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Date: Mon, 22 Mar 2010 16:04:25 -0400
From: Steven Rostedt <rostedt@...dmis.org>
To: Jason Baron <jbaron@...hat.com>
Cc: linux-kernel@...r.kernel.org, mingo@...e.hu,
mathieu.desnoyers@...ymtl.ca, hpa@...or.com, tglx@...utronix.de,
andi@...stfloor.org, roland@...hat.com, rth@...hat.com,
mhiramat@...hat.com, fweisbec@...il.com
Subject: Re: [PATCH 2/5] jump label: base patch
On Mon, 2010-03-22 at 12:07 -0400, Jason Baron wrote:
> base patch to implement 'jump labeling'. Based on a new 'asm goto' inline
> assembly gcc mechanism, we can now branch to labels from an 'asm goto'
> statment. This allows us to create a 'no-op' fastpath, which can subsequently
> be patched with a jump to the slowpath code. This is useful for code which
> might be rarely used, but which we'd like to be able to call, if needed.
> Tracepoints are the current usecase that these are being implemented for.
>
> Signed-off-by: Jason Baron <jbaron@...hat.com>
> ---
> include/asm-generic/vmlinux.lds.h | 10 ++-
> include/linux/jump_label.h | 57 +++++++++++++
> kernel/Makefile | 2 +-
> kernel/jump_label.c | 165 +++++++++++++++++++++++++++++++++++++
> 4 files changed, 232 insertions(+), 2 deletions(-)
> create mode 100644 include/linux/jump_label.h
> create mode 100644 kernel/jump_label.c
>
> diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
> index 67e6520..83a469d 100644
> --- a/include/asm-generic/vmlinux.lds.h
> +++ b/include/asm-generic/vmlinux.lds.h
> @@ -167,7 +167,8 @@
> BRANCH_PROFILE() \
> TRACE_PRINTKS() \
> FTRACE_EVENTS() \
> - TRACE_SYSCALLS()
> + TRACE_SYSCALLS() \
> + JUMP_TABLE() \
>
> /*
> * Data section helpers
> @@ -206,6 +207,7 @@
> *(__vermagic) /* Kernel version magic */ \
> *(__markers_strings) /* Markers: strings */ \
> *(__tracepoints_strings)/* Tracepoints: strings */ \
> + *(__jump_strings)/* Jump: strings */ \
> } \
> \
> .rodata1 : AT(ADDR(.rodata1) - LOAD_OFFSET) { \
> @@ -557,6 +559,12 @@
> #define BUG_TABLE
> #endif
>
> +#define JUMP_TABLE() \
> + . = ALIGN(64); \
> + VMLINUX_SYMBOL(__start___jump_table) = .; \
> + *(__jump_table) \
> + VMLINUX_SYMBOL(__stop___jump_table) = .; \
> +
> #ifdef CONFIG_PM_TRACE
> #define TRACEDATA \
> . = ALIGN(4); \
> diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
> new file mode 100644
> index 0000000..3d42e8c
> --- /dev/null
> +++ b/include/linux/jump_label.h
> @@ -0,0 +1,57 @@
> +#ifndef _LINUX_JUMP_LABEL_H
> +#define _LINUX_JUMP_LABEL_H
> +
> +#include <asm/jump_label.h>
> +
> +struct jump_entry {
> + unsigned long code;
> + unsigned long target;
> + char *name;
> +};
> +
> +enum jump_label_type {
> + JUMP_LABEL_ENABLE,
> + JUMP_LABEL_DISABLE
> +};
> +
> +#ifdef __HAVE_ARCH_JUMP_LABEL
> +
> +extern struct jump_entry __start___jump_table[];
> +extern struct jump_entry __stop___jump_table[];
> +
> +#define DEFINE_JUMP_LABEL(name) \
> + const char __jlstrtab_##name[] \
> + __used __attribute__((section("__jump_strings"))) = #name;
> +
> +extern void arch_jump_label_transform(struct jump_entry *entry,
> + enum jump_label_type type);
> +
> +extern int jump_label_update(const char *name, enum jump_label_type type);
> +
> +#define enable_jump_label(name) \
> + jump_label_update(name, JUMP_LABEL_ENABLE);
> +
> +#define disable_jump_label(name) \
> + jump_label_update(name, JUMP_LABEL_DISABLE);
> +
> +#else
> +
> +#define DEFINE_JUMP_LABEL(name)
> +
> +#define JUMP_LABEL(tag, label, cond) \
> + if (unlikely(cond)) \
> + goto label;
> +
> +static inline int enable_jump_label(const char *name)
> +{
> + return 0;
> +}
> +
> +static inline int disable_jump_label(const char *name)
> +{
> + return 0;
> +}
> +
> +#endif
> +
> +#endif
> diff --git a/kernel/Makefile b/kernel/Makefile
> index d5c3006..59ff12e 100644
> --- a/kernel/Makefile
> +++ b/kernel/Makefile
> @@ -10,7 +10,7 @@ obj-y = sched.o fork.o exec_domain.o panic.o printk.o \
> kthread.o wait.o kfifo.o sys_ni.o posix-cpu-timers.o mutex.o \
> hrtimer.o rwsem.o nsproxy.o srcu.o semaphore.o \
> notifier.o ksysfs.o pm_qos_params.o sched_clock.o cred.o \
> - async.o range.o
> + async.o range.o jump_label.o
> obj-$(CONFIG_HAVE_EARLY_RES) += early_res.o
> obj-y += groups.o
>
> diff --git a/kernel/jump_label.c b/kernel/jump_label.c
> new file mode 100644
> index 0000000..671fcbb
> --- /dev/null
> +++ b/kernel/jump_label.c
> @@ -0,0 +1,165 @@
> +/*
> + * jump label support
> + *
> + * Copyright (C) 2009 Jason Baron <jbaron@...hat.com>
> + *
> + */
> +#include <linux/jump_label.h>
> +#include <linux/memory.h>
