Fix critical SMP scheduler issues with CPU selection and ready-to-run list management

[hujun260]

Summary

This PR addresses critical issues in the SMP scheduler's CPU selection and task management logic:

1. Fixes incorrect CPU initialization in nxsched_select_cpu() preventing valid CPU selection
2. Removes redundant lock validation that prevented optimal CPU selection for bound tasks
3. Simplifies ready-to-run list addition by eliminating unnecessary bounds checking
4. Fixes improper task cleanup sequencing in nxsched_remove_self() operation

These changes ensure correct task placement on SMP systems and proper scheduler behavior during task removal.

Motivation and Problem

Issue 1: Invalid CPU Selection Initialization

• nxsched_select_cpu() initializes cpu = CONFIG_SMP_NCPUS (invalid sentinel value)
• No validation ensured valid CPU selection before return
• Tasks could be assigned to non-existent CPUs

Issue 2: Lock-Check Preventing Optimal Scheduling

• Condition !nxsched_islocked_tcb(rtcb) prevented selection of CPU-locked tasks
• CPU-locked tasks should be valid candidates for selection, not excluded
• This violated SMP affinity semantics

Issue 3: Redundant Condition in Ready-to-Run Addition

• Check if (target_cpu < CONFIG_SMP_NCPUS) was unnecessary after CPU selection
• nxsched_select_cpu() already guarantees valid CPU return (now with assertion)
• Removed dead-code conditional block

Issue 4: Incorrect Task Cleanup Sequencing

• nxsched_switch_running() called in nxsched_remove_self() after function return
• Proper cleanup requires call before exit
• Relocated to nxsched_remove_running() for correct sequencing

Changes Made

File: sched/sched/sched.h

1. Line 580: Changed CPU initialization from CONFIG_SMP_NCPUS to 0xff (invalid sentinel)

   • Allows detection of failed CPU selection

2. Line 603-604: Removed lock check from CPU selection condition

   • Before: else if (rtcb->sched_priority <= minprio && !nxsched_islocked_tcb(rtcb))
   • After: else if (rtcb->sched_priority <= minprio)
   • Rationale: CPU-locked tasks are valid candidates for selection

3. Line 613: Added DEBUGASSERT(cpu != 0xff) before return

   • Validates CPU selection always produces valid result
   • Catches failures in CPU selection algorithm

File: sched/sched/sched_addreadytorun.c

1. Line 166: Added priority check to switch_equal condition

   • Before: if (switch_equal)
   • After: if (switch_equal && sched_priority > 0)
   • Prevents invalid priority decrement for idle thread

2. Line 258: Moved tcb declaration outside conditional block

   • Now declared at function entry: FAR struct tcb_s *tcb = current_task(target_cpu);
   • Enables removal of bounds checking

3. Lines 267-276: Removed if (target_cpu < CONFIG_SMP_NCPUS) conditional

   • Removed dead-code check (CPU selection guarantees valid result)
   • Added btcb->cpu = target_cpu; assignment for CPU tracking
   • Simplified to direct comparison: if (tcb->sched_priority < btcb->sched_priority)

4. Line 267: Added comment explaining CPU assignment

   • "In some cases, such as setaffinity, cpu need to be used."

File: sched/sched/sched_process_delivered.c

1. Line 108: Removed bounds check from CPU comparison
   • Before: if (target_cpu < CONFIG_SMP_NCPUS && target_cpu != cpu && ...)
   • After: if (target_cpu != cpu && ...)
   • Rationale: CPU selection guarantees valid result

File: sched/sched/sched_removereadytorun.c

1. Line 166: Relocated nxsched_switch_running(tcb->cpu, false) call
   • Moved from nxsched_remove_self() to nxsched_remove_running()
   • Ensures proper cleanup sequencing before task exit
   • Prevents use-after-free scenarios

Impact

• Correctness: Fixes fundamental SMP scheduler bugs affecting task placement
• Performance: Eliminates unnecessary validation checks after guaranteed CPU selection
• Safety: Adds DEBUGASSERT to catch CPU selection failures in development
• Reliability: Corrects task cleanup sequencing to prevent race conditions

Verification Checklist

• Code compiles without warnings on all ARM architectures (ARMv7-A, ARMv7-R, ARM64)
• SMP scheduler logic validated for multi-CPU task placement
• CPU affinity masks properly respected in selection
• Task removal sequencing verified for safe cleanup
• Ready-to-run list operations tested in single and multi-CPU configurations
• No regression in single-CPU configurations (CONFIG_SMP=0)
• DEBUGASSERT validates CPU selection correctness

Testing Scenarios

1. SMP Task Placement

   • Create multiple tasks with CPU affinity masks
   • Verify correct CPU assignment via task introspection
   • Confirm tasks execute on assigned CPUs

2. Load Balancing

   • Verify CPU selection chooses least-loaded eligible CPU
   • Confirm priority ordering respected in multi-CPU scenario
   • Test with mixed priority levels

3. CPU-Locked Tasks

   • Verify CPU-locked tasks remain candidate for selection
   • Confirm proper sequencing when multiple locked tasks present
   • Test interaction with affinity-based scheduling

4. Task Removal

   • Verify proper cleanup on task exit
   • Confirm no use-after-free with current_task() calls
   • Test rapid task creation/destruction cycles

5. Edge Cases

   • Single eligible CPU scenario
   • All CPUs saturated with high-priority tasks
   • Task switching between CPUs during lifecycle

Technical Notes

CPU Selection Algorithm:

• Uses invalid sentinel (0xff) to detect selection failure
• Ensures every valid path through algorithm assigns valid CPU
• DEBUGASSERT provides development-time validation
• Production code operates with guaranteed correct CPU value

Ready-to-Run Addition:

• Dependency on valid CPU selection enables code simplification
• CPU assignment (btcb->cpu = target_cpu) critical for affinity tracking
• Removes defensive programming unnecessary after guarantee

Task Removal Sequencing:

• nxsched_switch_running() must execute while task TCB valid
• Moving call to nxsched_remove_running() prevents post-exit access
• Maintains exception safety for cleanup operations

Build Information

Compiler: ARM GCC 10.x and later
Architectures: ARMv7-A, ARMv7-R, ARM64, ARM Cortex-M series
Configurations: CONFIG_SMP enabled (also compatible with single-CPU)
Flags: -Wall -Wextra -Werror