在C中实现MLFQ（多级反馈队列）的最佳方法是什么？

Question

I have a function that looks like this that I need to implement. Threads call this function with these parameters. It's supposed to return with the correct time it accessed the CPU and if it can't access the CPU, it will wait until it can access it.

With regards to the correct time, I keep a global variable that gets updated each time, it calls it.

How do I implement the waits and synchronize it correctly.

int MLFQ(float currentTime, int tid, int remainingTime, int tprio)

My code looks something like this so far and it doesn't quite work.

update globalTime (globalTime = currentTime)
Mutex_lock
Add to MLFQ if needed (either to 5, 10, 15, 20, or 25 MLFQ)
if (canAccessCPU)
    getCPU
    unlock mutex
    return globalTime
else
    mutex_unlock
    return MLFQ(globalTime, tid, remainingTime, tprio);

Answer 1

Your post uses pseudo code, and there are some ambiguities, so comment if I make the wrong assumptions here:

How do I implement the waits and synchronize it correctly [?]

Waits ,

Waits in threads are often implemented in such a way as to not block other threads. Sleep() at the bottom of a thread worker function allows some time for the called thread to sleep, ie share time with other processes. In Windows, it is prototyped as:

VOID WINAPI Sleep(
  _In_  DWORD dwMilliseconds
);  

Linux sleep() here

Synchronizing :
Can be done in many ways. Assuming you are referring to keeping the order in which calls come in from several threads, you can create a simple struct that can be passed back as an argument that could contain a TRUE/FALSE indication of whether the uP was accessed, and the time the attempt was made:

In someheader.h file:

typedef struct  {
    int uPAccess;
    time_t time;
}UP_CALL;

extern UP_CALL uPCall, *pUPCall;

In all of the the .c file(s) you will use:

#include "someheader.h" 

In one of the .c files you must initialize the struct: perhaps in the main fucntion:

int main(void)
{
    pUPCall = &uPCall;
    //other code  
    return 0;
} 

You can now include a pointer to struct in the thread worker function, (normally globals are at risk of access contention between threads, but you are protecting using mutex ), to get time of access attempt, and success of attempt