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authorAndi Kleen <ak@suse.de>2008-01-30 13:33:17 +0100
committerIngo Molnar <mingo@elte.hu>2008-01-30 13:33:17 +0100
commitca74a6f84e68b44867022f4a4f3ec17c087c864e (patch)
treea5e84b251b1574b09288fb2636b4e4ea088ae70e /arch/x86/kernel/alternative.c
parent751752789162fde69474edfa15935d0a77c0bc17 (diff)
downloadop-kernel-dev-ca74a6f84e68b44867022f4a4f3ec17c087c864e.zip
op-kernel-dev-ca74a6f84e68b44867022f4a4f3ec17c087c864e.tar.gz
x86: optimize lock prefix switching to run less frequently
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. v1->v2: Rename max_cpus v1->v2: Fix off by one in UP check (Thomas Gleixner) Signed-off-by: Andi Kleen <ak@suse.de> Signed-off-by: Ingo Molnar <mingo@elte.hu> Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Diffstat (limited to 'arch/x86/kernel/alternative.c')
-rw-r--r--arch/x86/kernel/alternative.c16
1 files changed, 14 insertions, 2 deletions
diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index cdc4324..318a4f9 100644
--- a/arch/x86/kernel/alternative.c
+++ b/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(void)
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 || setup_max_cpus <= 1)
+ alternatives_smp_switch(0);
}
#endif
apply_paravirt(__parainstructions, __parainstructions_end);
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