[PATCH] pidhash: don't count idle threads

fork_idle() does unhash_process() just after copy_process().  Contrary,
boot_cpu's idle thread explicitely registers itself for each pid_type with nr
= 0.

copy_process() already checks p->pid != 0 before process_counts++, I think we
can just skip attach_pid() calls and job control inits for idle threads and
kill unhash_process().  We don't need to cleanup ->proc_dentry in fork_idle()
because with this patch idle threads are never hashed in
kernel/pid.c:pid_hash[].

We don't need to hash pid == 0 in pidmap_init().  free_pidmap() is never
called with pid == 0 arg, so it will never be reused.  So it is still possible
to use pid == 0 in any PIDTYPE_xxx namespace from kernel/pid.c's POV.

However with this patch we don't hash pid == 0 for PIDTYPE_PID case.  We still
have have PIDTYPE_PGID/PIDTYPE_SID entries with pid == 0: /sbin/init and
kernel threads which don't call daemonize().
Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>
Cc: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

arch/um/kernel/smp.c

@@ -143,7 +143,6 @@ void smp_prepare_cpus(unsigned int maxcpus)
 		idle = idle_thread(cpu);
 
 		init_idle(idle, cpu);
-		unhash_process(idle);
 
 		waittime = 200000000;
 		while (waittime-- && !cpu_isset(cpu, cpu_callin_map))

include/linux/sched.h

@@ -1214,8 +1214,6 @@ static inline int thread_group_empty(task_t *p)
 #define delay_group_leader(p) \
 		(thread_group_leader(p) && !thread_group_empty(p))
 
-extern void unhash_process(struct task_struct *p);
-
 /*
  * Protects ->fs, ->files, ->mm, ->ptrace, ->group_info, ->comm, keyring
  * subscriptions and synchronises with wait4().  Also used in procfs.  Also

kernel/exit.c

@@ -56,7 +56,6 @@ static void __unhash_process(struct task_struct *p)
 		detach_pid(p, PIDTYPE_SID);
 
 		list_del_init(&p->tasks);
-		if (p->pid)
 		__get_cpu_var(process_counts)--;
 	}
 
@@ -118,21 +117,6 @@ repeat:
 		goto repeat;
 }
 
-/* we are using it only for SMP init */
-
-void unhash_process(struct task_struct *p)
-{
-	struct dentry *proc_dentry;
-
-	spin_lock(&p->proc_lock);
-	proc_dentry = proc_pid_unhash(p);
-	write_lock_irq(&tasklist_lock);
-	__unhash_process(p);
-	write_unlock_irq(&tasklist_lock);
-	spin_unlock(&p->proc_lock);
-	proc_pid_flush(proc_dentry);
-}
-
 /*
  * This checks not only the pgrp, but falls back on the pid if no
  * satisfactory pgrp is found. I dunno - gdb doesn't work correctly

kernel/fork.c

@@ -1181,6 +1181,7 @@ static task_t *copy_process(unsigned long clone_flags,
 	 */
 	p->ioprio = current->ioprio;
 
+	if (likely(p->pid)) {
 		add_parent(p);
 		if (unlikely(p->ptrace & PT_PTRACED))
 			__ptrace_link(p, current->parent);
@@ -1193,13 +1194,13 @@ static task_t *copy_process(unsigned long clone_flags,
 			attach_pid(p, PIDTYPE_SID, p->signal->session);
 
 			list_add_tail(&p->tasks, &init_task.tasks);
-		if (p->pid)
 			__get_cpu_var(process_counts)++;
 		}
 		attach_pid(p, PIDTYPE_TGID, p->tgid);
 		attach_pid(p, PIDTYPE_PID, p->pid);
-
 		nr_threads++;
+	}
+
 	total_forks++;
 	spin_unlock(&current->sighand->siglock);
 	write_unlock_irq(&tasklist_lock);
@@ -1263,7 +1264,7 @@ task_t * __devinit fork_idle(int cpu)
 	if (!task)
 		return ERR_PTR(-ENOMEM);
 	init_idle(task, cpu);
-	unhash_process(task);
+
 	return task;
 }
 

kernel/pid.c

@@ -247,16 +247,8 @@ void __init pidhash_init(void)
 
 void __init pidmap_init(void)
 {
-	int i;
-
 	pidmap_array->page = (void *)get_zeroed_page(GFP_KERNEL);
+	/* Reserve PID 0. We never call free_pidmap(0) */
 	set_bit(0, pidmap_array->page);
 	atomic_dec(&pidmap_array->nr_free);
-
-	/*
-	 * Allocate PID 0, and hash it via all PID types:
-	 */
-
-	for (i = 0; i < PIDTYPE_MAX; i++)
-		attach_pid(current, i, 0);
 }