Re: [PATCH 1/3] pidhash: don't count idle threads

[Oleg Nesterov <oleg@tv-sign.ru>]

Oleg Nesterov wrote:
>
> I think we can just skip attach_pid() calls and job control inits
> for idle threads and kill unhash_process().

This means that fork_idle() should call REMOVE_LINKS(), updated
patch below.

[PATCH 1/3] 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().

Note: it is still possible to have PIDTYPE_PGID/PIDTYPE_SID entries
with nr == 0 (forked from init thread, including kernel threads
spawned from init/main.c:init()), this is ok because we never call
free_pidmap() with pid == 0 arg, so it will never be reused.

Question: why does unhash_process() call proc_pid_unhash()? Is not it
a bug to have ->proc_dentry != NULL after fork_idle()->copy_process()?

Signed-off-by: Oleg Nesterov <oleg@tv-sign.ru>

--- RC-1/kernel/pid.c~NOIPID	2006-01-19 18:13:07.000000000 +0300
+++ RC-1/kernel/pid.c	2006-01-30 19:50:33.000000000 +0300
@@ -277,16 +277,8 @@ void __init pidhash_init(void)
 
 void __init pidmap_init(void)
 {
-	int i;
-
 	pidmap_array->page = (void *)get_zeroed_page(GFP_KERNEL);
+	/* Reserve PID 0 */
 	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);
 }
--- RC-1/kernel/fork.c~NOIPID	2006-01-19 18:13:07.000000000 +0300
+++ RC-1/kernel/fork.c	2006-01-30 19:52:59.000000000 +0300
@@ -1135,19 +1135,20 @@ static task_t *copy_process(unsigned lon
 	if (unlikely(p->ptrace & PT_PTRACED))
 		__ptrace_link(p, current->parent);
 
-	attach_pid(p, PIDTYPE_PID, p->pid);
-	attach_pid(p, PIDTYPE_TGID, p->tgid);
-	if (thread_group_leader(p)) {
-		p->signal->tty = current->signal->tty;
-		p->signal->pgrp = process_group(current);
-		p->signal->session = current->signal->session;
-		attach_pid(p, PIDTYPE_PGID, process_group(p));
-		attach_pid(p, PIDTYPE_SID, p->signal->session);
-		if (p->pid)
+	if (p->pid) {
+		attach_pid(p, PIDTYPE_PID, p->pid);
+		attach_pid(p, PIDTYPE_TGID, p->tgid);
+		if (thread_group_leader(p)) {
+			p->signal->tty = current->signal->tty;
+			p->signal->pgrp = process_group(current);
+			p->signal->session = current->signal->session;
+			attach_pid(p, PIDTYPE_PGID, process_group(p));
+			attach_pid(p, PIDTYPE_SID, p->signal->session);
 			__get_cpu_var(process_counts)++;
+		}
+		nr_threads++;
 	}
 
-	nr_threads++;
 	total_forks++;
 	write_unlock_irq(&tasklist_lock);
 	proc_fork_connector(p);
@@ -1209,8 +1210,10 @@ task_t * __devinit fork_idle(int cpu)
 	task = copy_process(CLONE_VM, 0, idle_regs(&regs), 0, NULL, NULL, 0);
 	if (!task)
 		return ERR_PTR(-ENOMEM);
+
+	REMOVE_LINKS(task);
 	init_idle(task, cpu);
-	unhash_process(task);
+
 	return task;
 }
 
--- RC-1/include/linux/sched.h~NOIPID	2006-01-19 18:13:07.000000000 +0300
+++ RC-1/include/linux/sched.h	2006-01-29 23:35:44.000000000 +0300
@@ -1217,8 +1217,6 @@ static inline int thread_group_empty(tas
 #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
--- RC-1/kernel/exit.c~NOIPID	2006-01-19 18:13:07.000000000 +0300
+++ RC-1/kernel/exit.c	2006-01-29 23:30:02.000000000 +0300
@@ -52,8 +52,7 @@ static void __unhash_process(struct task
 	if (thread_group_leader(p)) {
 		detach_pid(p, PIDTYPE_PGID);
 		detach_pid(p, PIDTYPE_SID);
-		if (p->pid)
-			__get_cpu_var(process_counts)--;
+		__get_cpu_var(process_counts)--;
 	}
 
 	REMOVE_LINKS(p);
@@ -114,21 +113,6 @@ 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
--- RC-1/arch/um/kernel/smp.c~NOIPID	2005-03-19 14:16:40.000000000 +0300
+++ RC-1/arch/um/kernel/smp.c	2006-01-29 23:42:32.000000000 +0300
@@ -143,7 +143,6 @@ void smp_prepare_cpus(unsigned int maxcp
 		idle = idle_thread(cpu);
 
 		init_idle(idle, cpu);
-		unhash_process(idle);
 
 		waittime = 200000000;
 		while (waittime-- && !cpu_isset(cpu, cpu_callin_map))
-
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