Modification of a test file taken from https://www.cs.virginia.edu/~cr4bd/4414/S2021/lottery.html for `xv6` scheduling

fetch.c

#include "types.h"
#include "user.h"
#include "param.h"
#include "pstat.h"

#undef USE_YIELD
#define MAX_CHILDREN 32
#define LARGE_TICKET_COUNT 100000
#define MAX_YIELDS_FOR_SETUP 100

__attribute__((noreturn))
void yield_forever() {
    while (1) {
        yield();
    }
}

__attribute__((noreturn))
void run_forever() {
    while (1) {
        __asm__("");
    }
}

int spawn(int tickets) {
    int pid = fork();
    if (pid == 0) {
        settickets(tickets);
        yield();
#ifdef USE_YIELD
        yield_forever();
#else
        run_forever();
#endif
    } else if (pid != -1) {
        return pid;
    } else {
        printf(2, "error in fork\n");
        return -1;
    }
}

int find_index_of_pid(int *list, int list_size, int pid) {
    for (int i = 0; i < list_size; ++i) {
        if (list[i] == pid)
            return i;
    }
    return -1;
}

int get_num_process(int *inuse){
    int num_processes = 0;
    for(int i=0; i<NPROC; ++i){
        num_processes += inuse[i];
    }
    return num_processes;
}

void wait_for_ticket_counts(int num_children, int *pids, int *tickets) {
    for (int yield_count = 0; yield_count < MAX_YIELDS_FOR_SETUP; ++yield_count) {
        yield();
        int done = 1;
        struct pstat info;
        getpinfo(&info);
        for (int i = 0; i < num_children; ++i) {
            int index = find_index_of_pid(info.pid, get_num_process(info.inuse), pids[i]);
            if (info.tickets[index] != tickets[i]) done = 0;
        }
        if (done)
            break;
    }
}

int main(int argc, char *argv[])
{
    if (argc < 3) {
        printf(2, "usage: %s seconds tickets1 tickets2 ... ticketsN\n"
                  "       seconds is the number of time units to run for\n"
                  "       ticketsX is the number of tickets to give to subprocess N\n",
                  argv[0]);
        exit();
    }
    int tickets_for[MAX_CHILDREN];
    int active_pids[MAX_CHILDREN];
    int num_seconds = atoi(argv[1]);
    int num_children = argc - 2;
    if (num_children > MAX_CHILDREN) {
        printf(2, "only up to %d supported\n", MAX_CHILDREN);
        exit();
    }
    /* give us a lot of ticket so we don't get starved */
    settickets(LARGE_TICKET_COUNT);
    for (int i = 0; i < num_children; ++i) {
        int tickets = atoi(argv[i + 2]);
        tickets_for[i] = tickets;
        active_pids[i] = spawn(tickets);
    }
    wait_for_ticket_counts(num_children, active_pids, tickets_for);
    struct pstat before, after;
    getpinfo(&before);
    int before_num_processes = get_num_process(before.inuse);
    sleep(num_seconds);
    getpinfo(&after);
    int after_num_processes = get_num_process(after.inuse);
    for (int i = 0; i < num_children; ++i) {
        kill(active_pids[i]);
    }
    for (int i = 0; i < num_children; ++i) {
        wait();
    }
    if (before_num_processes >= NPROC || after_num_processes >= NPROC) {
        printf(2, "getpinfo's num_processes is greater than NPROC before parent slept\n");
        return 1;
    }
    printf(1, "TICKETS\tTICKS\n");
    for (int i = 0; i < num_children; ++i) {
        int before_index = find_index_of_pid(before.pid, before_num_processes, active_pids[i]);
        int after_index = find_index_of_pid(after.pid, after_num_processes, active_pids[i]);
        if (before_index == -1)
            printf(2, "child %d did not exist for getpinfo before parent slept\n", i);
        if (after_index == -1)
            printf(2, "child %d did not exist for getpinfo after parent slept\n", i);
        if (before_index == -1 || after_index == -1) {
            printf(1, "%d\t--unknown--\n", tickets_for[i]);
        } else {
            if (before.tickets[before_index] != tickets_for[i]) {
                printf(2, "child %d had wrong number of tickets in getpinfo before parent slept\n", i);
            }
            if (after.tickets[after_index] != tickets_for[i]) {
                printf(2, "child %d had wrong number of tickets in getpinfo after parent slept\n", i);
            }
            printf(1, "%d\t%d\n", tickets_for[i], after.ticks[after_index] - before.ticks[before_index]);
        }
    }
    exit();
}

note.md

• Add the above file and update Makefile to run fetch as a user program
• Add yield as a syscall

void
sys_yield(void)
{
  yield();
}

• Run the command fetch 1500 100 200 300 400 500

• for those who are getting the child %d had wrong number of tickets in getpinfo before parent slept

• tweak with the MAX_YIELDS_FOR_SETUP. Increasing it to a certain amount will make the parent process wait necessary times before it calls getpinfo for the before structure.