linux-kernel - [RFC PATCH 3/4] sched/topology: remove smt

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Date:   Wed, 29 Aug 2018 15:19:11 +0200
From:   Vincent Guittot <vincent.guittot@...aro.org>
To:     peterz@...radead.org, mingo@...hat.com,
        linux-kernel@...r.kernel.org
Cc:     Vincent Guittot <vincent.guittot@...aro.org>
Subject: [RFC PATCH 3/4] sched/topology: remove smt_gain

smt_gain is used to compute the capacity of CPUs of a SMT core with the
constraint 1 < smt_gain < 2 in order to be able to compute number of CPUs
per core. The field has_free_capacity of struct numa_stat, which was the
last user of this computation of number of CPUs per core, has been removed
by :

  commit 2d4056fafa19 ("sched/numa: Remove numa_has_capacity()")

We can now remove this constraint on core capacity and use the defautl value
SCHED_CAPACITY_SCALE for SMT CPUs. With this remove, SCHED_CAPACITY_SCALE
becomes the maximum compute capacity of CPUs on every systems. This should
help to simplify some code and remove fields like rd->max_cpu_capacity

Furthermore, arch_scale_cpu_capacity() is used with a NULL sd in several other
places in the code when it wants the capacity of a CPUs to scale
some metrics like in pelt, deadline or schedutil. In case on SMT, the value
returned is not the capacity of SMT CPUs but default SCHED_CAPACITY_SCALE.

Remove the smt_gain field from sched_domain struct

Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Ingo Molnar <mingo@...hat.com>
Cc: linux-kernel@...r.kernel.org (open list)
Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
---
 include/linux/sched/topology.h | 1 -
 kernel/sched/sched.h           | 3 ---
 kernel/sched/topology.c        | 2 --
 3 files changed, 6 deletions(-)

diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
index 2634774..212792b 100644
--- a/include/linux/sched/topology.h
+++ b/include/linux/sched/topology.h
@@ -89,7 +89,6 @@ struct sched_domain {
 	unsigned int newidle_idx;
 	unsigned int wake_idx;
 	unsigned int forkexec_idx;
-	unsigned int smt_gain;
 
 	int nohz_idle;			/* NOHZ IDLE status */
 	int flags;			/* See SD_* */
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 4a2e8ca..b1715b8 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1758,9 +1758,6 @@ unsigned long arch_scale_freq_capacity(int cpu)
 static __always_inline
 unsigned long arch_scale_cpu_capacity(struct sched_domain *sd, int cpu)
 {
-	if (sd && (sd->flags & SD_SHARE_CPUCAPACITY) && (sd->span_weight > 1))
-		return sd->smt_gain / sd->span_weight;
-
 	return SCHED_CAPACITY_SCALE;
 }
 #endif
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index 56a0fed..069c924 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -1129,7 +1129,6 @@ sd_init(struct sched_domain_topology_level *tl,
 
 		.last_balance		= jiffies,
 		.balance_interval	= sd_weight,
-		.smt_gain		= 0,
 		.max_newidle_lb_cost	= 0,
 		.next_decay_max_lb_cost	= jiffies,
 		.child			= child,
@@ -1155,7 +1154,6 @@ sd_init(struct sched_domain_topology_level *tl,
 	if (sd->flags & SD_SHARE_CPUCAPACITY) {
 		sd->flags |= SD_PREFER_SIBLING;
 		sd->imbalance_pct = 110;
-		sd->smt_gain = 1178; /* ~15% */
 
 	} else if (sd->flags & SD_SHARE_PKG_RESOURCES) {
 		sd->flags |= SD_PREFER_SIBLING;
-- 
2.7.4