Need help understanding the function Scheduler_SMP_Allocate_processor_lazy

[Gedare Bloom]

On Sat, Jun 20, 2020 at 6:12 AM Richi Dubey <richidubey at gmail.com> wrote:
>
> Hi,
>
> I've been learning how SMP scheduler word in RTEMS and I came across the following function definition:
> --------------------------------------------------------------------------------------------
>
> /**
>  * @brief Allocates the cpu for the scheduled thread.
>  *
>  * Attempts to prevent migrations but does not take into account affinity.
>  *
>  * @param context The scheduler context instance.
>  * @param scheduled The scheduled node that should be executed next.
>  * @param victim If the heir is this node's thread, no processor is allocated.
>  * @param[in, out] victim_cpu The cpu to allocate.
>  */
>
>  static inline void _Scheduler_SMP_Allocate_processor_lazy(
>   Scheduler_Context *context,
>   Scheduler_Node    *scheduled,
>   Scheduler_Node    *victim,
>   Per_CPU_Control   *victim_cpu
> )
> {
>   Thread_Control *scheduled_thread = _Scheduler_Node_get_user( scheduled );
>   Thread_Control *victim_thread = _Scheduler_Node_get_user( victim );
>   Per_CPU_Control *scheduled_cpu = _Thread_Get_CPU( scheduled_thread );
>   Per_CPU_Control *cpu_self = _Per_CPU_Get();
>   Thread_Control *heir;
>
>   _Assert( _ISR_Get_level() != 0 );
>
>   if ( _Thread_Is_executing_on_a_processor( scheduled_thread ) ) {
>     if ( _Scheduler_SMP_Is_processor_owned_by_us( context, scheduled_cpu ) ) {
>       heir = scheduled_cpu->heir;
>       _Thread_Dispatch_update_heir(
>         cpu_self,
>         scheduled_cpu,
>         scheduled_thread
>       );
>     } else {
>       /* We have to force a migration to our processor set */
>       heir = scheduled_thread;
>     }
>   } else {
>     heir = scheduled_thread;
>   }
>
>   if ( heir != victim_thread ) {
>     _Thread_Set_CPU( heir, victim_cpu );
>     _Thread_Dispatch_update_heir( cpu_self, victim_cpu, heir );
>   }
> }
>
>
> --------------------------------------------------------------------------------
> Can someone please help me understand what this function is trying to do and how exactly it is achieving that?  I am finding it hard to understand the meaning and use of the terms like cpu_self (why do we need cpu_self ? Is it the CPU for scheduled or for victim?),  _Thread_Is_executing_on_a_processor( scheduled_thread ): Why would we have the thread already executing when we need to allocate a processor to it? Why are we allocating a processor then?
>

Have you looked at how the function gets called/used? Or more
importantly, when it gets called?

you see cpu_self = Per_CPU_Get();
Have you looked at that function to see what it does yet? Does it need
better documentation/doxygen?

> Thanks,
> Richi.
>
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