x86/smp: Don't ever patch back to UP if we unplug cpus

We still patch SMP instructions to UP variants if we boot with a
single CPU, but not at any other time.  In particular, not if we
unplug CPUs to return to a single cpu.

Paul McKenney points out:

 mean offline overhead is 6251/48=130.2 milliseconds.

 If I remove the alternatives_smp_switch() from the offline
 path [...] the mean offline overhead is 550/42=13.1 milliseconds

Basically, we're never going to get those 120ms back, and the
code is pretty messy.

We get rid of:

 1) The "smp-alt-once" boot option. It's actually "smp-alt-boot", the
    documentation is wrong. It's now the default.

 2) The skip_smp_alternatives flag used by suspend.

 3) arch_disable_nonboot_cpus_begin() and arch_disable_nonboot_cpus_end()
    which were only used to set this one flag.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Paul McKenney <paul.mckenney@us.ibm.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Peter Zijlstra <a.p.zijlstra@chello.nl>
Link: http://lkml.kernel.org/r/87vcgwwive.fsf@rustcorp.com.au
Signed-off-by: Ingo Molnar <mingo@kernel.org>

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index ad7e2e5..7aef334 100644 (file)
--- a/Documentation/kernel-parameters.txt
+++ b/Documentation/kernel-parameters.txt
@@ -2638,9 +2638,6 @@ bytes respectively. Such letter suffixes can also be entirely omitted.
        smart2=         [HW]
                        Format: <io1>[,<io2>[,...,<io8>]]
 
-       smp-alt-once    [X86-32,SMP] On a hotplug CPU system, only
-                       attempt to substitute SMP alternatives once at boot.
-
        smsc-ircc2.nopnp        [HW] Don't use PNP to discover SMC devices
        smsc-ircc2.ircc_cfg=    [HW] Device configuration I/O port
        smsc-ircc2.ircc_sir=    [HW] SIR base I/O port

diff --git a/arch/x86/include/asm/alternative.h b/arch/x86/include/asm/alternative.h
index 7078068..444704c 100644 (file)
--- a/arch/x86/include/asm/alternative.h
+++ b/arch/x86/include/asm/alternative.h
@@ -60,7 +60,7 @@ extern void alternatives_smp_module_add(struct module *mod, char *name,
                                        void *locks, void *locks_end,
                                        void *text, void *text_end);
 extern void alternatives_smp_module_del(struct module *mod);
-extern void alternatives_smp_switch(int smp);
+extern void alternatives_enable_smp(void);
 extern int alternatives_text_reserved(void *start, void *end);
 extern bool skip_smp_alternatives;
 #else
@@ -68,7 +68,7 @@ static inline void alternatives_smp_module_add(struct module *mod, char *name,
                                               void *locks, void *locks_end,
                                               void *text, void *text_end) {}
 static inline void alternatives_smp_module_del(struct module *mod) {}
-static inline void alternatives_smp_switch(int smp) {}
+static inline void alternatives_enable_smp(void) {}
 static inline int alternatives_text_reserved(void *start, void *end)
 {
        return 0;

diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
index afb7ff7..af1f326 100644 (file)
--- a/arch/x86/kernel/alternative.c
+++ b/arch/x86/kernel/alternative.c
@@ -23,19 +23,6 @@
 
 #define MAX_PATCH_LEN (255-1)
 
-#ifdef CONFIG_HOTPLUG_CPU
-static int smp_alt_once;
-
-static int __init bootonly(char *str)
-{
-       smp_alt_once = 1;
-       return 1;
-}
-__setup("smp-alt-boot", bootonly);
-#else
-#define smp_alt_once 1
-#endif
-
 static int __initdata_or_module debug_alternative;
 
 static int __init debug_alt(char *str)
@@ -326,9 +313,6 @@ static void alternatives_smp_unlock(const s32 *start, const s32 *end,
 {
        const s32 *poff;
 
-       if (noreplace_smp)
-               return;
-
        mutex_lock(&text_mutex);
        for (poff = start; poff < end; poff++) {
                u8 *ptr = (u8 *)poff + *poff;
@@ -359,7 +343,7 @@ struct smp_alt_module {
 };
 static LIST_HEAD(smp_alt_modules);
 static DEFINE_MUTEX(smp_alt);
-static int smp_mode = 1;       /* protected by smp_alt */
+static bool uniproc_patched = false;   /* protected by smp_alt */
 
 void __init_or_module alternatives_smp_module_add(struct module *mod,
                                                  char *name,
@@ -368,19 +352,18 @@ void __init_or_module alternatives_smp_module_add(struct module *mod,
 {
        struct smp_alt_module *smp;
 
