On Tue, Jan 22, 2013 at 01:04:54PM +0530, Srivatsa S. Bhat wrote:
> There are places where preempt_disable() or local_irq_disable() are used
> to prevent any CPU from going offline during the critical section. Let us
> call them as "atomic hotplug readers" ("atomic" because they run in atomic,
> non-preemptible contexts).
>
> Today, preempt_disable() or its equivalent works because the hotplug writer
> uses stop_machine() to take CPUs offline. But once stop_machine() is gone
> from the CPU hotplug offline path, the readers won't be able to prevent
> CPUs from going offline using preempt_disable().
>
> So the intent here is to provide synchronization APIs for such atomic hotplug
> readers, to prevent (any) CPUs from going offline, without depending on
> stop_machine() at the writer-side. The new APIs will look something like
> this: get_online_cpus_atomic() and put_online_cpus_atomic()
>
> Some important design requirements and considerations:
> -----------------------------------------------------
>
> 1. Scalable synchronization at the reader-side, especially in the fast-path
>
> Any synchronization at the atomic hotplug readers side must be highly
> scalable - avoid global single-holder locks/counters etc. Because, these
> paths currently use the extremely fast preempt_disable(); our replacement
> to preempt_disable() should not become ridiculously costly and also should
> not serialize the readers among themselves needlessly.
>
> At a minimum, the new APIs must be extremely fast at the reader side
> atleast in the fast-path, when no CPU offline writers are active.
>
> 2. preempt_disable() was recursive. The replacement should also be recursive.
>
> 3. No (new) lock-ordering restrictions
>
> preempt_disable() was super-flexible. It didn't impose any ordering
> restrictions or rules for nesting. Our replacement should also be equally
> flexible and usable.
>
> 4. No deadlock possibilities
>
> Regular per-cpu locking is not the way to go if we want to have relaxed
> rules for lock-ordering. Because, we can end up in circular-locking
> dependencies as explained in https://lkml.org/lkml/2012/12/6/290
>
> So, avoid the usual per-cpu locking schemes (per-cpu locks/per-cpu atomic
> counters with spin-on-contention etc) as much as possible, to avoid
> numerous deadlock possibilities from creeping in.
>
>
> Implementation of the design:
> ----------------------------
>
> We use per-CPU reader-writer locks for synchronization because:
>
> a. They are quite fast and scalable in the fast-path (when no writers are
> active), since they use fast per-cpu counters in those paths.
>
> b. They are recursive at the reader side.
>
> c. They provide a good amount of safety against deadlocks; they don't
> spring new deadlock possibilities on us from out of nowhere. As a
> result, they have relaxed locking rules and are quite flexible, and
> thus are best suited for replacing usages of preempt_disable() or
> local_irq_disable() at the reader side.
>
> Together, these satisfy all the requirements mentioned above.
>
> I'm indebted to Michael Wang and Xiao Guangrong for their numerous thoughtful
> suggestions and ideas, which inspired and influenced many of the decisions in
> this as well as previous designs. Thanks a lot Michael and Xiao!
