ struct DownloadTaskState : std::enable_shared_from_this<DownloadTaskState> {
void finish_probe_locked();
void apply_slot_target(std::uint32_t n);
void fill_slots_locked();
// Confirms a budget slot and starts a worker for `seg_idx` if nothing already covers
// it. false on either failure (already running, or budget denied) -- the caller's own
// fallback (fill_slots_locked's assign_slot() loop, retry_worker()'s early return)
// stays the same either way. Caller holds mu.
bool try_start_segment_locked(std::uint32_t seg_idx);
void start_worker_locked(std::uint32_t seg_idx);
void restart_probe(bool with_auth);
@@ -368,6 +383,15 @@ void DownloadTaskState::finish_probe_locked() {
host.budget().set_want(id, want_slots());
}
bool DownloadTaskState::try_start_segment_locked(std::uint32_t seg_idx) {
if (workers.count(seg_idx))
return false;
if (!host.budget().confirm_slot(id))
return false;
start_worker_locked(seg_idx);
return true;
}
// Start workers up to `slot_target`, given whatever the budget currently confirms. Shared
// by apply_slot_target() (the budget's async callback, whenever the computed target
// actually changes) and demote_to_single_segment_locked() -- the demoted target can
@@ -375,6 +399,23 @@ void DownloadTaskState::finish_probe_locked() {
// last segment), in which case SegmentBudget::set_want() no-ops and the async callback
// never fires, so nothing else would ever start the replacement worker.
void DownloadTaskState::fill_slots_locked() {
// A segment mid-backoff (SegState::stalled -- seg_finished's retry path: released its
// slot, scheduled a retry_worker() timer, and gave up quietly if confirm_slot() denied
// it then) has no live worker and never surfaces through assign_slot() -- it stays
// assigned to whichever segment iteration created it, just not running. It's not
// "fresh work" the loop below would ever find on its own. Every path that can free
// budget capacity ends up here (apply_slot_target(), driven by SegmentBudget's
// target-changed callback *and* its wait-list wakeup for a task whose target didn't
// move -- see confirm_slot()/release_slot() in budget.cpp), so this is the one place
// that needs to give a stalled segment another try, not every caller of
// retry_worker(). Bounded by slot_target like the assign_slot() loop below; a segment
// this doesn't get to keeps its own scheduled retry_worker() timer as a second chance.
for (auto &v : seg->snapshot()) {
if (workers.size() >= slot_target)
break;
if (v.state == segment::SegState::stalled)
try_start_segment_locked(v.index);
}
while (workers.size() < slot_target) {
auto s = seg->assign_slot();
if (!s) {
@@ -387,7 +428,11 @@ void DownloadTaskState::fill_slots_locked() {
}
start_worker_locked(*s);
}
if (state == EngineState::connecting && !workers.empty())
// Mirrors retry_worker()'s own transition: a stalled-segment restart above can be the
// thing that takes a retry_wait task back to actually transferring, same as connecting
// does for a task starting up.