> +#include <linux/uaccess.h>
> +#include <linux/module.h>
> +#include <asm/alternative.h>
> +#include <linux/list.h>
> +#include <linux/jhash.h>
> +
> +#ifdef __HAVE_ARCH_JUMP_LABEL
> +
> +#define JUMP_LABEL_HASH_BITS 6
> +#define JUMP_LABEL_TABLE_SIZE (1 << JUMP_LABEL_HASH_BITS)
> +static struct hlist_head jump_label_table[JUMP_LABEL_TABLE_SIZE];
> +
> +/* mutex to protect coming/going of the the jump_label table */
> +static DEFINE_MUTEX(jump_label_mutex);
> +
> +struct jump_label_entry {
> + struct hlist_node hlist;
> + struct jump_entry *table;
> + int nr_entries;
> + /* hang modules off here */
> + struct hlist_head modules;
> + char name[0];
> +};
> +
> +static void swap_jump_label_entries(struct jump_entry *previous, struct jump_entry *next)
> +{
> + struct jump_entry tmp;
> +
> + tmp = *next;
> + *next = *previous;
> + *previous = tmp;
> +}
> +
> +static void sort_jump_label_entries(struct jump_entry *start, struct jump_entry *stop)
> +{
> + int swapped = 0;
> + struct jump_entry *iter;
> + struct jump_entry *iter_next;
> +
> + do {
> + swapped = 0;
> + iter = start;
> + iter_next = start;
> + iter_next++;
> + for (; iter_next < stop; iter++, iter_next++) {
> + if (strcmp(iter->name, iter_next->name) > 0) {
> + swap_jump_label_entries(iter, iter_next);
> + swapped = 1;
> + }
> + }
> + } while (swapped == 1);
I'm curious to how long this takes with a few hundred trace points
(which we currently have). Perhaps this could be sorted at compile time?
> +}
> +
> +static struct jump_label_entry *get_jump_label_entry(const char *name)
> +{
> + struct hlist_head *head;
> + struct hlist_node *node;
> + struct jump_label_entry *e;
> + u32 hash = jhash(name, strlen(name), 0);
> +
> + head = &jump_label_table[hash & (JUMP_LABEL_TABLE_SIZE - 1)];
> + hlist_for_each_entry(e, node, head, hlist) {
> + if (!strcmp(name, e->name))
> + return e;
> + }
> + return NULL;
> +}
> +
> +static struct jump_label_entry *add_jump_label_entry(const char *name, int nr_entries, struct jump_entry *table)
> +{
> + struct hlist_head *head;
> + struct hlist_node *node;
> + struct jump_label_entry *e;
> + size_t name_len = strlen(name) + 1;
> + u32 hash = jhash(name, name_len-1, 0);
> +
> + head = &jump_label_table[hash & (JUMP_LABEL_TABLE_SIZE - 1)];
> + hlist_for_each_entry(e, node, head, hlist) {
> + if (!strcmp(name, e->name))
> + return ERR_PTR(-EEXIST);
> + }
I see duplicate code here for hashing.
Why not:
e = get_jump_label_entry(name);
if (e)
return ERR_PTR(-EEXIST);
?
> + e = kmalloc(sizeof(struct jump_label_entry) + name_len, GFP_KERNEL);
> + if (!e)
> + return ERR_PTR(-ENOMEM);
> + memcpy(&e->name[0], name, name_len);
> + e->table = table;
> + e->nr_entries = nr_entries;
> + INIT_HLIST_HEAD(&(e->modules));
> + hlist_add_head(&e->hlist, head);
> + return e;
> +}
> +
> +static int build_jump_label_hashtable(struct jump_entry *start, struct jump_entry *stop)
> +{
> + struct jump_entry *iter, *iter_begin;
> + struct jump_label_entry *entry;
> + int count;
> +
> + sort_jump_label_entries(start, stop);
> + iter = start;
> + while (iter < stop) {
> + entry = get_jump_label_entry(iter->name);
> + if (!entry) {
> + iter_begin = iter;
> + count = 0;
> + while ((iter < stop) &&
> + (strcmp(iter->name, iter_begin->name) == 0)) {
> + iter++;
> + count++;
> + }
> + entry = add_jump_label_entry(iter_begin->name, count,
> + iter_begin);
> + if (IS_ERR(entry))
> + return PTR_ERR(entry);
> + continue;
> + }
> + WARN(1, KERN_ERR "build_jump_hashtable: unexpected entry!\n");
> + }
> + return 0;
> +}
> +
> +int jump_label_update(const char *name, enum jump_label_type type)
Non static function without kernel documentation.
-- Steve
> +{
> + struct jump_entry *iter;
> + struct jump_label_entry *entry;
> + struct hlist_node *module_node;
> + struct jump_label_module_entry *e_module;
> + int count;
> +
> + mutex_lock(&jump_label_mutex);
> + entry = get_jump_label_entry(name);
> + if (entry) {
> + count = entry->nr_entries;
> + iter = entry->table;
> + while (count--) {
> + if (kernel_text_address(iter->code))
> + arch_jump_label_transform(iter, type);
> + iter++;
> + }
> + }
> + mutex_unlock(&jump_label_mutex);
> + return 0;
> +}
> +
> +static int init_jump_label(void)
> +{
> + int ret;
> +
> + mutex_lock(&jump_label_mutex);
> + ret = build_jump_label_hashtable(__start___jump_table,
> + __stop___jump_table);
> + mutex_unlock(&jump_label_mutex);
> + return ret;
> +}
> +early_initcall(init_jump_label);
> +
> +#endif
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