Re: [PATCH v2] x86: don't ever patch back to UP if we unplug cpus.

From: Rusty Russell
Date: Thu Aug 23 2012 - 02:53:58 EST

On Wed, 22 Aug 2012 11:41:53 +0200, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Rusty Russell <rusty@xxxxxxxxxxxxxxx> wrote:
>
> > 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@xxxxxxxxxxxxxxx>
> > ---
> > Documentation/kernel-parameters.txt | 3 -
> > arch/x86/include/asm/alternative.h | 4 -
> > arch/x86/kernel/alternative.c | 104 +++++++-----------------------------
> > arch/x86/kernel/smpboot.c | 20 ------
> > kernel/cpu.c | 11 ---
> > 5 files changed, 27 insertions(+), 115 deletions(-)
>
> and this version breaks the build on allyesconfig-x86-64:
>
> arch/x86/xen/smp.c:380:3: error: implicit declaration of function âalternatives_smp_switchâ [-Werror=implicit-function-declaration]

Thanks, grepped harder. Xen fixed now, thanks.

Cheers,
Rusty.

Subject: 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@xxxxxxxxxxxxxxx>
Cc: Paul McKenney <paul.mckenney@xxxxxxxxxx>
Cc: Suresh Siddha <suresh.b.siddha@xxxxxxxxx>
Cc: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <a.p.zijlstra@xxxxxxxxx>
Signed-off-by: Ingo Molnar <mingo@xxxxxxxxxx>

diff --git a/Documentation/kernel-parameters.txt b/Documentation/kernel-parameters.txt
index ad7e2e5..7aef334 100644
--- 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
--- 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 ced4534..357475a 100644
--- 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
--- 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
--- 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 e615dfb..f560598 100644
--- a/kernel/cpu.c
+++ b/kernel/cpu.c
@@ -447,14 +447,6 @@ 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;
@@ -466,7 +458,6 @@ 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) {
@@ -482,8 +473,6 @@ 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 */
--
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