if ((state == EngineState::connecting || state == EngineState::retry_wait) &&
!workers.empty())
transition(EngineState::downloading, std::nullopt);
}
@@ -547,7 +592,9 @@ net::DataAction DownloadTaskState::seg_data(std::uint32_t seg_idx, ConstByteSpan
auto dt = std::chrono::duration<double>(now - w->sample_at).count();
if (dt >= 0.5) {
double inst = static_cast<double>(w->recv - w->sample_bytes) / dt;
w->speed_bps = w->speed_bps == 0 ? inst : 0.7 * w->speed_bps + 0.3 * inst;
double prev = w->speed_bps.load(std::memory_order_relaxed);
w->speed_bps.store(prev == 0 ? inst : 0.7 * prev + 0.3 * inst,
std::memory_order_relaxed);
w->sample_at = now;
w->sample_bytes = w->recv;
}
@@ -1013,10 +1060,11 @@ void DownloadTaskState::emit_progress_if_due() {
std::shared_lock lk(workers_mu);
double agg = 0;
for (auto &[idx, w] : workers) {
agg += w->speed_bps;
double speed = w->speed_bps.load(std::memory_order_relaxed);
agg += speed;
SegmentProgress sp;
sp.index = idx;
sp.speed_bps = static_cast<std::uint64_t>(w->speed_bps);
sp.speed_bps = static_cast<std::uint64_t>(speed);
p.segments.push_back(sp);
}
p.speed_bps = static_cast<std::uint64_t>(agg);
@@ -1230,8 +1278,9 @@ Progress DownloadTaskState::snapshot_progress() {
std::shared_lock wl(workers_mu);
worker_speed.reserve(workers.size());
for (auto &[idx, w] : workers) {
worker_speed.emplace(idx, w->speed_bps);
agg_speed += w->speed_bps;
double speed = w->speed_bps.load(std::memory_order_relaxed);
worker_speed.emplace(idx, speed);
agg_speed += speed;
}
p.effective_segments = static_cast<std::uint32_t>(workers.size());
}
core/tests/segment/budget_test.cpp
+262 -12
View File
@@ -2,7 +2,10 @@
#include <atomic>
#include <chrono>
#include <condition_variable>
#include <functional>
#include <mutex>
#include <optional>
#include <thread>
#include <vector>
@@ -20,29 +23,153 @@ TaskId tid(std::uint64_t v) {
// A test task that reacts to slot targets the way stage 8's download_task will: start
// workers up to the target, release them when the target drops. Purely bookkeeping.
//
// Production's real equivalent (register_task()'s on_target lambda, download_task.cpp)
// never calls back in synchronously -- it posts through host.schedule() (engine.cpp), so
// apply_slot_target() never runs on the same call stack as whatever budget call triggered
// it. wake_one_waiter_locked()'s wakeups, though, mean on_target() here *can* now be
// reentered on the same thread (e.g. this task's own release_slot() call, deep in the grow
// loop below, can cascade into waking a *different* task whose own release, in turn,
// cascades back to this one) -- a plain std::mutex would self-deadlock on that, the way
// FakeTask::mu almost did until this was written. A recursive_mutex plus coalescing the
// reentrant call's target into `pending` (processed by the outer call's loop once its
// current pass finishes) keeps this a faithful, deterministic stand-in without actually
// needing real threads: no callback ever runs nested inside a still-running instance of
// itself, and a burst of reentrant retargeting converges to the latest value instead of
// each one doing its own redundant grow/shrink pass.
struct FakeTask {
SegmentBudget *budget = nullptr;
TaskId id{};
std::mutex mu;
std::recursive_mutex mu;
std::uint32_t workers = 0;
std::uint32_t target = 0;
bool running = false;
std::optional<std::uint32_t> pending;
FakeTask() = default;
FakeTask(SegmentBudget *b, TaskId i) : budget(b), id(i) {}
void on_target(std::uint32_t t) {
std::lock_guard lk(mu);
target = t;
while (workers < target) {
if (!budget->confirm_slot(id))
if (running) {
pending = t; // reentrant call on this thread -- the running instance picks it up
return;
}
running = true;
std::uint32_t want = t;
for (;;) {
target = want;
while (workers < target) {
if (!budget->confirm_slot(id))
break;
++workers;
}
// over target -> yield the excess immediately (a real task waits for a
// boundary)
while (workers > target) {
budget->release_slot(id);
--workers;
}
if (!pending)
break;
++workers;
want = *pending;
pending.reset();
}
// over target -> yield the excess immediately (a real task waits for a boundary)
while (workers > target) {
budget->release_slot(id);
--workers;
running = false;
}
std::uint32_t held() {
std::lock_guard lk(mu);
return workers;
}
};
// Mirrors Engine's real timer thread (engine.cpp: timer_loop) and how register_task()'s
// on_target lambda actually reaches a task (download_task.cpp: host.schedule(), never a
// direct call). One dedicated thread drains queued closures one at a time -- the
// serialization that makes production immune to the cross-task deadlock risk FakeTask's
// synchronous, whatever-thread-triggered-it delivery has under heavy *concurrent* driving
// (budget_concurrent_confirm_release_stays_consistent, below, is the one test that
// actually exercises this: multiple threads calling set_want() for *different* tasks at
// once, each able to cascade into a wake for another task's callback -- two such cascades
// landing on two different FakeTask mutexes in opposite orders on two different threads is
// a real AB-BA deadlock a recursive_mutex alone doesn't prevent, since that only guards a
// single thread against re-entering itself). Every other test in this file drives the
// budget from one thread at a time, where that risk can't arise, so plain FakeTask is
// still the simpler, sufficient double there.
