linux-kernel - [PATCH v4 1/7] sched/fair: default to false in test_idle

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Date:   Sun, 19 Jun 2022 20:04:45 +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>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        K Prateek Nayak <kprateek.nayak@....com>,
        "Gautham R . Shenoy" <gautham.shenoy@....com>,
        linux-kernel@...r.kernel.org, Abel Wu <wuyun.abel@...edance.com>
Subject: [PATCH v4 1/7] 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 shared domain, and can bail out early if
it isn't available. Modern compilers seems capable of handling such
cases, so remove the tricky self-defined default return value.

Signed-off-by: Abel Wu <wuyun.abel@...edance.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 8bed75757e65..e5e8453e3ffb 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -1595,11 +1595,11 @@ 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;
 
 	/*
@@ -6206,7 +6206,7 @@ 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;
 
@@ -6214,7 +6214,7 @@ static inline bool test_idle_cores(int cpu, bool def)
 	if (sds)
 		return READ_ONCE(sds->has_idle_cores);
 
-	return def;
+	return false;
 }
 
 /*
@@ -6230,7 +6230,7 @@ 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)) {
@@ -6306,9 +6306,9 @@ 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)
@@ -6535,7 +6535,7 @@ 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