linux-kernel - [PATCH 2/5] sched/fair: default to false in test_idle

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Message-Id: <20220712082036.5130-3-wuyun.abel@bytedance.com>
Date:   Tue, 12 Jul 2022 16:20:33 +0800
From:   Abel Wu <wuyun.abel@...edance.com>
To:     Peter Zijlstra <peterz@...radead.org>,
        Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>
Cc:     Josh Don <joshdon@...gle.com>, Chen Yu <yu.c.chen@...el.com>,
        linux-kernel@...r.kernel.org, Abel Wu <wuyun.abel@...edance.com>
Subject: [PATCH 2/5] sched/fair: default to false in test_idle_cores

It's uncertain whether idle cores exist or not if shared sched-
domains are not ready, so returning "no idle cores" usually
makes sense.

While __update_idle_core() is an exception, it checks status
of this core and set to shared sched-domain if necessary. So
the whole logic depends on the existence of sds, and can bail
out early if !sds.

It's somehow a little tricky, and as Josh suggested that it
should be transient while the domain isn't ready. So remove
the self-defined default value to make things more clearer,
while introduce little overhead in idle path.

Signed-off-by: Abel Wu <wuyun.abel@...edance.com>
Reviewed-by: Josh Don <joshdon@...gle.com>
---
 kernel/sched/fair.c | 16 ++++++++--------
 1 file changed, 8 insertions(+), 8 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index cd758b3616bd..c72093f282ec 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1592,11 +1592,11 @@ numa_type numa_classify(unsigned int imbalance_pct,
 
 #ifdef CONFIG_SCHED_SMT
 /* Forward declarations of select_idle_sibling helpers */
-static inline bool test_idle_cores(int cpu, bool def);
+static inline bool test_idle_cores(int cpu);
 static inline int numa_idle_core(int idle_core, int cpu)
 {
 	if (!static_branch_likely(&sched_smt_present) ||
-	    idle_core >= 0 || !test_idle_cores(cpu, false))
+	    idle_core >= 0 || !test_idle_cores(cpu))
 		return idle_core;
 
 	/*
@@ -6260,7 +6260,7 @@ static inline void set_idle_cores(int cpu, int val)
 		WRITE_ONCE(sds->has_idle_cores, val);
 }
 
-static inline bool test_idle_cores(int cpu, bool def)
+static inline bool test_idle_cores(int cpu)
 {
 	struct sched_domain_shared *sds;
 
@@ -6268,7 +6268,7 @@ static inline bool test_idle_cores(int cpu, bool def)
 	if (sds)
 		return READ_ONCE(sds->has_idle_cores);
 
-	return def;
+	return false;
 }
 
 /*
@@ -6284,7 +6284,7 @@ void __update_idle_core(struct rq *rq)
 	int cpu;
 
 	rcu_read_lock();
-	if (test_idle_cores(core, true))
+	if (test_idle_cores(core))
 		goto unlock;
 
 	for_each_cpu(cpu, cpu_smt_mask(core)) {
@@ -6360,9 +6360,9 @@ static inline void set_idle_cores(int cpu, int val)
 {
 }
 
-static inline bool test_idle_cores(int cpu, bool def)
+static inline bool test_idle_cores(int cpu)
 {
-	return def;
+	return false;
 }
 
 static inline int select_idle_core(struct task_struct *p, int core, struct cpumask *cpus, int *idle_cpu)
@@ -6604,7 +6604,7 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
 		return target;
 
 	if (sched_smt_active()) {
-		has_idle_core = test_idle_cores(target, false);
+		has_idle_core = test_idle_cores(target);
 
 		if (!has_idle_core && cpus_share_cache(prev, target)) {
 			i = select_idle_smt(p, sd, prev);
-- 
2.31.1