Name: Don't Check for Dead Tasks on Every Schedule
Status: Booted on 2.6.7-rc2-bk3
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

Currently the scheduler checks if we just unscheduled a dead task, and
if so, it does the (probably final) put_task_struct.

We can remove this check by using a "one-behind" cache in exit.c: upon
exit, we free put_task_struct the task from the last exit.  In
addition, we prime the cache with the init_task, so we never have to
check if it's empty.

diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .18428-linux-2.6.7-rc2-bk3/init/main.c .18428-linux-2.6.7-rc2-bk3.updated/init/main.c
--- .18428-linux-2.6.7-rc2-bk3/init/main.c	2004-05-31 09:57:40.000000000 +1000
+++ .18428-linux-2.6.7-rc2-bk3.updated/init/main.c	2004-06-03 10:41:18.000000000 +1000
@@ -91,6 +91,7 @@ extern void free_initmem(void);
 extern void populate_rootfs(void);
 extern void driver_init(void);
 extern void prepare_namespace(void);
+extern void exit_init(void);
 
 #ifdef CONFIG_TC
 extern void tc_init(void);
@@ -414,6 +415,9 @@ asmlinkage void __init start_kernel(void
 	 */
 	smp_prepare_boot_cpu();
 
+	/* Set up exit task cache. */
+	exit_init();
+
 	/*
 	 * Set up the scheduler prior starting any interrupts (such as the
 	 * timer interrupt). Full topology setup happens at smp_init()
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .18428-linux-2.6.7-rc2-bk3/kernel/exit.c .18428-linux-2.6.7-rc2-bk3.updated/kernel/exit.c
--- .18428-linux-2.6.7-rc2-bk3/kernel/exit.c	2004-05-31 09:57:40.000000000 +1000
+++ .18428-linux-2.6.7-rc2-bk3.updated/kernel/exit.c	2004-06-03 10:41:18.000000000 +1000
@@ -23,6 +23,7 @@
 #include <linux/mount.h>
 #include <linux/proc_fs.h>
 #include <linux/mempolicy.h>
+#include <linux/percpu.h>
 
 #include <asm/uaccess.h>
 #include <asm/unistd.h>
@@ -33,6 +34,7 @@ extern void sem_exit (void);
 extern struct task_struct *child_reaper;
 
 int getrusage(struct task_struct *, int, struct rusage __user *);
+static DEFINE_PER_CPU(struct task_struct *, last_exit);
 
 static void __unhash_process(struct task_struct *p)
 {
@@ -751,6 +753,12 @@ static void exit_notify(struct task_stru
 	_raw_write_unlock(&tasklist_lock);
 	local_irq_enable();
 
+	/* We free up the previous one at this point, to ensure we
+	 * never get freed while we're exiting.  Preemption stays
+	 * disabled until after we've scheduled away. */
+	put_task_struct(__get_cpu_var(last_exit));
+	__get_cpu_var(last_exit) = tsk;
+	
 	/* If the process is dead, release it - nobody will wait for it */
 	if (state == TASK_DEAD)
 		release_task(tsk);
@@ -1163,6 +1171,17 @@ end_wait4:
 	return retval;
 }
 
+void __init exit_init(void)
+{
+	unsigned int i;
+
+	/* All this to save a branch in the exit path... */
+	for (i = 0; i < NR_CPUS; i++) {
+		get_task_struct(current);
+		per_cpu(last_exit, i) = current;
+	}
+}
+
 #ifdef __ARCH_WANT_SYS_WAITPID
 
 /*
diff -urpN --exclude TAGS -X /home/rusty/devel/kernel/kernel-patches/current-dontdiff --minimal .18428-linux-2.6.7-rc2-bk3/kernel/sched.c .18428-linux-2.6.7-rc2-bk3.updated/kernel/sched.c
--- .18428-linux-2.6.7-rc2-bk3/kernel/sched.c	2004-06-03 08:14:09.000000000 +1000
+++ .18428-linux-2.6.7-rc2-bk3.updated/kernel/sched.c	2004-06-03 10:41:47.000000000 +1000
@@ -1003,27 +1003,12 @@ static void finish_task_switch(task_t *p
 {
 	runqueue_t *rq = this_rq();
 	struct mm_struct *mm = rq->prev_mm;
-	unsigned long prev_task_flags;
 
 	rq->prev_mm = NULL;
 
-	/*
-	 * A task struct has one reference for the use as "current".
-	 * If a task dies, then it sets TASK_ZOMBIE in tsk->state and calls
-	 * schedule one last time. The schedule call will never return,
-	 * and the scheduled task must drop that reference.
-	 * The test for TASK_ZOMBIE must occur while the runqueue locks are
-	 * still held, otherwise prev could be scheduled on another cpu, die
-	 * there before we look at prev->state, and then the reference would
-	 * be dropped twice.
-	 *		Manfred Spraul <manfred@colorfullife.com>
-	 */
-	prev_task_flags = prev->flags;
 	finish_arch_switch(rq, prev);
 	if (mm)
 		mmdrop(mm);
-	if (unlikely(prev_task_flags & PF_DEAD))
-		put_task_struct(prev);
 }
 
 /**