[PATCH] shrink task_struct by removing per_cpu utime and stime

Patch from Bill Irwin.  It has the potential to break userspace
monitoring tools a little bit, and I'm a rater uncertain about
how useful the per-process per-cpu accounting is.

Bill sent this out as an RFC on July 29:

"These statistics severely bloat the task_struct and nothing in
 userspace can rely on them as they're conditional on CONFIG_SMP.  If
 anyone is using them (or just wants them around), please speak up."

And nobody spoke up.

If we apply this, the contents of /proc/783/cpu will go from

	cpu  1 1
	cpu0 0 0
	cpu1 0 0
	cpu2 1 1
	cpu3 0 0

to

	cpu  1 1

And we shall save 256 bytes from the ia32 task_struct.

On my SMP build with NR_CPUS=32:

Without this patch, sizeof(task_struct) is 1824, slab uses a 1-order
allocation and we are getting 2 task_structs per page.

With this patch, sizeof(task_struct) is 1568, slab uses a 2-order
allocation and we are getting 2.5 task_structs per page.

So it seems worthwhile.

(Maybe this highlights a shortcoming in slab.  For the 1824-byte case
it could have used a 0-order allocation)

2 files changed, 0 insertions, 11 deletions

diff --git a/kernel/fork.c b/kernel/fork.c
index 27cf572cedf8..9cc9718f4e39 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -795,15 +795,6 @@ static struct task_struct *copy_process(unsigned long clone_flags,
 	p->tty_old_pgrp = 0;
 	p->utime = p->stime = 0;
 	p->cutime = p->cstime = 0;
-#ifdef CONFIG_SMP
-	{
-		int i;
-
-		/* ?? should we just memset this ?? */
-		for(i = 0; i < NR_CPUS; i++)
-			p->per_cpu_utime[i] = p->per_cpu_stime[i] = 0;
-	}
-#endif
 	p->array = NULL;
 	p->lock_depth = -1;		/* -1 = no lock */
 	p->start_time = jiffies;
diff --git a/kernel/timer.c b/kernel/timer.c
index 19e4296c72bd..0b9913aad53b 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -694,8 +694,6 @@ static inline void do_it_prof(struct task_struct *p)
 void update_one_process(struct task_struct *p, unsigned long user,
 			unsigned long system, int cpu)
 {
-	p->per_cpu_utime[cpu] += user;
-	p->per_cpu_stime[cpu] += system;
 	do_process_times(p, user, system);
 	do_it_virt(p, user);
 	do_it_prof(p);