>
> Cc: Russell King <linux@xxxxxxxxxxxxxxxx>
> Cc: Mike Frysinger <vapier@xxxxxxxxxx>
> Cc: Tony Luck <tony.luck@xxxxxxxxx>
> Cc: Ralf Baechle <ralf@xxxxxxxxxxxxxx>
> Cc: David Howells <dhowells@xxxxxxxxxx>
> Cc: "James E.J. Bottomley" <jejb@xxxxxxxxxxxxxxxx>
> Cc: Benjamin Herrenschmidt <benh@xxxxxxxxxxxxxxxxxxx>
> Cc: Martin Schwidefsky <schwidefsky@xxxxxxxxxx>
> Cc: Paul Mundt <lethal@xxxxxxxxxxxx>
> Cc: "David S. Miller" <davem@xxxxxxxxxxxxx>
> Cc: "H. Peter Anvin" <hpa@xxxxxxxxx>
> Cc: x86@xxxxxxxxxx
> Cc: linux-arm-kernel@xxxxxxxxxxxxxxxxxxx
> Cc: uclinux-dist-devel@xxxxxxxxxxxxxxxxxxxx
> Cc: linux-ia64@xxxxxxxxxxxxxxx
> Cc: linux-mips@xxxxxxxxxxxxxx
> Cc: linux-am33-list@xxxxxxxxxx
> Cc: linux-parisc@xxxxxxxxxxxxxxx
> Cc: linuxppc-dev@xxxxxxxxxxxxxxxx
> Cc: linux-s390@xxxxxxxxxxxxxxx
> Cc: linux-sh@xxxxxxxxxxxxxxx
> Cc: sparclinux@xxxxxxxxxxxxxxx
> Signed-off-by: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
With the change suggested by Namhyung:
Reviewed-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> ---
>
> arch/arm/Kconfig | 1 +
> arch/blackfin/Kconfig | 1 +
> arch/ia64/Kconfig | 1 +
> arch/mips/Kconfig | 1 +
> arch/mn10300/Kconfig | 1 +
> arch/parisc/Kconfig | 1 +
> arch/powerpc/Kconfig | 1 +
> arch/s390/Kconfig | 1 +
> arch/sh/Kconfig | 1 +
> arch/sparc/Kconfig | 1 +
> arch/x86/Kconfig | 1 +
> include/linux/cpu.h | 4 +++
> kernel/cpu.c | 57 ++++++++++++++++++++++++++++++++++++++++++++++---
> 13 files changed, 69 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> index 67874b8..cb6b94b 100644
> --- a/arch/arm/Kconfig
> +++ b/arch/arm/Kconfig
> @@ -1616,6 +1616,7 @@ config NR_CPUS
> config HOTPLUG_CPU
> bool "Support for hot-pluggable CPUs"
> depends on SMP && HOTPLUG
> + select PERCPU_RWLOCK
> help
> Say Y here to experiment with turning CPUs off and on. CPUs
> can be controlled through /sys/devices/system/cpu.
> diff --git a/arch/blackfin/Kconfig b/arch/blackfin/Kconfig
> index b6f3ad5..83d9882 100644
> --- a/arch/blackfin/Kconfig
> +++ b/arch/blackfin/Kconfig
> @@ -261,6 +261,7 @@ config NR_CPUS
> config HOTPLUG_CPU
> bool "Support for hot-pluggable CPUs"
> depends on SMP && HOTPLUG
> + select PERCPU_RWLOCK
> default y
>
> config BF_REV_MIN
> diff --git a/arch/ia64/Kconfig b/arch/ia64/Kconfig
> index 3279646..c246772 100644
> --- a/arch/ia64/Kconfig
> +++ b/arch/ia64/Kconfig
> @@ -378,6 +378,7 @@ config HOTPLUG_CPU
> bool "Support for hot-pluggable CPUs (EXPERIMENTAL)"
> depends on SMP && EXPERIMENTAL
> select HOTPLUG
> + select PERCPU_RWLOCK
> default n
> ---help---
> Say Y here to experiment with turning CPUs off and on. CPUs
> diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
> index 2ac626a..f97c479 100644
> --- a/arch/mips/Kconfig
> +++ b/arch/mips/Kconfig
> @@ -956,6 +956,7 @@ config SYS_HAS_EARLY_PRINTK
> config HOTPLUG_CPU
> bool "Support for hot-pluggable CPUs"
> depends on SMP && HOTPLUG && SYS_SUPPORTS_HOTPLUG_CPU
> + select PERCPU_RWLOCK
> help
> Say Y here to allow turning CPUs off and on. CPUs can be
> controlled through /sys/devices/system/cpu.
