[PATCH 2/2] powerpc: implement arch_scale_smt_power for Power7

Joel Schopp
2010-01-20T20:04:14Z

On Power7 processors running in SMT4 mode with 2, 3, or 4 idle threads
there is performance benefit to idling the higher numbered threads in
the core.

This patch implements arch_scale_smt_power to dynamically update smt
thread power in these idle cases in order to prefer threads 0,1 over
threads 2,3 within a core.

Signed-off-by: Joel Schopp <jschopp@austin.ibm.com>
---
Index: linux-2.6.git/arch/powerpc/kernel/smp.c
===================================================================
--- linux-2.6.git.orig/arch/powerpc/kernel/smp.c
+++ linux-2.6.git/arch/powerpc/kernel/smp.c
@@ -617,3 +617,44 @@ void __cpu_die(unsigned int cpu)
smp_ops->cpu_die(cpu);
}
#endif
+
+static inline int thread_in_smt4core(int x)
+{
+ return x % 4;
+}
+unsigned long arch_scale_smt_power(struct sched_domain *sd, int cpu)
+{
+ int cpu2;
+ int idle_count = 0;
+
+ struct cpumask *cpu_map = sched_domain_span(sd);
+
+ unsigned long weight = cpumask_weight(cpu_map);
+ unsigned long smt_gain = sd->smt_gain;
+
+ if (cpu_has_feature(CPU_FTRS_POWER7) && weight == 4) {
+ for_each_cpu(cpu2, cpu_map) {
+ if (idle_cpu(cpu2))
+ idle_count++;
+ }
+
+ /* the following section attempts to tweak cpu power based
+ * on current idleness of the threads dynamically at runtime
+ */
+ if (idle_count == 2 || idle_count == 3 || idle_count == 4) {
+ if (thread_in_smt4core(cpu) == 0 ||
+ thread_in_smt4core(cpu) == 1) {
+ /* add 75 % to thread power */
+ smt_gain += (smt_gain >> 1) + (smt_gain >> 2);
+ } else {
+ /* subtract 75 % to thread power */
+ smt_gain = smt_gain >> 2;
+ }
+ }
+ }
+ /* default smt gain is 1178, weight is # of SMT threads */
+ smt_gain /= weight;
+
+ return smt_gain;
+
+}
Index: linux-2.6.git/kernel/sched_features.h
===================================================================
--- linux-2.6.git.orig/kernel/sched_features.h
+++ linux-2.6.git/kernel/sched_features.h
@@ -107,7 +107,7 @@ SCHED_FEAT(CACHE_HOT_BUDDY, 1)
/*
* Use arch dependent cpu power functions
*/
-SCHED_FEAT(ARCH_POWER, 0)
+SCHED_FEAT(ARCH_POWER, 1)

SCHED_FEAT(HRTICK, 0)
SCHED_FEAT(DOUBLE_TICK, 0)