class TestTimer {
public:
TestTimer() {
worker_ = std::jthread([this](std::stop_token st) { run(st); });
}
~TestTimer() {
worker_.request_stop();
cv_.notify_all();
}
void post(std::function<void()> fn) {
{
std::lock_guard lk(mu_);
queue_.push_back(std::move(fn));
}
cv_.notify_one();
}
// Blocks until the queue is empty and nothing is mid-run -- what a test needs before
// asserting on state this timer's closures mutate.
void drain() {
std::unique_lock lk(mu_);
cv_done_.wait(lk, [&] { return queue_.empty() && !running_; });
}
private:
void run(std::stop_token st) {
std::unique_lock lk(mu_);
while (true) {
cv_.wait(lk, st, [&] { return !queue_.empty(); });
if (st.stop_requested())
return;
auto fn = std::move(queue_.front());
queue_.erase(queue_.begin());
running_ = true;
lk.unlock();
fn();
lk.lock();
running_ = false;
if (queue_.empty())
cv_done_.notify_all();
}
}
std::mutex mu_;
std::condition_variable_any cv_; // _any: wait() below takes a stop_token predicate
std::condition_variable cv_done_;
std::vector<std::function<void()>> queue_;
bool running_ = false;
std::jthread worker_;
};
// Same grow/shrink logic as FakeTask, but on_target() only ever posts through a TestTimer
// instead of running synchronously -- see the comment above TestTimer for why that's the
// faithful model under concurrent driving.
struct AsyncFakeTask {
SegmentBudget *budget = nullptr;
TestTimer *timer = nullptr;
TaskId id{};
std::mutex mu; // only the timer thread ever touches workers/target -- plain suffices
std::uint32_t workers = 0;
std::uint32_t target = 0;
void on_target(std::uint32_t t) {
timer->post([this, t] {
std::lock_guard lk(mu);
target = t;
while (workers < target) {
if (!budget->confirm_slot(id))
break;
++workers;
}
while (workers > target) {
budget->release_slot(id);
--workers;
}
});
}
std::uint32_t held() {
std::lock_guard lk(mu);
@@ -235,12 +362,14 @@ VT_TEST(budget_on_changed_fires_on_starved_edge) {
VT_TEST(budget_concurrent_confirm_release_stays_consistent) {
SegmentBudget b({.max_active_segments = 16});
constexpr int kTasks = 6;
std::vector<std::unique_ptr<FakeTask>> ts;
TestTimer timer;
std::vector<std::unique_ptr<AsyncFakeTask>> ts;
for (int i = 0; i < kTasks; ++i) {
ts.push_back(std::make_unique<FakeTask>());
ts.push_back(std::make_unique<AsyncFakeTask>());
ts.back()->budget = &b;
ts.back()->timer = &timer;
ts.back()->id = tid(i + 1);
FakeTask *ft = ts.back().get();
AsyncFakeTask *ft = ts.back().get();
b.register_task(ft->id, {.host = "h", .per_task_cap = 6, .resumable = true},
[ft](std::uint32_t n) { ft->on_target(n); });
}
@@ -254,6 +383,7 @@ VT_TEST(budget_concurrent_confirm_release_stays_consistent) {
drivers.clear(); // join
for (auto &ft : ts)
b.set_want(ft->id, 0);
timer.drain(); // let every queued on_target actually run before asserting
// With everyone wanting nothing, the budget must be fully released.