-       if (noreplace_smp)
-               return;
+       mutex_lock(&smp_alt);
+       if (!uniproc_patched)
+               goto unlock;
 
-       if (smp_alt_once) {
-               if (boot_cpu_has(X86_FEATURE_UP))
-                       alternatives_smp_unlock(locks, locks_end,
-                                               text, text_end);
-               return;
-       }
+       if (num_possible_cpus() == 1)
+               /* Don't bother remembering, we'll never have to undo it. */
+               goto smp_unlock;
 
        smp = kzalloc(sizeof(*smp), GFP_KERNEL);
        if (NULL == smp)
-               return; /* we'll run the (safe but slow) SMP code then ... */
+               /* we'll run the (safe but slow) SMP code then ... */
+               goto unlock;
 
        smp->mod        = mod;
        smp->name       = name;
@@ -392,11 +375,10 @@ void __init_or_module alternatives_smp_module_add(struct module *mod,
                __func__, smp->locks, smp->locks_end,
                smp->text, smp->text_end, smp->name);
 
-       mutex_lock(&smp_alt);
        list_add_tail(&smp->next, &smp_alt_modules);
-       if (boot_cpu_has(X86_FEATURE_UP))
-               alternatives_smp_unlock(smp->locks, smp->locks_end,
-                                       smp->text, smp->text_end);
+smp_unlock:
+       alternatives_smp_unlock(locks, locks_end, text, text_end);
+unlock:
        mutex_unlock(&smp_alt);
 }
 
@@ -404,24 +386,18 @@ void __init_or_module alternatives_smp_module_del(struct module *mod)
 {
        struct smp_alt_module *item;
 
-       if (smp_alt_once || noreplace_smp)
-               return;
-
        mutex_lock(&smp_alt);
        list_for_each_entry(item, &smp_alt_modules, next) {
                if (mod != item->mod)
                        continue;
                list_del(&item->next);
-               mutex_unlock(&smp_alt);
-               DPRINTK("%s: %s\n", __func__, item->name);
                kfree(item);
-               return;
+               break;
        }
        mutex_unlock(&smp_alt);
 }
 
-bool skip_smp_alternatives;
-void alternatives_smp_switch(int smp)
+void alternatives_enable_smp(void)
 {
        struct smp_alt_module *mod;
 
@@ -436,34 +412,21 @@ void alternatives_smp_switch(int smp)
        pr_info("lockdep: fixing up alternatives\n");
 #endif
 
-       if (noreplace_smp || smp_alt_once || skip_smp_alternatives)
-               return;
-       BUG_ON(!smp && (num_online_cpus() > 1));
+       /* Why bother if there are no other CPUs? */
+       BUG_ON(num_possible_cpus() == 1);
 
        mutex_lock(&smp_alt);
 
-       /*
-        * 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) {
+       if (uniproc_patched) {
                pr_info("switching to SMP code\n");
+               BUG_ON(num_online_cpus() != 1);
                clear_cpu_cap(&boot_cpu_data, X86_FEATURE_UP);
                clear_cpu_cap(&cpu_data(0), X86_FEATURE_UP);
                list_for_each_entry(mod, &smp_alt_modules, next)
                        alternatives_smp_lock(mod->locks, mod->locks_end,
                                              mod->text, mod->text_end);
-       } else {
-               pr_info("switching to UP code\n");
-               set_cpu_cap(&boot_cpu_data, X86_FEATURE_UP);
-               set_cpu_cap(&cpu_data(0), X86_FEATURE_UP);
-               list_for_each_entry(mod, &smp_alt_modules, next)
-                       alternatives_smp_unlock(mod->locks, mod->locks_end,
-                                               mod->text, mod->text_end);
+               uniproc_patched = false;
        }
-       smp_mode = smp;
        mutex_unlock(&smp_alt);
 }
 
@@ -540,40 +503,22 @@ void __init alternative_instructions(void)
 
        apply_alternatives(__alt_instructions, __alt_instructions_end);
 
