Hi all,

This patch series improves the performance of the process freezer by
skipping zombie tasks during freezing.

In the suspend and hibernation paths, the freezer traverses all tasks
and attempts to freeze them. However, zombie tasks (EXIT_ZOMBIE with
PF_EXITING) are already dead — they are not schedulable and cannot enter
the refrigerator. Attempting to freeze such tasks is redundant and
unnecessarily increases freezing time.

In particular, on systems under fork storm conditions (e.g., many
short-lived processes quickly becoming zombies), the number of zombie tasks
can spike into the thousands or more. We observed that this causes the
freezer loop to waste significant time processing tasks that are guaranteed
to not need freezing.

Testing and Results

Platform:
- Architecture: x86_64
- CPU: ZHAOXIN KaiXian KX-7000
- RAM: 16 GB
- Kernel: v6.6.93

result without the patch:
dmesg | grep elap
[  219.608992] Freezing user space processes completed (elapsed 0.010 seconds)
[  219.617355] Freezing remaining freezable tasks completed (elapsed 0.008 seconds)
[  228.029119] Freezing user space processes completed (elapsed 0.013 seconds)
[  228.040672] Freezing remaining freezable tasks completed (elapsed 0.011 seconds)
[  236.879065] Freezing user space processes completed (elapsed 0.020 seconds)
[  236.897976] Freezing remaining freezable tasks completed (elapsed 0.018 seconds)
[  246.276679] Freezing user space processes completed (elapsed 0.026 seconds)
[  246.298636] Freezing remaining freezable tasks completed (elapsed 0.021 seconds)
[  256.221504] Freezing user space processes completed (elapsed 0.030 seconds)
[  256.248955] Freezing remaining freezable tasks completed (elapsed 0.027 seconds)
[  266.674987] Freezing user space processes completed (elapsed 0.040 seconds)
[  266.709811] Freezing remaining freezable tasks completed (elapsed 0.034 seconds)
[  277.701679] Freezing user space processes completed (elapsed 0.046 seconds)
[  277.742048] Freezing remaining freezable tasks completed (elapsed 0.040 seconds)
[  289.246611] Freezing user space processes completed (elapsed 0.046 seconds)
[  289.290753] Freezing remaining freezable tasks completed (elapsed 0.044 seconds)
[  301.516854] Freezing user space processes completed (elapsed 0.041 seconds)
[  301.576287] Freezing remaining freezable tasks completed (elapsed 0.059 seconds)
[  314.422499] Freezing user space processes completed (elapsed 0.043 seconds)
[  314.465804] Freezing remaining freezable tasks completed (elapsed 0.043 seconds)

result with the patch:
dmesg | grep elap
[   54.161674] Freezing user space processes completed (elapsed 0.007 seconds)
[   54.171536] Freezing remaining freezable tasks completed (elapsed 0.009 seconds)
[   62.556462] Freezing user space processes completed (elapsed 0.006 seconds)
[   62.566496] Freezing remaining freezable tasks completed (elapsed 0.010 seconds)
[   71.395421] Freezing user space processes completed (elapsed 0.009 seconds)
[   71.402820] Freezing remaining freezable tasks completed (elapsed 0.007 seconds)
[   80.785463] Freezing user space processes completed (elapsed 0.010 seconds)
[   80.793914] Freezing remaining freezable tasks completed (elapsed 0.008 seconds)
[   90.962659] Freezing user space processes completed (elapsed 0.012 seconds)
[   90.973519] Freezing remaining freezable tasks completed (elapsed 0.010 seconds)
[  101.435638] Freezing user space processes completed (elapsed 0.013 seconds)
[  101.449023] Freezing remaining freezable tasks completed (elapsed 0.013 seconds)
[  112.669786] Freezing user space processes completed (elapsed 0.015 seconds)
[  112.683540] Freezing remaining freezable tasks completed (elapsed 0.013 seconds)
[  124.585681] Freezing user space processes completed (elapsed 0.017 seconds)
[  124.599553] Freezing remaining freezable tasks completed (elapsed 0.013 seconds)
[  136.826635] Freezing user space processes completed (elapsed 0.016 seconds)
[  136.841840] Freezing remaining freezable tasks completed (elapsed 0.015 seconds)
[  149.686575] Freezing user space processes completed (elapsed 0.016 seconds)
[  149.701549] Freezing remaining freezable tasks completed (elapsed 0.014 seconds)

Here is the user-space fork storm simulator used for testing:

```c
// create_zombie.c

void usage(const char *prog) {
    fprintf(stderr, "Usage: %s <number_of_zombies>\n", prog);
    exit(EXIT_FAILURE);
}

int main(int argc, char *argv[]) {
    if (argc != 2) {
        usage(argv[0]);
    }

    long num_zombies = strtol(argv[1], NULL, 10);
    if (num_zombies <= 0 || num_zombies > 1000000) {
        fprintf(stderr, "Invalid number of zombies: %ld\n", num_zombies);
        exit(EXIT_FAILURE);
    }

    printf("Creating %ld zombie processes...\n", num_zombies);

    for (long i = 0; i < num_zombies; i++) {
        pid_t pid = fork();
        if (pid < 0) {
            perror("fork failed");
            exit(EXIT_FAILURE);
        } else if (pid == 0) {
            // Child exits immediately
            exit(0);
        }
        // Parent does NOT wait, leaving zombie
    }

    printf("All child processes created. Sleeping for 60 seconds...\n");
    sleep(60);

    printf("Parent exiting, zombies will be reaped by init.\n");
    return 0;
}
```

And we used a shell loop to suspend repeatedly:

```bash
LOOPS=10

echo none > /sys/power/pm_test
echo 5 > /sys/module/suspend/parameters/pm_test_delay
for ((i=1; i<=LOOPS; i++)); do
echo "===== Test round $i/$LOOPS ====="
./create_zombie $((i * 3000)) &
sleep 5
echo mem > /sys/power/state

pkill create_zombie
echo "Round $i complete. Waiting 5s..."
sleep 5

done
echo "==== All $LOOPS rounds complete ===="
```

Zihuan Zhang (1):
  PM / Freezer: Skip zombie/dead processes to reduce freeze latency

 kernel/power/process.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

-- 
2.25.1