x86, suspend: Avoid unnecessary smp alternatives switch during suspend/resume

During suspend, we disable all the non boot cpus. And during resume we bring
them all back again. So no need to do alternatives_smp_switch() in between.

On my core 2 based laptop, this speeds up the suspend path by 15msec and the
resume path by 5 msec (suspend/resume speed up differences can be attributed
to the different P-states that the cpu is in during suspend/resume).

Signed-off-by: Suresh Siddha <[e-mail]>
LKML-Reference: <[e-mail]>
Cc: Rafael J. Wysocki <[e-mail]>
Signed-off-by: H. Peter Anvin <[e-mail]>

arch/x86/include/asm/alternative.h

···
 extern void alternatives_smp_module_del(struct module *mod);
 extern void alternatives_smp_switch(int smp);
 extern int alternatives_text_reserved(void *start, void *end);
+extern bool skip_smp_alternatives;
 #else
 static inline void alternatives_smp_module_add(struct module *mod, char *name,
 					       void *locks, void *locks_end,

arch/x86/kernel/alternative.c

···
 	mutex_unlock(&smp_alt);
 }
 
+bool skip_smp_alternatives;
 void alternatives_smp_switch(int smp)
 {
 	struct smp_alt_module *mod;
···
 	printk("lockdep: fixing up alternatives.\n");
 #endif
 
-	if (noreplace_smp || smp_alt_once)
+	if (noreplace_smp || smp_alt_once || skip_smp_alternatives)
 		return;
 	BUG_ON(!smp && (num_online_cpus() > 1));
 

arch/x86/kernel/smpboot.c

···
 	preempt_enable();
 }
 
+void arch_disable_nonboot_cpus_begin(void)
+{
+	/*
+	 * Avoid the smp alternatives switch during the disable_nonboot_cpus().
+	 * In the suspend path, we will be back in the SMP mode shortly anyways.
+	 */
+	skip_smp_alternatives = true;
+}
+
+void arch_disable_nonboot_cpus_end(void)
+{
+	skip_smp_alternatives = false;
+}
+
 void arch_enable_nonboot_cpus_begin(void)
 {
 	set_mtrr_aps_delayed_init();

kernel/cpu.c

···
 #ifdef CONFIG_PM_SLEEP_SMP
 static cpumask_var_t frozen_cpus;
 
+void __weak arch_disable_nonboot_cpus_begin(void)
+{
+}
+
+void __weak arch_disable_nonboot_cpus_end(void)
+{
+}
+
 int disable_nonboot_cpus(void)
 {
 	int cpu, first_cpu, error = 0;
···
 	 * with the userspace trying to use the CPU hotplug at the same time
 	 */
 	cpumask_clear(frozen_cpus);
+	arch_disable_nonboot_cpus_begin();
 
 	printk("Disabling non-boot CPUs ...\n");
 	for_each_online_cpu(cpu) {
···
 			break;
 		}
 	}
+
+	arch_disable_nonboot_cpus_end();
 
 	if (!error) {
 		BUG_ON(num_online_cpus() > 1);