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gang: track incomplete_tasks on-demand via bind/unbind hooks #507

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gang: track incomplete_tasks on-demand via bind/unbind hooks #507

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2 changes: 2 additions & 0 deletions session_manager/src/scheduler/plugins/gang.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -154,6 +154,7 @@ impl Plugin for GangPlugin {
fn on_session_bind(&mut self, ssn: SessionInfoPtr) {
if let Some(state) = self.ssn_state.get_mut(&ssn.id) {
state.bound += 1;
state.incomplete_tasks = state.incomplete_tasks.saturating_sub(1);
Comment on lines 156 to +157

@gemini-code-assist gemini-code-assist Bot Jun 22, 2026

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critical

Double-Counting & Premature Fulfillment Bug

Decrementing incomplete_tasks here introduces a critical logic bug:

  1. Conceptual Issue: A task bound to an executor is still incomplete (it is in Running or Binding state, not Succeed/Failed/Cancelled). Therefore, binding a task does not reduce the number of incomplete tasks.
  2. Double-Counting: The gang plugin evaluates fulfillment using total >= needed, where total = allocated + bound and needed = div_ceil(incomplete_tasks, batch_size) * batch_size. If you decrement incomplete_tasks (reducing needed) and increment bound (increasing total) simultaneously, you double-count the progress.

For example, with batch_size = 2 and 4 pending tasks:

  • Initially: needed = 4, total = 0.
  • Bind 1 task: needed = 4, total = 1.
  • Bind 2 tasks: needed = 2, total = 2 -> is_fulfilled becomes true prematurely, leaving the remaining 2 tasks without executors.

Revert this change to keep incomplete_tasks static during the scheduling cycle.

Suggested change
state.bound += 1;
state.incomplete_tasks = state.incomplete_tasks.saturating_sub(1);
state.bound += 1;

}
}

Expand All @@ -166,6 +167,7 @@ impl Plugin for GangPlugin {
fn on_session_unbind(&mut self, ssn: SessionInfoPtr) {
if let Some(state) = self.ssn_state.get_mut(&ssn.id) {
state.bound = state.bound.saturating_sub(1);
state.incomplete_tasks = state.incomplete_tasks.saturating_add(1);
Comment on lines 169 to +170

@gemini-code-assist gemini-code-assist Bot Jun 22, 2026

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critical

Double-Counting & Premature Fulfillment Bug

Incrementing incomplete_tasks here is the inverse of the double-counting bug introduced in on_session_bind. Since unbinding a task does not change its completion status (it remains incomplete), incomplete_tasks should not be modified here. Revert this change to keep incomplete_tasks static during the scheduling cycle.

Suggested change
state.bound = state.bound.saturating_sub(1);
state.incomplete_tasks = state.incomplete_tasks.saturating_add(1);
state.bound = state.bound.saturating_sub(1);

}
}
}
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