sched: remove precise CPU load

CPU load calculations are statistical anyway, and there's little benefit
from having it calculated on every scheduling event. So remove this code,
it gets rid of a divide from the scheduler wakeup and context-switch
fastpath.

Signed-off-by: Ingo Molnar <mingo@elte.hu>
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Mike Galbraith <efault@gmx.de>
Reviewed-by: Thomas Gleixner <tglx@linutronix.de>

3 files changed, 9 insertions, 41 deletions

diff --git a/kernel/sched.c b/kernel/sched.c
index d4dabfcc776c..25cc9b2a8c15 100644
--- a/kernel/sched.c
+++ b/kernel/sched.c
@@ -1972,42 +1972,11 @@ unsigned long nr_active(void)
  */
 static void update_cpu_load(struct rq *this_rq)
 {
-        u64 fair_delta64, exec_delta64, idle_delta64, sample_interval64, tmp64;
         unsigned long total_load = this_rq->ls.load.weight;
         unsigned long this_load =  total_load;
-        struct load_stat *ls = &this_rq->ls;
         int i, scale;
 
         this_rq->nr_load_updates++;
-        if (unlikely(!(sysctl_sched_features & SCHED_FEAT_PRECISE_CPU_LOAD)))
-                goto do_avg;
-
-        /* Update delta_fair/delta_exec fields first */
-        update_curr_load(this_rq);
-
-        fair_delta64 = ls->delta_fair + 1;
-        ls->delta_fair = 0;
-
-        exec_delta64 = ls->delta_exec + 1;
-        ls->delta_exec = 0;
-
-        sample_interval64 = this_rq->clock - ls->load_update_last;
-        ls->load_update_last = this_rq->clock;
-
-        if ((s64)sample_interval64 < (s64)TICK_NSEC)
-                sample_interval64 = TICK_NSEC;
-
-        if (exec_delta64 > sample_interval64)
-                exec_delta64 = sample_interval64;
-
-        idle_delta64 = sample_interval64 - exec_delta64;
-
-        tmp64 = div64_64(SCHED_LOAD_SCALE * exec_delta64, fair_delta64);
-        tmp64 = div64_64(tmp64 * exec_delta64, sample_interval64);
-
-        this_load = (unsigned long)tmp64;
-
-do_avg:
 
         /* Update our load: */
         for (i = 0, scale = 1; i < CPU_LOAD_IDX_MAX; i++, scale += scale) {
@@ -2017,7 +1986,13 @@ do_avg:
 
                 old_load = this_rq->cpu_load[i];
                 new_load = this_load;
-
+                /*
+                 * Round up the averaging division if load is increasing. This
+                 * prevents us from getting stuck on 9 if the load is 10, for
+                 * example.
+                 */
+                if (new_load > old_load)
+                        new_load += scale-1;
                 this_rq->cpu_load[i] = (old_load*(scale-1) + new_load) >> i;
         }
 }
@@ -6484,7 +6459,6 @@ static inline void init_cfs_rq(struct cfs_rq *cfs_rq, struct rq *rq)
 
 void __init sched_init(void)
 {
-        u64 now = sched_clock();
         int highest_cpu = 0;
         int i, j;
 
@@ -6509,8 +6483,6 @@ void __init sched_init(void)
                 INIT_LIST_HEAD(&rq->leaf_cfs_rq_list);
                 list_add(&rq->cfs.leaf_cfs_rq_list, &rq->leaf_cfs_rq_list);
 #endif
-                rq->ls.load_update_last = now;
-                rq->ls.load_update_start = now;
 
                 for (j = 0; j < CPU_LOAD_IDX_MAX; j++)
                         rq->cpu_load[j] = 0;
diff --git a/kernel/sched_debug.c b/kernel/sched_debug.c
index fd080f686f18..6b789dae7fdf 100644
--- a/kernel/sched_debug.c
+++ b/kernel/sched_debug.c
@@ -145,8 +145,6 @@ static void print_cpu(struct seq_file *m, int cpu)
         P(nr_running);
         SEQ_printf(m, "  .%-30s: %lu\n", "load",
                    rq->ls.load.weight);
-        P(ls.delta_fair);
-        P(ls.delta_exec);
         P(nr_switches);
         P(nr_load_updates);
         P(nr_uninterruptible);
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index 2138c40f4836..105d57b41aa2 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -94,16 +94,14 @@ enum {
         SCHED_FEAT_FAIR_SLEEPERS        = 1,
         SCHED_FEAT_SLEEPER_AVG          = 2,
         SCHED_FEAT_SLEEPER_LOAD_AVG     = 4,
-        SCHED_FEAT_PRECISE_CPU_LOAD     = 8,
-        SCHED_FEAT_START_DEBIT          = 16,
-        SCHED_FEAT_SKIP_INITIAL         = 32,
+        SCHED_FEAT_START_DEBIT          = 8,
+        SCHED_FEAT_SKIP_INITIAL         = 16,
 };
 
 const_debug unsigned int sysctl_sched_features =
                 SCHED_FEAT_FAIR_SLEEPERS        *1 |
                 SCHED_FEAT_SLEEPER_AVG          *0 |
                 SCHED_FEAT_SLEEPER_LOAD_AVG     *1 |
-                SCHED_FEAT_PRECISE_CPU_LOAD     *1 |
                 SCHED_FEAT_START_DEBIT          *1 |
                 SCHED_FEAT_SKIP_INITIAL         *0;