> diff --git a/arch/mn10300/Kconfig b/arch/mn10300/Kconfig
> index e70001c..a64e488 100644
> --- a/arch/mn10300/Kconfig
> +++ b/arch/mn10300/Kconfig
> @@ -60,6 +60,7 @@ config ARCH_HAS_ILOG2_U32
>
> config HOTPLUG_CPU
> def_bool n
> + select PERCPU_RWLOCK
>
> source "init/Kconfig"
>
> diff --git a/arch/parisc/Kconfig b/arch/parisc/Kconfig
> index b77feff..6f55cd4 100644
> --- a/arch/parisc/Kconfig
> +++ b/arch/parisc/Kconfig
> @@ -226,6 +226,7 @@ config HOTPLUG_CPU
> bool
> default y if SMP
> select HOTPLUG
> + select PERCPU_RWLOCK
>
> config ARCH_SELECT_MEMORY_MODEL
> def_bool y
> diff --git a/arch/powerpc/Kconfig b/arch/powerpc/Kconfig
> index 17903f1..56b1f15 100644
> --- a/arch/powerpc/Kconfig
> +++ b/arch/powerpc/Kconfig
> @@ -336,6 +336,7 @@ config HOTPLUG_CPU
> bool "Support for enabling/disabling CPUs"
> depends on SMP && HOTPLUG && EXPERIMENTAL && (PPC_PSERIES || \
> PPC_PMAC || PPC_POWERNV || (PPC_85xx && !PPC_E500MC))
> + select PERCPU_RWLOCK
> ---help---
> Say Y here to be able to disable and re-enable individual
> CPUs at runtime on SMP machines.
> diff --git a/arch/s390/Kconfig b/arch/s390/Kconfig
> index b5ea38c..a9aafb4 100644
> --- a/arch/s390/Kconfig
> +++ b/arch/s390/Kconfig
> @@ -299,6 +299,7 @@ config HOTPLUG_CPU
> prompt "Support for hot-pluggable CPUs"
> depends on SMP
> select HOTPLUG
> + select PERCPU_RWLOCK
> help
> Say Y here to be able to turn CPUs off and on. CPUs
> can be controlled through /sys/devices/system/cpu/cpu#.
> diff --git a/arch/sh/Kconfig b/arch/sh/Kconfig
> index babc2b8..8c92eef 100644
> --- a/arch/sh/Kconfig
> +++ b/arch/sh/Kconfig
> @@ -765,6 +765,7 @@ config NR_CPUS
> config HOTPLUG_CPU
> bool "Support for hot-pluggable CPUs (EXPERIMENTAL)"
> depends on SMP && HOTPLUG && EXPERIMENTAL
> + select PERCPU_RWLOCK
> help
> Say Y here to experiment with turning CPUs off and on. CPUs
> can be controlled through /sys/devices/system/cpu.
> diff --git a/arch/sparc/Kconfig b/arch/sparc/Kconfig
> index 9f2edb5..e2bd573 100644
> --- a/arch/sparc/Kconfig
> +++ b/arch/sparc/Kconfig
> @@ -253,6 +253,7 @@ config HOTPLUG_CPU
> bool "Support for hot-pluggable CPUs"
> depends on SPARC64 && SMP
> select HOTPLUG
> + select PERCPU_RWLOCK
> help
> Say Y here to experiment with turning CPUs off and on. CPUs
> can be controlled through /sys/devices/system/cpu/cpu#.
> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> index 79795af..a225d12 100644
> --- a/arch/x86/Kconfig
> +++ b/arch/x86/Kconfig
> @@ -1689,6 +1689,7 @@ config PHYSICAL_ALIGN
> config HOTPLUG_CPU
> bool "Support for hot-pluggable CPUs"
> depends on SMP && HOTPLUG
> + select PERCPU_RWLOCK
> ---help---
> Say Y here to allow turning CPUs off and on. CPUs can be
> controlled through /sys/devices/system/cpu.
> diff --git a/include/linux/cpu.h b/include/linux/cpu.h
> index ce7a074..cf24da1 100644
> --- a/include/linux/cpu.h
> +++ b/include/linux/cpu.h
> @@ -175,6 +175,8 @@ extern struct bus_type cpu_subsys;
>
> extern void get_online_cpus(void);
> extern void put_online_cpus(void);
> +extern void get_online_cpus_atomic(void);
> +extern void put_online_cpus_atomic(void);
> #define hotcpu_notifier(fn, pri) cpu_notifier(fn, pri)
> #define register_hotcpu_notifier(nb) register_cpu_notifier(nb)
> #define unregister_hotcpu_notifier(nb) unregister_cpu_notifier(nb)
> @@ -198,6 +200,8 @@ static inline void cpu_hotplug_driver_unlock(void)
>
> #define get_online_cpus() do { } while (0)
> #define put_online_cpus() do { } while (0)
> +#define get_online_cpus_atomic() do { } while (0)
> +#define put_online_cpus_atomic() do { } while (0)
> #define hotcpu_notifier(fn, pri) do { (void)(fn); } while (0)
> /* These aren't inline functions due to a GCC bug. */
> #define register_hotcpu_notifier(nb) ({ (void)(nb); 0; })
> diff --git a/kernel/cpu.c b/kernel/cpu.c
> index 3046a50..1c84138 100644
> --- a/kernel/cpu.c
> +++ b/kernel/cpu.c
> @@ -1,6 +1,18 @@
> /* CPU control.
