[PATCH x86] [12/16] Optimize lock prefix switching to run less frequently

[Andi Kleen <ak@suse.de>]

On VMs implemented using JITs that cache translated code changing the lock 
prefixes is a quite costly operation that forces the JIT to throw away and 
retranslate a lot of code. 

Previously a SMP kernel would rewrite the locks once for each CPU which
is quite unnecessary. This patch changes the code to never switch at boot in
the normal case (SMP kernel booting with >1 CPU) or only once for SMP kernel 
on UP.

This makes a significant difference in boot up performance on AMD SimNow!
Also I expect it to be a little faster on native systems too because a smp 
switch does a lot of text_poke()s which each synchronize the pipeline.

Signed-off-by: Andi Kleen <ak@suse.de>

---
 arch/x86/kernel/alternative.c |   16 ++++++++++++++--
 include/linux/smp.h           |    2 ++
 init/main.c                   |    2 +-
 3 files changed, 17 insertions(+), 3 deletions(-)

Index: linux/arch/x86/kernel/alternative.c
===================================================================
--- linux.orig/arch/x86/kernel/alternative.c
+++ linux/arch/x86/kernel/alternative.c
@@ -273,6 +273,7 @@ struct smp_alt_module {
 };
 static LIST_HEAD(smp_alt_modules);
 static DEFINE_SPINLOCK(smp_alt);
+static int smp_mode = 1;	/* protected by smp_alt */
 
 void alternatives_smp_module_add(struct module *mod, char *name,
 				 void *locks, void *locks_end,
@@ -354,7 +355,14 @@ void alternatives_smp_switch(int smp)
 	BUG_ON(!smp && (num_online_cpus() > 1));
 
 	spin_lock_irqsave(&smp_alt, flags);
-	if (smp) {
+
+	/*
+ 	 * Avoid unnecessary switches because it forces JIT based VMs to
+	 * throw away all cached translations, which can be quite costly.
+	 */
+	if (smp == smp_mode) {
+		/* nothing */
+	} else if (smp) {
 		printk(KERN_INFO "SMP alternatives: switching to SMP code\n");
 		clear_cpu_cap(&boot_cpu_data, X86_FEATURE_UP);
 		clear_cpu_cap(&cpu_data(0), X86_FEATURE_UP);
@@ -369,6 +377,7 @@ void alternatives_smp_switch(int smp)
 			alternatives_smp_unlock(mod->locks, mod->locks_end,
 						mod->text, mod->text_end);
 	}
+	smp_mode = smp;
 	spin_unlock_irqrestore(&smp_alt, flags);
 }
 
@@ -441,7 +450,10 @@ void __init alternative_instructions(voi
 		alternatives_smp_module_add(NULL, "core kernel",
 					    __smp_locks, __smp_locks_end,
 					    _text, _etext);
-		alternatives_smp_switch(0);
+
+		/* Only switch to UP mode if we don't immediately boot others */
+		if (num_possible_cpus() == 1 || max_cpus == 0)
+			alternatives_smp_switch(0);
 	}
 #endif
  	apply_paravirt(__parainstructions, __parainstructions_end);
Index: linux/include/linux/smp.h
===================================================================
--- linux.orig/include/linux/smp.h
+++ linux/include/linux/smp.h
@@ -78,6 +78,8 @@ int on_each_cpu(void (*func) (void *info
  */
 void smp_prepare_boot_cpu(void);
 
+extern unsigned int max_cpus;
+
 #else /* !SMP */
 
 /*
Index: linux/init/main.c
===================================================================
--- linux.orig/init/main.c
+++ linux/init/main.c
@@ -128,7 +128,7 @@ static char *ramdisk_execute_command;
 
 #ifdef CONFIG_SMP
 /* Setup configured maximum number of CPUs to activate */
-static unsigned int __initdata max_cpus = NR_CPUS;
+unsigned int __initdata max_cpus = NR_CPUS;
 
 /*
  * Setup routine for controlling SMP activation