sched: optimize effective_load()
s_i = S * rw_i / \Sum_j rw_j
-> \Sum_j rw_j = S * rw_i / s_i
-> s'_i = S * (rw_i + w) / (\Sum_j rw_j + w)
delta s = s' - s = S * (rw + w) / ((S * rw / s) + w)
= s * (S * (rw + w) / (S * rw + s * w) - 1)
a = S*(rw+w), b = S*rw + s*w
delta s = s * (a-b) / b
IOW, trade one divide for two multiplies
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Cc: Srivatsa Vaddagiri <vatsa@linux.vnet.ibm.com>
Cc: Mike Galbraith <efault@gmx.de>
Signed-off-by: Ingo Molnar <mingo@elte.hu>
diff --git a/kernel/sched_fair.c b/kernel/sched_fair.c
index a169444..0d197be 100644
--- a/kernel/sched_fair.c
+++ b/kernel/sched_fair.c
@@ -1082,16 +1082,16 @@
for_each_sched_entity(se) {
#define D(n) (likely(n) ? (n) : 1)
- long S, Srw, rw, s, sn;
+ long S, rw, s, a, b;
S = se->my_q->tg->shares;
s = se->my_q->shares;
rw = se->my_q->load.weight;
- Srw = S * rw / D(s);
- sn = S * (rw + wl) / D(Srw + wg);
+ a = S*(rw + wl);
+ b = S*rw + s*wg;
- wl = sn - s;
+ wl = s*(a-b)/D(b);
wg = 0;
#undef D
}