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Date: Tue, 3 Jan 2017 14:39:29 -0500
From: Steven Rostedt <rostedt@...dmis.org>
To: Jason Baron <jbaron@...mai.com>
Cc: peterz@...radead.org, mingo@...nel.org,
linux-kernel@...r.kernel.org, Joe Perches <joe@...ches.com>
Subject: Re: [PATCH] jump_label: reduce the size of struct static_key
On Tue, 3 Jan 2017 13:59:09 -0500
Jason Baron <jbaron@...mai.com> wrote:
> In order to identify if the static_key->entries pointer contains a
> struct static_key_mod or a struct jump_entry pointer, bit 2 of
> static_key->entries is set to 1 if it points to a struct static_key_mod and
> is 0 if it points to a struct jump_entry. We were already using bit 1 in a
> similar way to store the initial value of the static_key. This does mean
> that allocations of struct static_key_mod and that the struct jump_entry
> tables need to be at least 4-byte aligned in memory. As far as I can tell
> all arches meet this criteria.
Yes it's OK. We've used the LSB two bits in other places as well.
>
> For my .config, the patch increased the text by 253 bytes, but reduced
> the data + bss size by 19,136, for a net savings of 18,883 bytes.
>
> text data bss dec hex filename
> 8315249 5061176 794624 14171049 d83ba9 vmlinux.pre
> 8315502 5046136 790528 14152166 d7f1e6 vmlinux.post
>
>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ingo Molnar <mingo@...nel.org>
> Cc: Joe Perches <joe@...ches.com>
> Signed-off-by: Jason Baron <jbaron@...mai.com>
> ---
> Documentation/static-keys.txt | 4 +-
> include/linux/jump_label.h | 17 +++++----
> kernel/jump_label.c | 89 +++++++++++++++++++++++++++++++++----------
> 3 files changed, 82 insertions(+), 28 deletions(-)
>
> diff --git a/Documentation/static-keys.txt b/Documentation/static-keys.txt
> index ea8d7b4..32a25fa 100644
> --- a/Documentation/static-keys.txt
> +++ b/Documentation/static-keys.txt
> @@ -155,7 +155,9 @@ or:
>
> There are a few functions and macros that architectures must implement in order
> to take advantage of this optimization. If there is no architecture support, we
> -simply fall back to a traditional, load, test, and jump sequence.
> +simply fall back to a traditional, load, test, and jump sequence. Also, the
> +struct jump_entry table must be at least 4-byte aligned because the
> +static_key->entry field makes use of the two least significant bits.
>
> * select HAVE_ARCH_JUMP_LABEL, see: arch/x86/Kconfig
>
> diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h
> index a0547c5..738d61e 100644
> --- a/include/linux/jump_label.h
> +++ b/include/linux/jump_label.h
> @@ -89,11 +89,13 @@ extern bool static_key_initialized;
>
> struct static_key {
> atomic_t enabled;
> -/* Set lsb bit to 1 if branch is default true, 0 ot */
> +/*
> + * bit 1 => 1 if key is initially true
> + * 0 if initially false
> + * bit 2 => 1 if points to struct static_key_mod
> + * 0 if points to struct jump_entry
> + */
That should be "bit 0" and "bit 1". Bits start at zero.
Also, I believe it may be even more readable if we make that into a
union:
union {
unsigned long type;
struct jump_entry *entries;
struct static_key_mod *next;
};
Then we can remove the typecasts.
> struct jump_entry *entries;
> -#ifdef CONFIG_MODULES
> - struct static_key_mod *next;
> -#endif
> };
>
> #else
> @@ -118,9 +120,10 @@ struct module;
>
> #ifdef HAVE_JUMP_LABEL
>
> -#define JUMP_TYPE_FALSE 0UL
> -#define JUMP_TYPE_TRUE 1UL
> -#define JUMP_TYPE_MASK 1UL
> +#define JUMP_TYPE_FALSE 0UL
> +#define JUMP_TYPE_TRUE 1UL
> +#define JUMP_TYPE_LINKED 2UL
> +#define JUMP_TYPE_MASK 3UL
>
> static __always_inline bool static_key_false(struct static_key *key)
> {
> diff --git a/kernel/jump_label.c b/kernel/jump_label.c
> index 93ad6c1..2eb1ed5 100644
> --- a/kernel/jump_label.c
> +++ b/kernel/jump_label.c
> @@ -234,7 +234,22 @@ static inline struct jump_entry *static_key_entries(struct static_key *key)
>
> static inline bool static_key_type(struct static_key *key)
> {
> - return (unsigned long)key->entries & JUMP_TYPE_MASK;
> + return (unsigned long)key->entries & JUMP_TYPE_TRUE;
return key->type & JUMP_TYPE_TRUE;
> +}
> +
> +static inline bool static_key_linked(struct static_key *key)
> +{
> + return (unsigned long)key->entries & JUMP_TYPE_LINKED;
return key->type & JUMP_TYPE_LINKED;
> +}
> +
> +static inline void static_key_clear_linked(struct static_key *key)
> +{
> + *(unsigned long *)&key->entries &= ~JUMP_TYPE_LINKED;
key->type &= ~JUMP_TYPE_LINKED;
> +}
> +
> +static inline void static_key_set_linked(struct static_key *key)
> +{
> + *(unsigned long *)&key->entries |= JUMP_TYPE_LINKED;
key->type |= JUMP_TYPE_LINKED;
> }
>
> static inline struct static_key *jump_entry_key(struct jump_entry *entry)
> @@ -307,12 +322,9 @@ void __init jump_label_init(void)
>
> key = iterk;
> /*
> - * Set key->entries to iter, but preserve JUMP_LABEL_TRUE_BRANCH.
