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 fe365c9a08e98392d17a0f54ac1c7209db5f90b2..5ba96d9ddbde5f4e06def3bcf392795b9432e9cb 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5774,6 +5774,18 @@ 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) {
@@ -5871,22 +5883,6 @@ 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();
@@ -5908,8 +5904,6 @@ 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 2f0a0be4d344d7de76211b01c6883c10fbb2ba89..9c219f7b097086d952c42eb7f8640e4f894ef4e5 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6237,6 +6237,7 @@ 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)
 {