fix(scheduler): SERIAL task avg/max stats corrupted by unflagged USB TX waits - #1392
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scheduler() updated movingSumExecutionTime (feeds the CLI tasks command's avg/us) unconditionally, while only maxExecutionTime was gated behind the ignoreCurrentTaskExecTime flag. serial_usb_vcp.c sets that flag during a USB CDC TX backpressure stall at connect time, so a connect burst could inflate avg while being excluded from max, producing avg > max. Move the movingSumExecutionTime update inside the same guard as maxExecutionTime. totalExecutionTime stays unconditional; it is a genuine cumulative wall-clock counter, unlike the moving average which characterizes recent task behavior. BF 4.5-maintenance's scheduler.c has the identical unconditional/gated structure, but has zero callers of schedulerIgnoreTaskExecTime() anywhere in its tree, so BF's version of this gap is unreachable. EF's serial_usb_vcp.c (added by PR emuflight#1285) is what makes it reachable, so this is an intentional EF-only divergence from BF's scheduler.c to fix a real, reproducible bug. Fixes emuflight#1382.
usbVcpWriteBuf()/usbVcpFlush() (drivers/serial_usb_vcp.c) only call schedulerIgnoreTaskExecTime() when CDC_Send_DATA() returns a partial count. CDC_Send_DATA() (both the F7/H7 HAL backend and the F4 STDPERIPH backend) can wait on CDC_Send_FreeBytes() == 0 and still return a full count if the wait clears within its own per-byte deadline, so the caller's partial-count check never sees it. A large CLI/MSP response right after USB connect (host not yet polling the CDC IN endpoint at full speed) triggers many small buffered flushes, each silently absorbing part of that per-byte deadline. None register as backpressure individually, but the sum inflates TASK_SERIAL's maxExecutionTime/maxload with real, uncounted wait time. Call schedulerIgnoreTaskExecTime() directly inside CDC_Send_DATA's own wait loop, on every iteration a wait occurs, regardless of whether the byte ultimately clears within budget. Confirmed via temporary micros()-based instrumentation (not included in this commit): SERIAL task max/us on the first tasks CLI read after a fresh connect dropped from 400-940% maxload to normal double/triple-digit microsecond values on STELLARH7DEV, TMOTORF7, and TUNERCF405. Completes the fix for emuflight#1382; the scheduler.c commit in this branch fixed the avg > max inversion, this fixes the residual max-inflation. Same gap confirmed present in Betaflight master by code analysis, filed as betaflight/betaflight#15577.
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📝 WalkthroughWalkthroughThe scheduler now always records cumulative execution time while excluding ignored USB transmit waits from moving-sum and maximum statistics. Both USB CDC implementations mark transmit-buffer waits as ignored. Unit tests verify the statistic behavior. ChangesScheduler and USB accounting
Estimated code review effort: 3 (Moderate) | ~20 minutes Merge Risk: 🔵 Low · up to The PR corrects scheduler execution-time accounting and USB CDC wait handling, with reported unit, build, and hardware validation. Merge is reasonable with owner awareness that the scheduler test should more directly verify that ignored waits cannot inflate maximum execution time. Sequence Diagram(s)sequenceDiagram
participant CDC_Send_DATA
participant Scheduler
participant TaskStatistics
CDC_Send_DATA->>Scheduler: mark USB transmit wait as ignored
CDC_Send_DATA->>CDC_Send_DATA: wait for transmission or TX buffer space
Scheduler->>TaskStatistics: add duration to totalExecutionTime
Scheduler->>TaskStatistics: skip moving-sum and maximum updates
🚥 Pre-merge checks | ✅ 4 | ❌ 1❌ Failed checks (1 warning)
✅ Passed checks (4 passed)
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@coderabbitai, AI generated comment Please analyze this PR's two fixes with particular attention to:
