sched: add smt_gain

The idea is that multi-threading a core yields more work capacity than
a single thread, provide a way to express a static gain for threads.

[ dino: backport to 31-rt ]
Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Dinakar Guniguntala <dino@in.ibm.com>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Darren Hart <dvhltc@us.ibm.com>
Cc: John Kacur <jkacur@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

include/linux/sched.h

@@ -966,6 +966,7 @@ struct sched_domain {
 	unsigned int newidle_idx;
 	unsigned int wake_idx;
 	unsigned int forkexec_idx;
+	unsigned int smt_gain;
 	int flags;			/* See SD_* */
 	enum sched_domain_level level;
 

include/linux/topology.h

@@ -99,6 +99,7 @@ int arch_update_cpu_topology(void);
 				| SD_SHARE_CPUPOWER,	\
 	.last_balance		= jiffies,		\
 	.balance_interval	= 1,			\
+	.smt_gain		= 1178,	/* 15% */	\
 }
 #endif
 #endif /* CONFIG_SCHED_SMT */

kernel/sched.c

@@ -8729,9 +8729,15 @@ static void init_sched_groups_power(int cpu, struct sched_domain *sd)
 		weight = cpumask_weight(sched_domain_span(sd));
 		/*
 		 * SMT siblings share the power of a single core.
+		 * Usually multiple threads get a better yield out of
+		 * that one core than a single thread would have,
+		 * reflect that in sd->smt_gain.
 		 */
-		if ((sd->flags & SD_SHARE_CPUPOWER) && weight > 1)
+		if ((sd->flags & SD_SHARE_CPUPOWER) && weight > 1) {
+			power *= sd->smt_gain;
 			power /= weight;
+			power >>= SCHED_LOAD_SHIFT;
+		}
 		sg_inc_cpu_power(sd->groups, power);
 		return;
 	}