VT_CHECK_EQ(b.budget().active, 0u);
@@ -262,3 +392,123 @@ VT_TEST(budget_concurrent_confirm_release_stays_consistent) {
sum += b.segments_active(ft->id);
VT_CHECK_EQ(sum, 0u);
}
// --- wait-list wakeup: tools/bench heap-profile / load found a real task time out
// waiting on a slot its own target already said it should have (core/docs/m7-baseline.md,
// docs/adr/0012). Root cause: confirm_slot()'s engine-wide cap check (needed so active_
// never exceeds max_active_segments -- a real over-admission bug, not just this liveness
// gap) can deny a task whose target is already correct, when a *different* task is
// legitimately still holding more than its own just-lowered target (yield is deferred to
// a segment boundary, ADR 0011 A1). Nothing in the plain target-changed callback
// mechanism ever revisits a task whose target didn't change -- it was already right. ---
namespace {
// Unlike FakeTask above, on_target() here only enqueues -- it never calls back into the
// budget synchronously. This matches production exactly: register_task()'s on_target
// lambda (download_task.cpp) posts through host.schedule() (engine.cpp), so
// apply_slot_target() never runs on the same call stack as whatever budget call triggered
// it. The test drives delivery explicitly (deliver_one()) instead of a background thread
// so the race this test exists to force -- confirm_slot() denied before the task that's
// over its target has processed its own shrink -- is deterministic, not a timing gamble.
struct QueuedTask {
SegmentBudget *budget = nullptr;
TaskId id{};
std::uint32_t workers = 0;
std::uint32_t target = 0;
std::vector<std::uint32_t> pending;
QueuedTask(SegmentBudget *b, TaskId i) : budget(b), id(i) {}
void on_target(std::uint32_t n) { pending.push_back(n); }
// Delivers the oldest queued target, applying it the way a real task's
// apply_slot_target()/fill_slots_locked() would: try to grow to it (confirm_slot()
// may deny), or shed down to it. Returns false (nothing to deliver) if the queue was
// empty -- the condition VT_REQUIRE checks to prove a wakeup was actually queued.
bool deliver_one() {
if (pending.empty())
return false;
target = pending.front();
pending.erase(pending.begin());
while (workers < target) {
if (!budget->confirm_slot(id))
break;
++workers;
}
while (workers > target) {
budget->release_slot(id);
--workers;
}
return true;
}
};
} // namespace
VT_TEST(budget_release_wakes_a_denied_waiter) {
// Sanity baseline: max_active=1, A (higher priority) holds it, B wants one too and is
// fairly denied -- its target stays 0 while A outranks it and still wants its slot.
// Once A stops wanting one, B's target rises and B is woken via the plain
// target-changed path -- no wait-list needed for this simple case. The harder case
// below is what actually needs it.
SegmentBudget b({.max_active_segments = 1});
QueuedTask A{&b, tid(1)}, B{&b, tid(2)};
b.register_task(A.id, {.host = "h", .per_task_cap = 1, .resumable = true},
[&](std::uint32_t n) { A.on_target(n); });
b.set_task_order(std::vector<TaskId>{A.id, B.id});
b.set_want(A.id, 1);
VT_REQUIRE(A.deliver_one());
VT_CHECK_EQ(A.workers, 1u);
// B registers and wants one too, but with A (higher priority) already holding the
// only slot and still wanting it, B's fairly computed target stays 0 -- unchanged
// from its just-registered value, so no callback is queued for it yet.
b.register_task(B.id, {.host = "h", .per_task_cap = 1, .resumable = true},
[&](std::uint32_t n) { B.on_target(n); });
b.set_want(B.id, 1);
VT_CHECK(!B.deliver_one());
VT_CHECK_EQ(B.workers, 0u);
b.set_want(A.id, 0); // A is done wanting a slot
VT_REQUIRE(A.deliver_one()); // A's target dropped to 0 -- sheds its held slot
VT_CHECK_EQ(A.workers, 0u);
VT_REQUIRE(B.deliver_one()); // B's target rose to 1 -- the plain target-changed path
VT_CHECK_EQ(B.workers, 1u); // B took the freed slot
}
VT_TEST(budget_wait_list_wakes_a_task_whose_target_never_changed) {
// The real gap. max_active=2. Y alone, holds both (target=2). X arrives wanting 1:
// this recompute correctly drops Y's target to 1 (giving X its guaranteed slot) and
// raises X's target to 1 -- both real target changes, both queued. Deliver X's
// *first*: X's target says grow, but Y still physically holds 2 (hasn't processed
// its own shrink yet) -- confirm_slot() must deny X here (active_ == max_active_),
// which is the correctness fix (over-admission is the real RSS bug). Then Y
// processes its shrink and actually releases. X's target never changes again -- it
// was already correctly 1 -- so nothing in the plain mechanism ever revisits X.