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Tip For best results, initiate chat on the files or code changes. 🧩 Analysis chain🏁 Script executed: #!/bin/bash
set -euo pipefail
echo '=== PR diff for requested files ==='
git diff --unified=80 HEAD^ HEAD -- \
src/main/scheduler/scheduler.c \
src/main/vcp_hal/usbd_cdc_interface.c \
src/main/vcpf4/usbd_cdc_vcp.c \
|| true
echo '=== Scheduler symbol map ==='
ast-grep outline src/main/scheduler/scheduler.c --items all --match 'scheduler|ignoreCurrentTaskExecTime|movingSumExecutionTime' || true
echo '=== Scheduler flag declarations, resets, writers, and accounting ==='
rg -n -C 10 \
'ignoreCurrentTaskExecTime|schedulerIgnoreTaskExecTime\s*\(|movingSumExecutionTime|maxExecutionTime|totalExecutionTime' \
src/main/scheduler/scheduler.c src/main \
|| true
echo '=== USB backend call sites and CDC wait-loop context ==='
rg -n -C 18 \
'CDC_Send_DATA\s*\(|CDC_Send_FreeBytes\s*\(|schedulerIgnoreTaskExecTime\s*\(' \
src/main/vcp_hal/usbd_cdc_interface.c \
src/main/vcpf4/usbd_cdc_vcp.c \
src/main/drivers/serial_usb_vcp.c \
|| true
echo '=== Scheduler tests changed by this PR ==='
git diff --name-only HEAD^ HEAD | rg -n 'scheduler|Scheduler|test|Test' || true
rg -n -C 12 \
'TestIgnoredExecutionTimeExcludedFromMovingSumAndMax|IgnoreTaskExecTime|movingSumExecutionTime' \
src/test src/main 2>/dev/null \
|| trueLength of output: 132162 🏁 Script executed: #!/bin/bash
set -euo pipefail
echo '=== All direct CDC_Send_DATA callers, excluding its two definitions ==='
rg -n -C 6 '\bCDC_Send_DATA\s*\(' src \
| sed -n '/CDC_Send_DATA/,+12p'
echo '=== All schedulerIgnoreTaskExecTime callers ==='
rg -n -C 5 '\bschedulerIgnoreTaskExecTime\s*\(' src \
| sed -n '/schedulerIgnoreTaskExecTime/,+10p'
echo '=== Task serial definition and registration ==='
rg -n -C 15 '\btaskHandleSerial\s*\(|TASK_SERIAL' \
src/main/fc/fc_tasks.c src/main 2>/dev/null \
| head -n 500
echo '=== USB interrupt handlers and calls into CDC send APIs ==='
rg -n -C 10 \
'(IRQHandler|HAL_PCD|USBD_LL|USBD_CDC|VCP_DataTx|CDC_Send_DATA|usbVcpWriteBuf|usbVcpFlush)' \
src/main/vcp_hal src/main/vcpf4 src/main/drivers/serial_usb_vcp.c \
| head -n 800
echo '=== delay implementation/declaration context ==='
rg -n -C 12 '\bdelay\s*\(' \
src/main/drivers/time.c src/main/drivers/time.h src/main 2>/dev/null \
| head -n 500
echo '=== Test section, including ignored-run reset setup ==='
sed -n '490,540p' src/test/unit/scheduler_unittest.ccLength of output: 118850
1. USB wait-loop callsThe new calls are safe in the observed call path.
The code must retain this contract. If an ISR later calls The F4-specific 2. Moving average and reset behaviorThe flag does not leak from one selected task invocation to the next. In However, the stated stale-average conclusion is not fully correct:
This behavior is consistent with treating ignored executions as no sample in a moving average. It is not consistent with a moving window that advances for every scheduler invocation. The existing unit test verifies one ignored run. It does not cover repeated ignored runs or define the intended stale-value behavior. For this PR, I do not see this as a blocker. The change correctly prevents excluded USB wait time from entering 3. General correctness and efficiencyThe changes are correct and minimal.
No reentrancy, ordering, or volatile issue was found for the current task-context call path. You are interacting with an AI system. |
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Inline comments:
In `@src/test/unit/scheduler_unittest.cc`:
- Around line 53-58: Update taskHandleSerial so the triggerIgnoreTaskExecTime
path adds a longer simulated duration than the baseline run, then adjust the
cumulative-time assertion in the corresponding scheduler test to expect that
longer duration. Keep the non-ignored run at TEST_HANDLE_SERIAL_TIME and ensure
both affected test cases validate the longer ignored execution.
