From ca74a6f84e68b44867022f4a4f3ec17c087c864e Mon Sep 17 00:00:00 2001 From: Andi Kleen Date: Wed, 30 Jan 2008 13:33:17 +0100 Subject: [PATCH] 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 Signed-off-by: Ingo Molnar Signed-off-by: Thomas Gleixner --- arch/x86/kernel/alternative.c | 16 ++++++++++++++-- include/linux/smp.h | 2 ++ init/main.c | 16 ++++++++-------- 3 files changed, 24 insertions(+), 10 deletions(-) diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c index cdc43242da92..318a4f9b7ece 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); diff --git a/include/linux/smp.h b/include/linux/smp.h index c25e66bcecf3..55232ccf9cfd 100644 --- a/include/linux/smp.h +++ b/include/linux/smp.h @@ -78,6 +78,8 @@ int on_each_cpu(void (*func) (void *info), void *info, int retry, int wait); */ void smp_prepare_boot_cpu(void); +extern unsigned int setup_max_cpus; + #else /* !SMP */ /* diff --git a/init/main.c b/init/main.c index 3f8aba291ed3..5843fe996703 100644 --- a/init/main.c +++ b/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 setup_max_cpus = NR_CPUS; /* * Setup routine for controlling SMP activation @@ -146,7 +146,7 @@ static inline void disable_ioapic_setup(void) {}; static int __init nosmp(char *str) { - max_cpus = 0; + setup_max_cpus = 0; disable_ioapic_setup(); return 0; } @@ -155,8 +155,8 @@ early_param("nosmp", nosmp); static int __init maxcpus(char *str) { - get_option(&str, &max_cpus); - if (max_cpus == 0) + get_option(&str, &setup_max_cpus); + if (setup_max_cpus == 0) disable_ioapic_setup(); return 0; @@ -164,7 +164,7 @@ static int __init maxcpus(char *str) early_param("maxcpus", maxcpus); #else -#define max_cpus NR_CPUS +#define setup_max_cpus NR_CPUS #endif /* @@ -393,7 +393,7 @@ static void __init smp_init(void) /* FIXME: This should be done in userspace --RR */ for_each_present_cpu(cpu) { - if (num_online_cpus() >= max_cpus) + if (num_online_cpus() >= setup_max_cpus) break; if (!cpu_online(cpu)) cpu_up(cpu); @@ -401,7 +401,7 @@ static void __init smp_init(void) /* Any cleanup work */ printk(KERN_INFO "Brought up %ld CPUs\n", (long)num_online_cpus()); - smp_cpus_done(max_cpus); + smp_cpus_done(setup_max_cpus); } #endif @@ -824,7 +824,7 @@ static int __init kernel_init(void * unused) __set_special_pids(1, 1); cad_pid = task_pid(current); - smp_prepare_cpus(max_cpus); + smp_prepare_cpus(setup_max_cpus); do_pre_smp_initcalls(); -- 2.20.1