> * (C) 2001, 2002, 2003, 2004 Rusty Russell
> *
> + * Rework of the CPU hotplug offline mechanism to remove its dependence on
> + * the heavy-weight stop_machine() primitive, by Srivatsa S. Bhat and
> + * Paul E. McKenney.
> + *
> + * Copyright (C) IBM Corporation, 2012-2013
> + * Authors: Srivatsa S. Bhat <srivatsa.bhat@xxxxxxxxxxxxxxxxxx>
> + * Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
> + *
> + * With lots of invaluable suggestions from:
> + * Oleg Nesterov <oleg@xxxxxxxxxx>
> + * Tejun Heo <tj@xxxxxxxxxx>
> + *
> * This code is licenced under the GPL.
> */
> #include <linux/proc_fs.h>
> @@ -19,6 +31,7 @@
> #include <linux/mutex.h>
> #include <linux/gfp.h>
> #include <linux/suspend.h>
> +#include <linux/percpu-rwlock.h>
>
> #include "smpboot.h"
>
> @@ -133,6 +146,38 @@ static void cpu_hotplug_done(void)
> mutex_unlock(&cpu_hotplug.lock);
> }
>
> +/*
> + * Per-CPU Reader-Writer lock to synchronize between atomic hotplug
> + * readers and the CPU offline hotplug writer.
> + */
> +DEFINE_STATIC_PERCPU_RWLOCK(hotplug_pcpu_rwlock);
> +
> +/*
> + * Invoked by atomic hotplug reader (a task which wants to prevent
> + * CPU offline, but which can't afford to sleep), to prevent CPUs from
> + * going offline. So, you can call this function from atomic contexts
> + * (including interrupt handlers).
> + *
> + * Note: This does NOT prevent CPUs from coming online! It only prevents
> + * CPUs from going offline.
> + *
> + * You can call this function recursively.
> + *
> + * Returns with preemption disabled (but interrupts remain as they are;
> + * they are not disabled).
> + */
> +void get_online_cpus_atomic(void)
> +{
> + percpu_read_lock_irqsafe(&hotplug_pcpu_rwlock);
> +}
> +EXPORT_SYMBOL_GPL(get_online_cpus_atomic);
> +
> +void put_online_cpus_atomic(void)
> +{
> + percpu_read_unlock_irqsafe(&hotplug_pcpu_rwlock);
> +}
> +EXPORT_SYMBOL_GPL(put_online_cpus_atomic);
> +
> #else /* #if CONFIG_HOTPLUG_CPU */
> static void cpu_hotplug_begin(void) {}
> static void cpu_hotplug_done(void) {}
> @@ -246,15 +291,21 @@ struct take_cpu_down_param {
> static int __ref take_cpu_down(void *_param)
> {
> struct take_cpu_down_param *param = _param;
> - int err;
> + unsigned long flags;
> + int err = 0;
> +
> + percpu_write_lock_irqsave(&hotplug_pcpu_rwlock, &flags);
>
> /* Ensure this CPU doesn't handle any more interrupts. */
> err = __cpu_disable();
> if (err < 0)
> - return err;
> + goto out;
>
> cpu_notify(CPU_DYING | param->mod, param->hcpu);
> - return 0;
> +
> +out:
> + percpu_write_unlock_irqrestore(&hotplug_pcpu_rwlock, &flags);
> + return err;
> }
>
> /* Requires cpu_add_remove_lock to be held */
>
--
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