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src/main/scheduler/scheduler.csrc/main/vcp_hal/usbd_cdc_interface.csrc/main/vcpf4/usbd_cdc_vcp.csrc/test/unit/scheduler_unittest.cc
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🔇 Additional comments (3)
src/main/scheduler/scheduler.c (1)
347-351: LGTM!src/main/vcp_hal/usbd_cdc_interface.c (1)
51-51: LGTM!Also applies to: 376-381
src/main/vcpf4/usbd_cdc_vcp.c (1)
33-33: LGTM!Also applies to: 171-187
Both runs in TestIgnoredExecutionTimeExcludedFromMovingSumAndMax added the same TEST_HANDLE_SERIAL_TIME duration, so a regression that dropped the ignoreCurrentTaskExecTime gate entirely would still pass: max/movingSum would coincidentally equal maxBaseline/movingSumBaseline either way. Double the ignored run's duration so a missing gate produces a visibly different value. Verified: reverting the scheduler.c fix now makes this test fail (movingSum 90 vs expected 30); with the fix, it passes. Addresses CodeRabbit finding on PR emuflight#1392.
1 participant
AI Generated pull-request
Summary
Closes #1382.
Two independent bugs in
TASK_SERIAL's scheduler stats, both stemming from gaps in howUSB VCP TX-backpressure waits are excluded from task execution time accounting.
1.
avg/uscould exceedmax/usscheduler()(src/main/scheduler/scheduler.c) updatedmovingSumExecutionTime(feeds the
tasksCLI command'savg/us) unconditionally, while onlymaxExecutionTimewas gated behind theignoreCurrentTaskExecTimeflag. A USBconnect-time CDC backpressure burst could inflate
avgwhile being excluded frommax,producing the logically-impossible
avg > max.Fix: move the
movingSumExecutionTimeupdate inside the same guard asmaxExecutionTime.totalExecutionTimestays unconditional — it's a genuine cumulativewall-clock counter, unlike the moving average.
Added
SchedulerUnittest.TestIgnoredExecutionTimeExcludedFromMovingSumAndMax, verifiedto fail without the fix (movingSum absorbed a gated run, 30→60) and pass with it.
2.
max/us/maxloadstill showing 400-940% right after connectFound via live hardware iteration this session, after confirming bug 1's fix alone
didn't fully resolve the symptom on H7/F7 hardware.
usbVcpWriteBuf()/usbVcpFlush()(src/main/drivers/serial_usb_vcp.c) only callschedulerIgnoreTaskExecTime()whenCDC_Send_DATA()returns a partial count. ButCDC_Send_DATA()— both the F7/H7 HAL backend (vcp_hal/usbd_cdc_interface.c) and theF4 STDPERIPH backend (
vcpf4/usbd_cdc_vcp.c) — can wait onCDC_Send_FreeBytes() == 0and still return a full count if the wait clears within its own per-byte deadline.
When that happens the caller's partial-count check never fires, so the wait is never
flagged, despite real wall-clock time having been spent.
A large CLI/MSP response right after USB connect (host not yet polling the CDC IN
endpoint at full speed) triggers many small buffered flushes. None register as
backpressure individually, but the sum inflates
TASK_SERIAL'smaxExecutionTime/maxloadwith real, uncounted wait time.Fix: call
schedulerIgnoreTaskExecTime()directly insideCDC_Send_DATA's own waitloop, on every iteration a wait occurs — not only on final timeout/partial-return.
Hardware verification
Fresh-boot → connect →
tasks(first read), repeated across multiple boot cycles:SERIALmax/us ~40000-41300, ~400-412% maxloadDiagnostic
micros()-based instrumentation was used to isolate the exact call siteduring this session, then fully removed before committing — not part of this PR.
Notes
scheduler.chas the identical unconditional/gated asymmetry asbug 1, but BF's own USB VCP driver never calls
schedulerIgnoreTaskExecTime()anywhere, so BF's version of that gap is unreachable dead code. EF's own
serial_usb_vcp.c(from a prior PR) is what makes it reachable — bug 1's fix is anintentional EF-only divergence from BF's
scheduler.cto fix a real, reachable bug.(
src/platform/STM32/serial_usb_vcp.c+vcp_hal/usbd_cdc_interface.c+vcpf4/usbd_cdc_vcp.c'sVCP_DataTx) — confirmed by code analysis, not BF-hardware-tested. Filed as needs validation: USB VCP TX-backpressure wait silently untracked by scheduler task stats betaflight/betaflight#15577.
Test plan
make clean_test && make test— 47/47 test binaries clean, including the newscheduler unit test
FOXEERF722V4, FOXEERF405, APEXF7, TMOTORF7, STELLARH7DEV, SITL — 10/10 succeeded,
0 failed, no warnings
Summary by CodeRabbit
Bug Fixes
Tests