SegmentBudget b({.max_active_segments = 2});
QueuedTask Y{&b, tid(1)}, X{&b, tid(2)};
b.register_task(Y.id, {.host = "h", .per_task_cap = 2, .resumable = true},
[&](std::uint32_t n) { Y.on_target(n); });
b.set_want(Y.id, 2);
VT_REQUIRE(Y.deliver_one());
VT_CHECK_EQ(Y.workers, 2u);
b.register_task(X.id, {.host = "h", .per_task_cap = 1, .resumable = true},
[&](std::uint32_t n) { X.on_target(n); });
b.set_task_order(std::vector<TaskId>{Y.id, X.id});
b.set_want(X.id, 1);
VT_REQUIRE(X.deliver_one());
VT_CHECK_EQ(X.workers, 0u); // denied: active_ == max_active_, even though X's target is 1
VT_REQUIRE(Y.deliver_one());
VT_CHECK_EQ(Y.workers, 1u); // Y actually releases its excess now
// The bug: without a wait-list, X.pending is empty here -- nothing was ever queued
// for it, because X's target never changed again. X would wait forever despite its
// target correctly saying it should hold a slot.
VT_REQUIRE(X.deliver_one());
VT_CHECK_EQ(X.workers, 1u);
VT_CHECK_EQ(b.budget().active, 2u);
}
docs/adr/0017-segment-budget-admission-and-wait-list-wakeup.md
+102
View File
@@ -0,0 +1,102 @@
# 17. SegmentBudget over-admission, and a wait-list to wake denied tasks
Status: accepted
## Context
`core/docs/m7-baseline.md`'s first pass measured ~70 MB RSS for the M1/M7 DoD's "20 active
downloads, `max_active_segments=32`" scenario, against a 60 MB target `docs/adr/0012`
estimated would hold with ~45–50 MB of margin. `tools/bench heap-profile` (added to chase
this down — no massif/heaptrack in this environment) found the actual cause directly:
`Engine::segment_budget().budget().active` read 56–86 against a `total` of 32. The engine
was not over budget by a measurement artifact or an ADR 0012 arithmetic gap; it was
genuinely running more concurrent segments — and their 1 MiB ring buffers — than
`max_active_segments` allows.
`SegmentBudget::confirm_slot(id)` checked only `t.held < t.target` — a per-task check.
`reallocate_locked()`'s two-pass fairness allocation does correctly bound
`sum(target) <= max_active_` at the moment it computes a plan, but that bound says nothing
about `sum(held)`: a task can be legitimately holding more than its own just-lowered
target for a while, because yield is deliberately deferred to a segment boundary, never
mid-segment (ADR 0011 A1). When that happens, another task's target can correctly rise to
claim the capacity the first task is *about* to give back, and both `confirm_slot()` calls
can succeed against their own, individually-correct targets while the sum of what's
*physically held* exceeds `max_active_`. Reproduced deterministically in
`core/tests/segment/budget_test.cpp` (`budget_active_never_exceeds_max_active_segments_
under_concurrent_load`, `budget_wait_list_wakes_a_task_whose_target_never_changed`)
independent of any real network I/O.
## Decision
**`confirm_slot()` also checks `active_ < max_active_`, unconditionally**, as a backstop
that doesn't depend on any task's target bookkeeping being perfectly in sync with what
every other task currently holds. This is the fix for the invariant itself.
