sched/smt: Update sched_smt_present at runtime

commit ba2591a5993eabcc8e874e30f361d8ffbb10d6d4 upstream

The static key sched_smt_present is only updated at boot time when SMT
siblings have been detected. Booting with maxcpus=1 and bringing the
siblings online after boot rebuilds the scheduling domains correctly but
does not update the static key, so the SMT code is not enabled.

Let the key be updated in the scheduler CPU hotplug code to fix this.

Signed-off-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 211890edf37e48feefea9f3f1166a3ba2c58270b..ec945451b9ef16561586e3709046b890556f49dc 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5788,6 +5788,18 @@ int sched_cpu_activate(unsigned int cpu)
        struct rq *rq = cpu_rq(cpu);
        struct rq_flags rf;
 
+#ifdef CONFIG_SCHED_SMT
+       /*
+        * The sched_smt_present static key needs to be evaluated on every
+        * hotplug event because at boot time SMT might be disabled when
+        * the number of booted CPUs is limited.
+        *
+        * If then later a sibling gets hotplugged, then the key would stay
+        * off and SMT scheduling would never be functional.
+        */
+       if (cpumask_weight(cpu_smt_mask(cpu)) > 1)
+               static_branch_enable_cpuslocked(&sched_smt_present);
+#endif
        set_cpu_active(cpu, true);
 
        if (sched_smp_initialized) {
@@ -5885,22 +5897,6 @@ int sched_cpu_dying(unsigned int cpu)
 }
 #endif
 
-#ifdef CONFIG_SCHED_SMT
-DEFINE_STATIC_KEY_FALSE(sched_smt_present);
-
-static void sched_init_smt(void)
-{
-       /*
-        * We've enumerated all CPUs and will assume that if any CPU
-        * has SMT siblings, CPU0 will too.
-        */
-       if (cpumask_weight(cpu_smt_mask(0)) > 1)
-               static_branch_enable(&sched_smt_present);
-}
-#else
-static inline void sched_init_smt(void) { }
-#endif
-
 void __init sched_init_smp(void)
 {
        sched_init_numa();
@@ -5922,8 +5918,6 @@ void __init sched_init_smp(void)
        init_sched_rt_class();
        init_sched_dl_class();
 
-       sched_init_smt();
-
        sched_smp_initialized = true;
 }
 

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 79f574dba09657316468174e478ea9dcda7b1230..183068d2284931712aa25188ec74d36a24796a92 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6183,6 +6183,7 @@ static inline int find_idlest_cpu(struct sched_domain *sd, struct task_struct *p
 }
 
 #ifdef CONFIG_SCHED_SMT
+DEFINE_STATIC_KEY_FALSE(sched_smt_present);
 
 static inline void set_idle_cores(int cpu, int val)
 {