From: Nick Piggin <nickpiggin@yahoo.com.au>

Use hlists for the PID hashes.  This halves the memory footprint of these
hashes.  No benchmarks, but I think this is a worthy improvement because
the hashes are something that would be likely to have significant portions
loaded into the cache of every CPU on some workloads.

This comes at the "expense" of
	1. reintroducing the memory  prefetch into the hash traversal loop;
	2. adding new pids to the head of the list instead of the tail. I
	   suspect that if this was a big problem then the hash isn't sized
	   well or could benefit from moving hot entries to the head.

Also, account for all the pid hashes when reporting hash memory usage.

Signed-off-by: Andrew Morton <akpm@osdl.org>
---

 25-akpm/include/linux/pid.h |    2 +-
 25-akpm/kernel/pid.c        |   19 ++++++++++---------
 2 files changed, 11 insertions(+), 10 deletions(-)

diff -puN include/linux/pid.h~use-hlist-for-pid-hash include/linux/pid.h
--- 25/include/linux/pid.h~use-hlist-for-pid-hash	2004-08-21 23:56:06.861648056 -0700
+++ 25-akpm/include/linux/pid.h	2004-08-21 23:58:05.706580880 -0700
@@ -16,7 +16,7 @@ struct pid
 	atomic_t count;
 	struct task_struct *task;
 	struct list_head task_list;
-	struct list_head hash_chain;
+	struct hlist_node hash_chain;
 };
 
 struct pid_link
diff -puN kernel/pid.c~use-hlist-for-pid-hash kernel/pid.c
--- 25/kernel/pid.c~use-hlist-for-pid-hash	2004-08-21 23:56:06.862647904 -0700
+++ 25-akpm/kernel/pid.c	2004-08-21 23:56:06.866647296 -0700
@@ -27,7 +27,7 @@
 #include <linux/hash.h>
 
 #define pid_hashfn(nr) hash_long((unsigned long)nr, pidhash_shift)
-static struct list_head *pid_hash[PIDTYPE_MAX];
+static struct hlist_head *pid_hash[PIDTYPE_MAX];
 static int pidhash_shift;
 
 int pid_max = PID_MAX_DEFAULT;
@@ -150,11 +150,11 @@ failure:
 
 fastcall struct pid *find_pid(enum pid_type type, int nr)
 {
-	struct list_head *elem, *bucket = &pid_hash[type][pid_hashfn(nr)];
+	struct hlist_node *elem;
 	struct pid *pid;
 
-	__list_for_each(elem, bucket) {
-		pid = list_entry(elem, struct pid, hash_chain);
+	hlist_for_each_entry(pid, elem,
+			&pid_hash[type][pid_hashfn(nr)], hash_chain) {
 		if (pid->nr == nr)
 			return pid;
 	}
@@ -181,7 +181,8 @@ int fastcall attach_pid(task_t *task, en
 		INIT_LIST_HEAD(&pid->task_list);
 		pid->task = task;
 		get_task_struct(task);
-		list_add(&pid->hash_chain, &pid_hash[type][pid_hashfn(nr)]);
+		hlist_add_head(&pid->hash_chain,
+				&pid_hash[type][pid_hashfn(nr)]);
 	}
 	list_add_tail(&task->pids[type].pid_chain, &pid->task_list);
 	task->pids[type].pidptr = pid;
@@ -200,7 +201,7 @@ static inline int __detach_pid(task_t *t
 		return 0;
 
 	nr = pid->nr;
-	list_del(&pid->hash_chain);
+	hlist_del(&pid->hash_chain);
 	put_task_struct(pid->task);
 
 	return nr;
@@ -282,9 +283,9 @@ void __init pidhash_init(void)
 	pidhash_shift = min(12, pidhash_shift);
 	pidhash_size = 1 << pidhash_shift;
 
-	printk("PID hash table entries: %d (order %d: %Zd bytes)\n",
+	printk("PID hash table entries: %d (order: %d, %Zd bytes)\n",
 		pidhash_size, pidhash_shift,
-		pidhash_size * sizeof(struct list_head));
+		PIDTYPE_MAX * pidhash_size * sizeof(struct hlist_head));
 
 	for (i = 0; i < PIDTYPE_MAX; i++) {
 		pid_hash[i] = alloc_bootmem(pidhash_size *
@@ -292,7 +293,7 @@ void __init pidhash_init(void)
 		if (!pid_hash[i])
 			panic("Could not alloc pidhash!\n");
 		for (j = 0; j < pidhash_size; j++)
-			INIT_LIST_HEAD(&pid_hash[i][j]);
+			INIT_HLIST_HEAD(&pid_hash[i][j]);
 	}
 #ifdef CONFIG_KGDB
 	kgdb_pid_init_done++;
_