-       /* switch to patch-once-at-boottime-only mode and free the
-        * tables in case we know the number of CPUs will never ever
-        * change */
-#ifdef CONFIG_HOTPLUG_CPU
-       if (num_possible_cpus() < 2)
-               smp_alt_once = 1;
-#endif
-
 #ifdef CONFIG_SMP
-       if (smp_alt_once) {
-               if (1 == num_possible_cpus()) {
-                       pr_info("switching to UP code\n");
-                       set_cpu_cap(&boot_cpu_data, X86_FEATURE_UP);
-                       set_cpu_cap(&cpu_data(0), X86_FEATURE_UP);
-
-                       alternatives_smp_unlock(__smp_locks, __smp_locks_end,
-                                               _text, _etext);
-               }
-       } else {
+       /* Patch to UP if other cpus not imminent. */
+       if (!noreplace_smp && (num_present_cpus() == 1 || setup_max_cpus <= 1)) {
+               uniproc_patched = true;
                alternatives_smp_module_add(NULL, "core kernel",
                                            __smp_locks, __smp_locks_end,
                                            _text, _etext);
-
-               /* Only switch to UP mode if we don't immediately boot others */
-               if (num_present_cpus() == 1 || setup_max_cpus <= 1)
-                       alternatives_smp_switch(0);
        }
-#endif
-       apply_paravirt(__parainstructions, __parainstructions_end);
 
-       if (smp_alt_once)
+       if (!uniproc_patched || num_possible_cpus() == 1)
                free_init_pages("SMP alternatives",
                                (unsigned long)__smp_locks,
                                (unsigned long)__smp_locks_end);
+#endif
+
+       apply_paravirt(__parainstructions, __parainstructions_end);
 
        restart_nmi();
 }

diff --git a/arch/x86/kernel/smpboot.c b/arch/x86/kernel/smpboot.c
index 7c5a8c3..c80a33b 100644 (file)
--- a/arch/x86/kernel/smpboot.c
+++ b/arch/x86/kernel/smpboot.c
@@ -665,7 +665,8 @@ static int __cpuinit do_boot_cpu(int apicid, int cpu, struct task_struct *idle)
        unsigned long boot_error = 0;
        int timeout;
 
-       alternatives_smp_switch(1);
+       /* Just in case we booted with a single CPU. */
+       alternatives_enable_smp();
 
        idle->thread.sp = (unsigned long) (((struct pt_regs *)
                          (THREAD_SIZE +  task_stack_page(idle))) - 1);
@@ -1053,20 +1054,6 @@ out:
        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();
@@ -1256,9 +1243,6 @@ void native_cpu_die(unsigned int cpu)
                if (per_cpu(cpu_state, cpu) == CPU_DEAD) {
                        if (system_state == SYSTEM_RUNNING)
                                pr_info("CPU %u is now offline\n", cpu);
-
-                       if (1 == num_online_cpus())
-                               alternatives_smp_switch(0);
                        return;
                }
                msleep(100);

diff --git a/arch/x86/xen/smp.c b/arch/x86/xen/smp.c
index f58dca7..353c50f 100644 (file)
--- a/arch/x86/xen/smp.c
+++ b/arch/x86/xen/smp.c
@@ -377,7 +377,8 @@ static int __cpuinit xen_cpu_up(unsigned int cpu, struct task_struct *idle)
                return rc;
 
        if (num_online_cpus() == 1)
-               alternatives_smp_switch(1);
+               /* Just in case we booted with a single CPU. */
+               alternatives_enable_smp();
 
        rc = xen_smp_intr_init(cpu);
        if (rc)
@@ -424,9 +425,6 @@ static void xen_cpu_die(unsigned int cpu)
        unbind_from_irqhandler(per_cpu(xen_irq_work, cpu), NULL);
        xen_uninit_lock_cpu(cpu);
        xen_teardown_timer(cpu);
-
-       if (num_online_cpus() == 1)
-               alternatives_smp_switch(0);
 }
 
 static void __cpuinit xen_play_dead(void) /* used only with HOTPLUG_CPU */

diff --git a/kernel/cpu.c b/kernel/cpu.c
index 14d3258..f6bfe3e 100644 (file)
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -439,14 +439,6 @@ EXPORT_SYMBOL_GPL(cpu_up);
 #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;
@@ -458,7 +450,6 @@ int disable_nonboot_cpus(void)
         * 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) {
@@ -474,8 +465,6 @@ int disable_nonboot_cpus(void)
                }
        }
 
-       arch_disable_nonboot_cpus_end();
-
        if (!error) {
                BUG_ON(num_online_cpus() > 1);
                /* Make sure the CPUs won't be enabled by someone else */