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 <[email protected]>
include/linux/sched.h | 1 +
kernel/sched.c | 16 ++++++++++++++++
kernel/timer.c | 8 +++++++-
3 files changed, 24 insertions(+), 1 deletion(-)
Index: linux/include/linux/sched.h
===================================================================
--- linux.orig/include/linux/sched.h 2006-03-16 16:17:55.982340489 -0600
+++ linux/include/linux/sched.h 2006-03-16 16:26:01.137096980 -0600
@@ -98,6 +98,7 @@ extern int last_pid;
DECLARE_PER_CPU(unsigned long, process_counts);
extern int nr_processes(void);
extern unsigned long nr_running(void);
+extern unsigned long nr_active(void);
extern unsigned long nr_uninterruptible(void);
extern unsigned long nr_iowait(void);
Index: linux/kernel/sched.c
===================================================================
--- linux.orig/kernel/sched.c 2006-03-16 16:17:56.376832371 -0600
+++ linux/kernel/sched.c 2006-03-16 16:26:01.141002841 -0600
@@ -1655,6 +1655,22 @@ 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
/*
Index: linux/kernel/timer.c
===================================================================
--- linux.orig/kernel/timer.c 2006-03-16 16:17:56.385620556 -0600
+++ linux/kernel/timer.c 2006-03-16 16:26:01.141979306 -0600
@@ -849,7 +849,7 @@ 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;
}
/*
-
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