That backstop creates a liveness question the original design never had to answer: a task
denied *only* by this new check has a target that's already correct — it doesn't change
again on its own, so `reallocate_locked()`'s plain "fire a callback when a task's target
changes" mechanism never revisits it. Nothing was watching for "this specific task is owed
a slot"; **`Task` gained a `waiting_for_slot` flag**, set by `confirm_slot()` on exactly
this denial and cleared on the task's next successful confirm however that retry was
triggered. `release_slot()` and `deregister_task()` — the only two places that can free
real capacity — now call `wake_one_waiter_locked()`, which hands the newly-freed slot to
the highest-priority `waiting_for_slot` task (other than the one that just released) via a
direct retry hint, if `reallocate_locked()`'s own plan didn't already produce one.
On the `download_task.cpp` side, a woken task's `apply_slot_target()` → `fill_slots_locked()`
also needed a fix: a segment that had backed off mid-retry (`SegState::stalled`, its
`release_slot()` already called, a `retry_worker()` timer already scheduled and possibly
already denied once) has no live worker and never surfaces through
`Segmenter::assign_slot()` — it stays *assigned*, just not running, and assign_slot() only
ever hands out unassigned or fresh ranges. `fill_slots_locked()` now restarts any stalled
segment with no live worker directly (bounded by `slot_target`, same as its `assign_slot()`
loop) before looking for new work; a segment it doesn't get to keeps its own scheduled
`retry_worker()` timer as a second chance, so this is additive, not a replacement for that
path.
### What this isn't
Production's real `on_target` callback (`register_task()`'s lambda, `download_task.cpp`)
never runs synchronously with whatever budget call triggered it — it posts through
`host.schedule()` (`engine.cpp`'s single timer thread), so a task's own `mu` can never be
re-entered on the same call stack, and two tasks' callbacks can never race each other into
an AB-BA lock order either (only one ever runs at a time, on one thread). An earlier
version of this fix tried to defend against a same-thread reentrancy hazard that,
diagnosed correctly, doesn't exist in production at all — it exists only if a *test double*
calls back into the budget synchronously from inside `on_target`, which none of production
does. `core/tests/segment/budget_test.cpp`'s `FakeTask` does exactly that (by design — it's
simple and every other test in the file drives the budget from one thread at a time, where
that's harmless); the one test that drives it from *multiple* concurrent threads
(`budget_concurrent_confirm_release_stays_consistent`) uses a separate `AsyncFakeTask` that
posts through a small `TestTimer`, mirroring `host.schedule()` / the engine's timer thread
for real, rather than adding synchronization machinery to `SegmentBudget` itself to paper
over a test double being unlike production. `wake_one_waiter_locked()`'s `exclude`
parameter is the one piece of that defense that's independently justified either way — a
task doesn't need to be told to retry a slot it just gave back itself — and is the only
piece that stayed.
## Consequences
- The M7 RSS number now clears the DoD line: `heap-profile` reports 45.41 MiB for the
20-task/8-segment/32-cap scenario (`core/docs/m7-baseline.md`), and
`budget.active` never exceeds `budget.total` regardless of contention.
- `docs/adr/0016`'s TSan-only load-test straggler (a *different* subsystem —
`rate::RateLimiter`'s byte-pacing, not `SegmentBudget`'s segment-admission) is now
suspected to have been this same root cause rather than the `CURLOPT_LOW_SPEED_TIME`
guess offered there, since the symptom (a task that simply never resumes) matches
exactly. Not reverified at the DoD's full shape under `--preset tsan` in this change —
`tools/bench`'s sanitizer-preset `load` registration still runs at reduced concurrency.
Worth a follow-up run before closing that ADR's postscript.
- `docs/adr/0016`'s actual subject — `rate::RateLimiter::TokenBucket`'s own peek/commit
race, unrelated to `SegmentBudget` — is untouched by this change and remains open.
- `wake_one_waiter_locked()` wakes exactly one task per actual release, by priority order.
Under sustained heavy oversubscription (this bench's 20 tasks × 8 segments = 160 wanted
against 32 available) a low-priority task can still wait a long time for its fair share
— that's the two-pass allocator's own fairness policy working as designed, not a
liveness bug: it will get there, just not fast, and every fresh `reallocate_locked()`
call (any task's `set_want`, registration, or departure) reconsiders everyone from
scratch. No test in this change measures *how* long; if that ever needs a stronger
guarantee (bounded wait time, not just eventual service), it's a fairness-policy change,
not a wiring fix.
Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>