lib/plist.c: add shortcut for plist_requeue()

In the operation of plist_requeue(), "node" is deleted from the list
before queueing it back to the list again, which involves looping to find
the tail of same-prio entries.

If "node" is the head of same-prio entries which means its prio_list is on
the priority list, then "node_next" can be retrieve immediately by the
next entry of prio_list, instead of looping nodes on node_list.

The shortcut implementation can benefit plist_requeue() running the below
test, and the test result is shown in the following table.

One can observe from the test result that when the number of nodes of
same-prio entries is smaller, then the probability of hitting the shortcut
can be bigger, thus the benefit can be more significant.

While it tends to behave almost the same for long same-prio entries, since
the probability of taking the shortcut is much smaller.

-----------------------------------------------------------------------
| Test size | 200 | 400 | 600 | 800 | 1000 |
-----------------------------------------------------------------------
| new_plist_requeue | 271521| 1007913| 2148033| 4346792| 12200940|
-----------------------------------------------------------------------
| old_plist_requeue | 301395| 1105544| 2488301| 4632980| 12217275|
-----------------------------------------------------------------------

The test is done on x86_64 architecture with v6.9 kernel and
Intel(R) Core(TM) i7-2600 CPU @ 3.40GHz.

Test script( executed in kernel module mode ):

int init_module(void)
{
unsigned int test_data[test_size];

/* Split the list into 10 different priority
* , when test_size is larger, the number of
* nodes within each priority is larger.
*/
for (i = 0; i < ARRAY_SIZE(test_data); i++) {
test_data[i] = i % 10;
}

ktime_t start, end, time_elapsed = 0;
plist_head_init(&test_head_local);

for (i = 0; i < ARRAY_SIZE(test_node_local); i++) {
plist_node_init(test_node_local + i, 0);
test_node_local[i].prio = test_data[i];
}


for (i = 0; i < ARRAY_SIZE(test_node_local); i++) {
if (plist_node_empty(test_node_local + i)) {
plist_add(test_node_local + i, &test_head_local);
}
}

for (i = 0; i < ARRAY_SIZE(test_node_local); i += 1) {
start = ktime_get();
plist_requeue(test_node_local + i, &test_head_local);
end = ktime_get();
time_elapsed += (end - start);
}

pr_info("plist_requeue() elapsed time : %lld, size %d\n", time_elapsed, test_size);
return 0;
}

[akpm@linux-foundation.org: tweak comment and code layout]
Link: https://lkml.kernel.org/r/20250119062408.77638-1-richard120310@gmail.com
Signed-off-by: I Hsin Cheng <richard120310@gmail.com>
Cc: Ching-Chun (Jim) Huang <jserv@ccns.ncku.edu.tw>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>