> + * Set key->entries to iter preserve JUMP_TYPE_MASK bits
> */
> *((unsigned long *)&key->entries) += (unsigned long)iter;
Could probably use key->type here too.
> -#ifdef CONFIG_MODULES
> - key->next = NULL;
> -#endif
> }
> static_key_initialized = true;
> jump_label_unlock();
> @@ -356,13 +368,20 @@ static int __jump_label_mod_text_reserved(void *start, void *end)
>
> static void __jump_label_mod_update(struct static_key *key)
> {
> - struct static_key_mod *mod;
> + struct jump_entry *stop;
> + struct static_key_mod *mod =
> + (struct static_key_mod *)static_key_entries(key);
Have a separate function here:
struct static_key_mod *mod = static_key_mod(key);
>
> - for (mod = key->next; mod; mod = mod->next) {
> + while (mod) {
> struct module *m = mod->mod;
>
> - __jump_label_update(key, mod->entries,
> - m->jump_entries + m->num_jump_entries);
> + if (!m)
> + stop = __stop___jump_table;
> + else
> + stop = m->jump_entries + m->num_jump_entries;
> + if (mod->entries)
> + __jump_label_update(key, mod->entries, stop);
> + mod = mod->next;
> }
> }
>
> @@ -397,7 +416,7 @@ static int jump_label_add_module(struct module *mod)
> struct jump_entry *iter_stop = iter_start + mod->num_jump_entries;
> struct jump_entry *iter;
> struct static_key *key = NULL;
> - struct static_key_mod *jlm;
> + struct static_key_mod *jlm, *jlm2;
>
> /* if the module doesn't have jump label entries, just return */
> if (iter_start == iter_stop)
> @@ -415,19 +434,36 @@ static int jump_label_add_module(struct module *mod)
> key = iterk;
> if (within_module(iter->key, mod)) {
> /*
> - * Set key->entries to iter, but preserve JUMP_LABEL_TRUE_BRANCH.
> + * Set key->entries to iter preserve JUMP_TYPE_MASK bits
> */
> *((unsigned long *)&key->entries) += (unsigned long)iter;
> - key->next = NULL;
> continue;
> }
> jlm = kzalloc(sizeof(struct static_key_mod), GFP_KERNEL);
> if (!jlm)
> return -ENOMEM;
> + if (!static_key_linked(key)) {
> + jlm2 = kzalloc(sizeof(struct static_key_mod),
> + GFP_KERNEL);
> + if (!jlm2) {
> + kfree(jlm);
> + return -ENOMEM;
> + }
> + preempt_disable();
> + jlm2->mod = __module_address((unsigned long)key);
> + preempt_enable();
> + jlm2->entries = static_key_entries(key);
> + jlm2->next = NULL;
> + *(unsigned long *)&key->entries = (unsigned long)jlm2 |
> + static_key_type(key);
Could use key->type here too.
> + static_key_set_linked(key);
> + }
> jlm->mod = mod;
> jlm->entries = iter;
> - jlm->next = key->next;
> - key->next = jlm;
> + jlm->next = (struct static_key_mod *)static_key_entries(key);
> + *(unsigned long *)&key->entries = (unsigned long)jlm |
> + static_key_type(key);
Here as well.
> + static_key_set_linked(key);
>
> /* Only update if we've changed from our initial state */
> if (jump_label_type(iter) != jump_label_init_type(iter))
> @@ -454,16 +490,28 @@ static void jump_label_del_module(struct module *mod)
> if (within_module(iter->key, mod))
> continue;
>
> - prev = &key->next;
> - jlm = key->next;
> + prev = (struct static_key_mod **)&key->entries;
prev = &key->mod;
> + jlm = (struct static_key_mod *)static_key_entries(key);
jlm = static_key_mod(key);
-- Steve
>
> while (jlm && jlm->mod != mod) {
> prev = &jlm->next;
> jlm = jlm->next;
> }
>
> - if (jlm) {
> - *prev = jlm->next;
> + if (!jlm)
> + continue;
> +
> + *prev = (struct static_key_mod *)((unsigned long)jlm->next |
> + ((unsigned long)*prev & JUMP_TYPE_MASK));
> + kfree(jlm);
> +
> + jlm = (struct static_key_mod *)static_key_entries(key);
> + /* if only one etry is left, fold it back into the static_key */
> + if (jlm->next == NULL) {
> + *(unsigned long *)&key->entries =
> + (unsigned long)jlm->entries |
> + static_key_type(key);
> + static_key_clear_linked(key);
> kfree(jlm);
> }
> }
> @@ -558,7 +606,8 @@ static void jump_label_update(struct static_key *key)
> #ifdef CONFIG_MODULES
> struct module *mod;
>
> - __jump_label_mod_update(key);
> + if (static_key_linked(key))
> + __jump_label_mod_update(key);
>
> preempt_disable();
> mod = __module_address((unsigned long)key);
> @@ -567,7 +616,7 @@ static void jump_label_update(struct static_key *key)
> preempt_enable();
> #endif
> /* if there are no users, entry can be NULL */
> - if (entry)
> + if (entry && !static_key_linked(key))
> __jump_label_update(key, entry, stop);
> }
>
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