[PATCH] sched: reduce overhead of calc_load

Currently, count_active_tasks() calls both nr_running() &
nr_interruptible().  Each of these functions does a "for_each_cpu" & reads
values from the runqueue of each cpu.  Although this is not a lot of
instructions, each runqueue may be located on different node.  Depending on
the architecture, a unique TLB entry may be required to access each
runqueue.

Since there may be more runqueues than cpu TLB entries, a scan of all
runqueues can trash the TLB.  Each memory reference incurs a TLB miss &
refill.

In addition, the runqueue cacheline that contains nr_running &
nr_uninterruptible may be evicted from the cache between the two passes.
This causes unnecessary cache misses.

Combining nr_running() & nr_interruptible() into a single function
substantially reduces the TLB & cache misses on large systems.  This should
have no measureable effect on smaller systems.

On a 128p IA64 system running a memory stress workload, the new function
reduced the overhead of calc_load() from 605 usec/call to 324 usec/call.

Signed-off-by: Jack Steiner <steiner@sgi.com>
Acked-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

3 files changed, 17 insertions, 1 deletions

diff --git a/include/linux/sched.h b/include/linux/sched.h
index d04186d8cc68..ab84adf5bb9a 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -100,6 +100,7 @@ DECLARE_PER_CPU(unsigned long, process_counts);
 extern int nr_processes(void);
 extern unsigned long nr_running(void);
 extern unsigned long nr_uninterruptible(void);
+extern unsigned long nr_active(void);
 extern unsigned long nr_iowait(void);
 
 #include <linux/time.h>
diff --git a/kernel/sched.c b/kernel/sched.c
index a9ecac398bb9..6e52e0adff80 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1658,6 +1658,21 @@ unsigned long nr_iowait(void)
         return sum;
 }
 
+unsigned long nr_active(void)
+{
+        unsigned long i, running = 0, uninterruptible = 0;
+
+        for_each_online_cpu(i) {
+                running += cpu_rq(i)->nr_running;
+                uninterruptible += cpu_rq(i)->nr_uninterruptible;
+        }
+
+        if (unlikely((long)uninterruptible < 0))
+                uninterruptible = 0;
+
+        return running + uninterruptible;
+}
+
 #ifdef CONFIG_SMP
 
 /*
diff --git a/kernel/timer.c b/kernel/timer.c
index 9062a82ee8ec..6b812c04737b 100644
--- a/kernel/timer.c
+++ b/kernel/timer.c
@@ -825,7 +825,7 @@ void update_process_times(int user_tick)
  */
 static unsigned long count_active_tasks(void)
 {
-        return (nr_running() + nr_uninterruptible()) * FIXED_1;
+        return nr_active() * FIXED_1;
 }
 
 /*